Formation and Dynamical Evolution of Exoplanets

March 26 - April 1, 2017 | Aspen Center for Physics

Schedule of Talks

All talk sessions will be at the Aspen Center for Physics.
Morning sessions start at 8:30 AM.
Evening sessions start at 4:30 PM.


MONDAY Morning — Observations I  [Chair: Fred Rasio]

    Shangfei Liu (Rice U): Characterizing Newborn Solar Systems with ALMA

    Recently ALMA has discovered ringed structures in a handful of protoplanetary disks. In particular, ALMA's observations of HD 163296 disk trace the spatial distribution of millimeter-sized particles and cold molecular gas on spatial scales as small as 25 AU. By comparing observations with results of two-fluid (gas + dust) hydrodynamic simulations coupled with radiative transfer calculations, we conclude that the middle and outer rings could be due to gravitational torque exerted by two Saturn-mass planets orbiting at 100 and 160 AU. Further numerical studies suggest that the inner dust gap can be interpreted as a Neptune-mass planet orbiting in a MRI dead zone. The technique described here can be applied to other ringed systems that resemble the early stage of the Solar System.

    Michael Meyer (U Michigan): Exoplanet Demographics versus Host Star Mass: Clues to Formation from Direct Imaging

    The distribution of planetary companion masses and the surface density of companions as a function of orbital separation provides a wealth of information concerning planet formation processes as well as the subsequent dynamical evolution of planetary systems. Here we confront recent direct imaging results with predictions for planetary mass distributions as well as very low mass brown dwarf binaries suggesting a local minimum in the companion mass ratio distribution (Reggiani et al. 2016). In addition, we explore predictions of gravitational instability models which include scattering, suggesting this is not the dominant channel for gas giant planet formation (Vigan et al. submitted). We also present the very latest results from the on-going “next generation” SPHERE GTO SHINE imaging survey on the ESO VLT. We explore a new model that fits constraints on the surface density distribution of gas giant planets surrounding M dwarfs combining results from radial velocity, microlensing, and imaging surveys (Meyer et al. submitted). We discuss the implications of these results (local minimum in planet mass function and peak in the orbital distribution) as a function of stellar mass. Model degeneracies can be broken if observations include large samples over a wide range in host star mass.

    Josh Eisner (U Arizona): Observing Planets in their Infancy

    While many exoplanetary systems are now known, observations of planets still in the process of formation are rare. Such observations are challenging, because young planets (and young stars) are typically found at distances >100 pc, and hence the angular separation between star and planet is small. On the other hand, accretion onto forming planets can render them much brighter than more mature exoplanets. Novel techniques used on the largest telescopes are now opening a window through which we can view the physics of planet formation. I will describe recent observations of LkCa 15, a young transition disk with a multiple-planet system contained in the disk clearing. With measured planet positions over a 7-year time-baseline consistent with Keplerian orbtial motion, and planet fluxes and colors compatible with models of accreting planets, this system may be a model of a forming planetary system. I will also highlight a spurious initial detection in another system, and discuss potential false signals and how we can rule them out in our datasets. Finally, I will illustrate how near-future observational capabilities will significantly expand the parameter space for detection of forming planetary systems.

    Lisa Prato (Lowell Obs): A Short-Period Giant Planet in the Young CI Tau Star + Disk System

    Radial velocity variations of the 2 Myr old classical T Tauri star CI Tau show periodic variability of 9 days in both optical and infrared light over a decade. These observations are consistent with a giant planet in a close orbit, just inside the circumstellar disk corotation radius. This represents the first detection of a short-period planet + young star + disk system. We have also maintained a program of optical photometry for this system; the periodicity of the flux variations is variable and likely reflects stellar activity such as spots, flares, accretion events, and possible disk-planet interactions. The radial velocity amplitude in conjunction with inclination measurements of CI Tau's disk yield a planet mass of 12 Jupiters, assuming alignment of the planetary orbital plane with that of the disk observed in millimeter continuum.

    Kevin Wagner (U Arizona): Direct Imaging of a Dynamically Active Young Exoplanet in a Multi-Star System

    HD 131399Ab is a Jovian exoplanet in a 16 Myr old multi-star system that our team recently discovered using direct imaging with VLT/SPHERE. The planet is surprisingly close to the classical boundary of orbital stability at ~1/3 the separation of the hierarchical triple star system (80 AU and 310 AU from the primary star, respectively), raising the question of how this peculiar world arrived at its present orbit and (for now) escaped ejection. I will present our on-going efforts to refine our model of the planet’s orbit, and will highlight how these results will inform our understanding of this unique system’s formation and evolution.

    John Henry Boisvert (UNLV): Uncovering System Architectures Near the 2:1 Resonance

    Uncovering the architectures of planetary systems give insight into their formation and evolution. For example, the protoplanetary disk in multi-planet systems can drive adjacent planets into mean-motion resonances (such as the 2:1), while simultaneously damping their eccentricities. On the other hand, planet-planet scattering will produce single planets with eccentric orbits.

    In the RV signal, there is a degeneracy between models with two planets on circular orbits near the 2:1 period ratio and single planets with an eccentric orbit. Historically, the single planet models have been favored on simplicity grounds. However, the prominence of 2:1 period ratio for systems observed by Kepler motivates additional scrutiny for single eccentric systems.

    We analyzed 118 single planet systems from the Exoplanets Data Explorer to compare three models: a single Keplerian, a circular double with a period ratio of 2:1, and a circular double with a period ratio near 2.17. We used a new Fully Marginalized Likelihood (FML) code to compute the Bayes factors for each of these models in order to determine which model is more likely given the data. A significant fraction of these systems prefer the double planet models---suggesting that disk-migration may be more important than the currently reported parameters suggest.

    Vincent Van Eylen (Leiden Obs): Orbital Dynamics of Single and Multi-Planet Systems

    Eccentricity is a fundamental orbital parameter which holds information about planet formation and evolution as well as habitability. Surprisingly, many massive gas giant planets travel on highly elliptical orbits, in contrast to the orbits of solar system planets which are nearly circular. So far, the orbital shape of smaller, more terrestrial, exoplanets remained largely elusive, because the stellar radial velocity caused by these small planets is extremely challenging to measure. I sidestepped this problem by using photometry from the Kepler satellite and utilizing a method relying on Kepler's second law, which relates the duration of a planetary transit to its orbital eccentricity, if the stellar density is known. I focused on systems where the stellar density is known from asteroseismology. This approach enabled me to measure the eccentricity of planets even smaller than Earth, much smaller than what was previously possible.

    I present eccentricity measurements for 74 planets in multi-planet systems, and 50 systems with a single transiting planet. The multi-planet systems are nearly circular, in full agreement with solar system eccentricities, but in contrast to the eccentricity distributions previously derived for exoplanets from radial velocity studies. The systems with a single transiting planet have significantly higher eccentricities. I relate this findings to obliquity measurements for multi- and single-planet systems. Finally, I link these findings to planet formation and evolution theory and argue that the eccentricity of systems with a single transiting planet may be related to the presence of non-transiting planets on inclined orbits.

    Ben Bar-Or (IAS): Mutual Inclination of Planets near Low-order Mean Motion Resonance

    Many of the planets found by the Kepler mission have an observable transit time variations (TTV). These TTVs are usually due to a second planet, near a low-order mean motion resonance. In most cases, the second planet is not transiting and therefore is unobserved. We use the existence of these unobserved planets, together with the observed ones, to constrain their mutual inclinations.


MONDAY Evening  — Observations II  [Chair: Hilke Schlichting]

    Eric Ford (PSU): The Occurrence Rate of Planetary Architectures

    Published studies of exoplanet occurrence rates based on Kepler observations have focused on the rate of planets within a given range of orbital periods and planet size or mass. Due to a combination of geometric transit probability and detection efficiency, not all planets in a given planetary system will be detected. Therefore, likelihood-based methods for characterizing the occurrence rate of planetary systems would require marginalizing over implausibly high-dimensional parameter space and are not computationally feasible. Approximate Bayesian Computing provides a rigorous framework for making likelihood-free inferences about the rate and distribution of various planetary system architectures. Our research team has begun applying Hierarchical Bayesian models (HBMs) and Approximate Bayesian Computing (ABC) to address a variety of questions about the occurrence rates of planets and planetary architectures, including orbital eccentricities, inclinations, multiplicity and the exoplanet mass-radius relationship. We show this method yields excellent agreement for easily detected planets, but find a significant increase in the occurrence rate for small planets at long orbital periods. We report results of recent effort to characterize the occurrence rate of planetary systems and architectures around FGK stars. We hope (but can't promise) to also have new results on the exoplanet-mass radius relationship based on transit timing variations, while accounting for the dominant detection biases.

    Ethan Kruse (U Washington): Hundreds of New K2 Planet Candidates

    We present our planet search results for K2 campaigns 0-8. We use the Everest light curves, which have better photometric quality than other K2 reductions. Our search more than doubles the number of K2 planet candidates, and we are sensitive to a wide variety of planets. We have found more single transit events, ultra-short period planets, multi-planet systems (including 5+ planet systems), and planets with transit timing variations (TTVs) than any other K2 search to date. We will give a broad overview of the search and results, then highlight some of the most interesting systems.

    Erik Petigura (Caltech) : Dynamics of a Resonant System of Sub-Saturns: Insights from TTVs and RVs

    The Kepler mission enabled measurements of mass and radius for planets as small as Earth, revealing a diversity of planet core and envelope masses. Transit Timing Variations (TTVs) and Radial Velocities (RVs) are the two principle observational techniques for measuring planet mass, but they probe nearly completely disjoint samples of planets. I will present an analysis of K2-24, a rare system amenable to both techniques, using RVs from Keck/HIRES and TTVs from K2 and Spitzer. These two observational techniques offer complimentary views of planet properties such as mass, eccentricity, and resonant configuration, which in turn shed light on their compositions and formation histories.

    Tsevi Mazeh (Tel Aviv U): The Planetary Mass-Radius Relation and its Dependence on Orbital Period as Measured by TTVs and RVs

    The two most common techniques for measuring planetary masses---the radial velocity (RV) and the transit timing variations (TTVs) techniques, have been observed to yield systematically different masses for planets of similar radii. Following Steffen (2016), we consider the effects of the observational biases of the two methods as a possible cause for this difference. We find that at short orbital periods (P<11 day), the two methods produce statistically similar results, whereas at long periods (P>11 day) the RV masses are systematically higher than the TTV ones. We suggest that this is consistent with an RV detection-sensitivity bias for longer periods.

    On the other hand, we do find an apparently significant difference between the short and the long-period planets, obtained by both observing techniques---the mass-radius relationship parameterized as a power law has a steeper index at short periods than at long periods.

    We also point out another anticipated observational bias between the two techniques---multiple planet systems with derived RV masses have substantially larger period ratios than the systems with TTV mass derivation.

    Lauren Weiss (U Montreal): New Insights on Planet Formation in WASP-47 from a Simultaneous Analysis of RVs and TTVs

    Measuring precise planet masses, densities, and orbital dynamics in individual planetary systems is an important pathway toward understanding planet formation. The WASP-47 system has an unusual architecture that invites a complex formation theory. The system includes a hot Jupiter ("b") neighbored by interior ("e") and exterior ("d") sub-Neptunes, and a long-period eccentric giant planet ("c"). We simultaneously modeled the K2 transit times and 118 radial velocities to determine precise masses, densities, and Keplerian orbital elements of the WASP-47 planets. For the transiting inner planetary system, we obtain M_e = 9.1+/-1.0 Earth masses (rho = 9.2 +/- 1.1 gcc), M_b = 358+/-12 Earth masses (rho = 1.02 +/- 0.02 gcc), and M_d =13.6 +/- 2.0 Earth masses (rho = 1.6 +/- 0.2 gcc). Combining RVs and TTVs provides a substantially better estimate of the mass of planet d (13.6 +/- 2.0 Earth masses) than obtained with only RVs (12.75 +/- 2.70 Earth masses) or TTVs (16.1 +/- 3.8 Earth masses). Planets e and d have high densities for their size, consistent with a history of photo-evaporation and/or formation in a volatile-poor environment. Through our RV and TTV analysis, we find that the planets are profoundly circular: e_e < 0.06, e_b < 0.011, and e_d < 0.025 (95% confidence). The WASP-47 system has three similarities to our own solar system: (1) the planetary orbits are nearly circular and coplanar, (2) the planets are not trapped in mean motion resonances, and (3) the planets have diverse compositions. None of the current single-process exoplanet formation theories adequately reproduce these three characteristics of the WASP-47 system (or our solar system). We propose that WASP-47, like the solar system, formed in two stages: first, the giant planets formed in a gas-rich disk and migrated to their present locations, and second, the high-density sub-Neptunes formed in situ in a gas-poor environment.

    George Zhou (Harvard-Smithsonian CfA): Planets around A-stars as Anchors for Planet Migration

    Very few planets have been confirmed around early type stars, since these are often too rapidly rotating for precise radial velocities to be obtained. Of the ~1900 transiting planets known today, only four have been confirmed to transit stars hotter than T>7000K. However, planets around high mass stars are key pieces of the planet formation puzzle, they are important to understanding giant planet formation and migration. For example, the protoplanetary disk mass around A-type stars should be significantly higher than that around solar-type stars, leading to a higher planet occurrence rate, and more compact systems. The stellar multiplicity rate is also higher for higher mass stars. Will the dynamically hotter environment around A-type planet-hosting stars result in different hot-Jupiter migrational pathways than around solar-type stars? We are using the Doppler tomography technique to confirm and characterize these planets around A-stars. I will detail our recent discoveries with the K2, KELT, and HAT surveys, and describe the next steps in defining the planet migration pathways that lead to these hot-Jupiters in the context of TESS.

    Kento Masuda (Princeton): Eccentric and Non-Transiting Close Friends of Two Kepler Warm Jupiters

    We report the discovery of non-transiting close companions to two transiting warm Jupiters observed with Kepler, Kepler-448/KOI-12b (P=17.9days, R=1.23RJup) and Kepler-693/KOI-824b (P=15.4days, R=0.91RJup), via dynamical modeling of their transit timing and duration variations (TTVs and TDVs). The companions are likely a brown dwarf (M=22MJup and a=4.2AU for Kepler-448) and a low-mass star (M=150MJup and a=2.8AU for Kepler-693), both orbiting close to the inner warm Jupiters in highly eccentric (e=0.65 and e=0.47) orbits. The TDVs of Kepler-693b also point to a significant mutual inclination between the inner and outer orbits, while for the Kepler-448 system evidence for the orbit–orbit misalignment is weak and the data are also consistent with the coplanar configuration. Based on the system architectures from the TTVs and TDVs, we compute long-term evolution of the inner orbits and find that their eccentricities can exhibit large oscillations due to the secular perturbation from the companions. These two systems may therefore serve as evidence for the tidal migration of warm Jupiters.

    Re'em Sari (Hebrew U): Geometric Approach to TTV

    We discuss a geometric approach to evaluating Transit Time Variations for planets which are on nearby orbits but not locked in a resonance. This approach, nullifies the need to choose a nearby resonance, simplifies the calculations and illuminates some confusing aspects.



TUESDAY Morning  —  Long-term Dynamics and Stability of Multi-planet Systems  [Chair: Eric Ford]

    Thomas Barclay (NASA Goddard): Exploring the Role that Giant Planets Play in Terrestrial Planet Formation and Evolution

    Terrestrial planets are ubiquitous in our Galaxy with most Sun-like stars home to at least one planet within 1 AU. Giant planets orbiting more than a few AU from their star occur less frequently. Here we investigate how the absence of giant siblings affects properties such as water delivery and impact rates onto terrestrial planets using N-body simulations. We find that these lonely worlds experience significantly more impacts from planetesimals and that these impacts occur much later into the formation process compared to simulations that include giant planets. However, there are far fewer of the most massive collisions that could sterilize a planet and wipe out emerging life. We also find that, without perturbations from giant planets, very little material is ejected from the system on Gyr timescales. We predict that residual material that does not get accreted by the planets or star ends up in a Nano-Oort cloud surrounding the star. Upcoming infrared space telescopes have the potential to detect Nano-Oort clouds if present.

    Ruobing Dong (U Arizona): Observational Planet Formation

    Planets form in gaseous protoplanetary disks surrounding newborn stars. As such, the best way to learn how they form from observations is to directly watch them forming in disks. Thanks to a fleet of powerful instruments with unprecedented resolving power that have come online recently, we are now able to unveil features in resolve observations of protoplanetary disks, such as gaps and spiral arms, that are most likely produced by embedded (unseen) planets. By comparing observations with theoretical models of planet-disk interactions, the locations and masses of these still forming planets may be constrained. Once a large enough sample of high resolution disk observations are accumulated in the next few years, we will be able to infer the statistics of planet formation from these disk observations. By comparing this statistics with the occurrence rate of planets around older stars from direct imaging surveys, the evolution of planetary systems can be probed.

    Andrew Shannon (PSU): Planet Migration in the Solar System: Testing with Oort Cloud Asteroids

    Migration may play a significant role in setting the dynamical state of exoplanetary systems. Determining whether migration takes place, and if so, how much, remains challenging. Due to the wealth of data available, the Solar system can provide an excellent test bed for planetary migration models. As the era of planet migration has ended in the Solar system, modelling is critical to deciphering the signatures of migration that remain. We model the dynamical and collisional formation and evolution of the Oort cloud, with particular emphasis on the injection of asteroids from the inner Solar system into the Oort cloud, so we can generate testable predictions for models of planetary migration histories. We find that the number of Oort cloud asteroids is particularly sensitive to cases where the giant planets migrate through the terrestrial region, such as the Grand Tack model. The discovery of the first confirmed Oort cloud asteroid, C/2014 S3, already represents a tentative detection of such an event.

    Rob Wittenmyer (U S Queensland): Curb your Enthusiasm: Using Dynamical Stability as a Sanity Check for New Planetary Systems

    The discovery of new exoplanets, particularly multi-planet systems, has become almost commonplace in recent years. Advances in radial velocity detection techniques are driving discoveries toward more ``hairy edge'' cases, where the configuration of the planetary system is not always clear from the available (noisy and ill-sampled) data. The parameter space is extremely complex, and the best-fit solution may not be physically feasible. We make a call for vigilance, discussing exemplar systems for which we have performed detailed dynamical stability simulations across a wide range of parameter space around the nominal best-fit. We highlight systems in which a slightly less-favoured solution delivers an eminently more dynamically feasible configuration.

    Hanno Rein (U Toronto Scarborough): Exploring the Long-term Stability of Planetary Systems with New Numerical Tools

    Our own Solar System remains the most studied and most constrained planetary system. To understand extrasolar planetary systems, it is therefore important to put our Solar System into context. I will present a large new set of high accuracy long-term integrations of the Solar System. The results confirm that the Solar System is on the cusp of instability and collisions between planets might occur within its lifetime. I discuss recent developments in the area of numerical integration methods that enabled these simulations. Finally, I will describe a new paradigm for running, analyzing and sharing datasets of N-body simulations.

    Christa Van Laerhoven (UBC): Packing Planets Together: When Neighbors Turn Against Each Other

    Many of the multi-planet systems discovered to date have planets packed much closer together than the planets in our solar system. Notably, these systems are generally not young, indicating that some fraction of very closely packed systems can survive for an extended period of time. We have used numerical simulations to investigate how quickly closely packed planetary systems go unstable. Thus far, we have concentrated on hypothetical planetary systems comprised of a number of Earth-mass planets on placed on initially circular orbits, evenly spaced in mutual Hill Radii, around either a Sun-like star or an M dwarf star (M0 or M5). Generally, the farther apart the planets are, the longer it takes for them to go unstable. In agreement with previous authors who have worked on this problem, we find that for separations between 3 and 8 mutual Hill Radii, the log of the time to instability scales approximately linearly with spacing in mutual Hill Radii. However, we have enough simulations to resolve further structure superimposed on this relation. We see that a system's lifetime can differ by several orders of magnitude over a small difference in planet spacing. We also see that the relationship between stability time and spacing changes for separations larger than about 8 mutual Hill Radii. We will discuss this structure and the implications for stability of closely packed planetary systems.

    Daniel Tamayo (U Toronto): A Machine Learns to Predict the Stability of Tightly Packed Planetary Systems

    The Kepler mission has uncovered hundreds of compact multi-planet systems. The dynamical pathways to instability in these systems and particularly their associated timescales are not well understood theoretically. However, long-term stability is often used as a constraint to narrow down the space of orbital solutions from the transit data. This requires large suites of N-body integrations that can each take several weeks to complete. As a result, it is currently impossible to fully sample the relevant phase space over the lifetimes of Kepler systems.

    We present a new approach using machine-learning algorithms that have enjoyed success across a broad range of high-dimensional industry applications. We have generated large sets of direct N-body integrations of synthetic compact planetary systems to train a variety of machine learning models to classify their stability on timescales of 10 million orbits. We find that by generating informative system features from short integrations, a gradient-boosted-decision-tree classifier can significantly outperform previously proposed heuristic criteria for stability, and is 1000 times faster than direct N-body integration (Tamayo et al., 2016, ApJL).

    We will also present our latest results from a follow-up computational campaign to extend our training set to timescales comparable to the ages of observed systems, and discuss how such models can particularly help characterize multi-planet systems discovered by TESS, given its shorter observation time baselines.

    Jack Lissauer (NASA Ames): Stability of Multi-Planet Systems Orbiting in the Alpha Centauri Stellar System

    We evaluate the extent of the regions within the alpha Centauri AB star system where small planets are able to orbit for billion-year timescales (Quarles & Lissauer 2016, Astron. J. 151, 111), as well as how closely-spaced planetary orbits can be (Quarles & Lissauer, in preparation). Although individual planets on low inclination, low eccentricity, orbits can survive throughout the habitable zones of both stars, perturbations from the companion star imply that the spacing of planets in multi-planet systems within the habitable zones of each star must be significantly larger than the spacing of similar multi-planet systems orbiting single stars in order to be long-lived. Because the binary companion induces a forced eccentricity upon the orbits of planets in orbit around either star, appropriately-aligned circumstellar orbits with small initial eccentricities are stable to slightly larger initial semimajor axes than are initially circular orbits. Initial eccentricities close to forced eccentricities can have a much larger effect on how closely planetary orbits can be spaced, and therefore on how many planets may remain in the habitable zones, although the required spacing remains significantly higher than for planets orbiting single stars.


TUESDAY Evening  — Orbital Resonances and Chaos  [Chair: Jason Steffen]

    Margaret Pan (MIT): Where are the Resonant Exoplanet Pairs?

    Of the ~1400 planets now known to reside in multiple planet systems, the vast majority do not lie in or near mean motion resonances. This appears to contradict the established wisdom that pairs of planets undergoing convergent migration due to disk-planet interactions should efficiently capture into resonance. Several explanations have been proposed for this, including escape from resonance due to overstable librations or disk turbulence; inefficient capture due to eccentricity excitation by disk warps; or even a complete absence of migration, ie. in situ formation. I will discuss another possibility: decreased resonance capture probabilities for smaller planets and gravitational stirring among neighboring planets may conspire to make resonant planet pairs unlikely around solar-type stars but more likely in M dwarf systems.

    Sam Hadden (Northwestern): Resonance Overlap and the Origins of Chaos in Multi-planet Systems

    Determining the orbital stability of planetary systems is longstanding open problem, even in the simplest case of two planet. Resonance overlap, which gives a heuristic criterion for the onset of chaos in conservative dynamical systems, provides one of the few analytic tools with which to attack the problem. Resonance overlap has previously been applied to derive analytic results for the transition to chaos in closely packed two-planet systems. However, past resonance overlap criteria have considered only first-order mean motion resonances, limiting their applicability to eccentric planets. I will describe a resonance overlap criterion that includes mean motion resonances at all orders in eccentricity in order to predict the transition to chaos for pairs of massive, eccentric planets. I will also discuss results on mapping resonance overlap and chaos in three-planet systems.

    Sarah Millholland (Yale): Constraints on Planet Nine within a Mean-motion Resonant Framework

    A number of authors have proposed that the statistically significant orbital alignment of the most distant Kuiper Belt Objects (KBOs) is evidence of an as-yet undetected planet in the outer solar system, referred to colloquially as “Planet Nine”. Dynamical simulations by Batygin & Brown (2016) have provided constraints on the range of the planet's possible orbits and sky locations. We have recently extended these investigations by exploring the suggestion of Malhtora et al. (2016) that Planet Nine is in small integer ratio mean-motion resonances (MMRs) with several of the most distant KBOs (Millholland & Laughlin 2017). We show that the observed KBO semi-major axes present a set of commensurabilities with an unseen planet at ~654 AU that has a greater than 98% chance of stemming from a sequence of MMRs rather than from a random distribution. We describe the constraints resulting from a Monte-Carlo optimization scheme that used billion-year dynamical integrations of the outer solar system to pinpoint the orbital properties of a planet that is capable of maintaining the KBOs' apsidal alignment. We also demonstrate a connection between the KBOs’ ability to maintain alignment and orbital stability and their participation in resonance. Finally, we discuss constraints on the planet’s current sky position.

    Andreas Quirrenbach (U Heidelberg): Two Brown Dwarfs Orbiting the Giant Star nu Oph in a 6:1 Mean Motion Resonance

    We present radial-velocity (RV) measurements for the K giant nu Oph, which reveal two brown dwarf companions with a period ratio close to 6:1. For our orbital analysis we use 150 precise RV measurements taken at Lick Observatory between 2000 and 2011, combined with literature data. Using a stellar mass of M=2.7Msun for nu Oph and applying a self-consistent N-body model we estimate the minimum companion masses to be 22.2Mjup and 24.7Mjup, with orbital periods of 530d and 3185d, respectively. We study a large set of potential orbital configurations for this system, employing a bootstrap analysis and a systematic chi2 grid-search coupled with our dynamical fitting model, and we examine their long-term stability. We find that the system is indeed locked in a 6:1 mean motion resonance (MMR), with all six resonance angles librating around 0deg. This is the highest-order MMR known. We also test a large set of coplanar inclined configurations, and we find that the system will remain in a stable resonance for most of them. The nu Oph system is important for probing planetary formation and evolution scenarios. It seems very likely that the two brown dwarf companions of nu Oph formed like planets in a circumstellar disk around the star and have been trapped in a MMR by smooth migration capture. This capture process is investigated in more detail in the accompanying paper by T. Trifonov.

    Trifon Trifonov (MPIA): Formation and Resonance Architecture of the HD202206 and ν Oph Brown Dwarf Systems

    The K giant ν Oph has two brown dwarf companions locked in a strong 6:1 mean-motion resonance (MMR), while the main-sequence star HD 202206 has an eccentric inner brown dwarf companion and an outer Jovian mass planet forming a 5:1 MMR pair. These are the highest order MMRs discovered to date, and they are important for probing planetary formation and evolution scenarios. In particular, they hint that brown dwarfs can form like planets in a circumstellar disk around the star and be captured into MMR by disk induced migration.

    To determine the probabilities of capture at their current 5:1 and 6:1 MMR, we have performed a series of three-body simulations using SyMBA modified to include a forced migration rate ((da/dt)/a) in Jacobi orbital elements. Starting with different migration rates (da/dt)/a and initial orbital geometries beyond the MMR ratio, we find that the resonances of ν Oph and HD 202206 can be accurately reproduced, if the initial orbits are eccentric and the migration rate is rather slow.

    Our analysis showed that for (da/dt)/a = 10^-6 to 10^-7 the probability of capture into the 6:1 and 5:1 MMR are approximately 15% and 20%, respectively.

    Darin Ragozzine (BYU): Improved Understanding of Planetary Spacings by including Observational Biases

    NASA's Kepler Space Telescope has identified ~700 systems of multiple transiting exoplanets. We provide a detailed update on the architectures of these systems with a focus on the period ratio distribution and a clear understanding of the biases inherent in multi-transiting geometry. Kepler systems have precisely measured period ratios and we will present a detailed fit to the period ratio distribution. Our fit quantitatively identifies the strength of several excesses (including the well-known excesses near the 3:2 and 2:1 resonances) and probabilistically assigns Kepler planet pairs to these excesses or to the remaining "continuum" population. The demographics of the near-resonance populations are explored in detail including the demographics of Transit Timing Variations. We present evidence that near-resonance "excesses" are not due to an actual enhancement in frequency, but due to their systematically lower mutual inclinations (which leads to a higher detection probability). We show strong evidence that Radial Velocity observations are frequently missing small intermediate planets, with serious implications for the completeness of RV identification of planets in STIPs. We extend the period ratio spacing work to entire systems by "normalizing" all systems to a single period/radius scale. Basic debiasing with CORBITS (Brakensiek & Ragozzine 2016) allows us to identify features of the planetary architectures including clear preferences for equally-spaced periods and larger radii at longer periods.

    Federico Mogavero (IAP): Towards the Statistical Mechanics of Planet Orbits in the Solar System

    The chaotic nature of planet dynamics in the solar system suggests the relevance of a statistical approach to planetary orbits. In such a statistical description, the time-dependent position and velocity of the planets are replaced by the probability density function (PDF) of their orbital elements. It is quite natural to set up this kind of approach in the framework of statistical mechanics. Indeed, Tremaine (2015) has recently addressed the statistical mechanics of terrestrial planet formation. In the present study, I focus on the collisionless excitation of eccentricities and inclinations by gravitational interactions in a planetary system. The prototype of such dynamics are the future planet trajectories in the solar system. I thus address the problem of constructing the statistical mechanics of the solar system planet orbits by investigating an ergodic ansatz based on conservation of angular momentum and energy. I compare the resulting predictions to the PDFs numerically constructed by Laskar (2008). I also revisit his random walk ansatz to account for the time dependence of these PDFs. Such a statistical theory could be combined with direct numerical simulations of planet trajectories in the context of planet formation, which is likely to be a chaotic process.

    David Nesvorny (SwRI): Dynamics and Transit Variations of Resonant Exoplanets

    The Transit Timing Variations (TTVs) are deviations of the measured mid-transit times from the exact periodicity. One of the most interesting causes of TTVs is the gravitational interaction between planets. Here we consider a case of two planets in a mean motion resonance (orbital periods in a ratio of small integers). This case is important because the resonant interaction can amplify the TTV effect and allow planets to be detected more easily. We develop an analytic model of the resonant dynamics valid for small orbital eccentricities and use it to derive the principal TTV terms. We find that a resonant system should show TTV terms with two basic periods (and their harmonics). The resonant TTV period is proportional (m/M*)^{-2/3}, where m and M_* are the planetary and stellar masses. For m=10^{-4} M*, for example, the TTV period exceeds the orbital period by ~2 orders of magnitude. The amplitude of the resonant TTV terms scales linearly with the libration amplitude. The ratio of the TTV amplitudes of two resonant planets is inversely proportional to the ratio of their masses. These and other relationships obtained in this work can be used to aid the interpretation of TTV observations.



WEDNESDAY Morning  — Strong Planet–Planet Interactions and Collisions  [Chair: Christa Van Laerhoven]

    Amaury Triaud (Cambridge U): Update on the TRAPPIST-1 System

    I will briefly review our observations of the TRAPPIST-1 system and present updates on our understanding of the system. I will describe how we predicted the orbit of planet h, and recovered it successfully from the K2 data, by assuming it was in a Laplace resonance, like all other triplets of planets. I will also discuss the occurrence rate of planets orbiting very low-mass stars, and compare the TRAPPIST-1 planets to those orbiting Sun-like stars, which gives us clues about their formation. I will also detail our plans to refine the planetary masses by measuring transit timings throughout the year.

    Jason Steffen (UNLV): Observable Consequences of Orbital Inclination and the Stability of Planetary Systems

    Instability plays a central role in the late stages of planet formation. For small, close-in planets the instability is primarily manifest through planet-planet collisions. Most studies of instability invoke coplanar systems and define the timescale to instability as the time to the first close encounter between planets. Through a suite of numerical simulations, we find significantly different behavior of systems where the planets have mutual inclinations of even a few degrees. For mutually inclined systems, the collision timescale can often exceed the encounter timescale by two orders of magnitude and as the initial mutual inclinations increase, the distribution of collision timescales evolves from mostly prompt collisions following a close encounter to mostly delayed collisions. We present the results of our investigation into these effects and discuss their implications for the final architectures of exoplanetary systems and the interpretation of exoplanet data.

    Hilke Schlichting (MIT/UCLA): Stealing the Gas: Giant Impacts and the Large Diversity in Exoplanet Densities

    Super-Earths and mini-Neptunes display a large diversity in their gas-mass fractions and bulk densities. This diversity is especially surprising for observed exoplanets residing in tightly-packed multiple-planet systems. These observations are challenging to explain by gas accretion and subsequent sculpting by photo-evaporation alone. We suggest that the large observed range in exoplanet bulk densities maybe due to one or two giant impacts that occurred late in their evolution once the gas disk dissipated. We show that giant impacts can modify the bulk composition of a super-Earth by factors of a few and, in some cases, lead to complete atmospheric loss. Such late giant impacts are likely to be common because super-Earths must have formed in the presence of the gas disk and their dynamical interaction with the disk is expected to have resulted in migration and efficient eccentricity damping. This leads to densely-packed planetary systems. As the gas disk dissipates, mutual gravitational excitations between the planets cause their eccentricity to growth culminating in one or two giant impacts before reaching long-term orbital stability.

    Sourav Chatterjee (Northwestern): Are Dynamical Processes Responsible for the Structural Diversity of Sub-Neptunes?

    Low average densities and small Hill radii of most planets found by Kepler make planet-planet collisions and planetesimal accretion by planets very important. Since the mass is dominated by a small dense core but the volume is dominated by a gaseous envelope, the sticky-sphere approximation, usually adopted in Nbody models, is particularly bad. Instead, planet-planet collisions may lead to many different outcomes including hit-and-run, gas expulsion, gas exchange (between planets), and even binary planets. Similarly, the period-ratio distribution of adjacent planet pairs indicate that planets may have accreted planetesimals few-10% of its mass after gas disk dispersal. If true, the total mass in accreted planetesimals is comparable to the mass of typical envelopes. Such accretion can alter a planet's total gas mass, and its final average density. We will present our latest results from simulations combining dynamical modeling of planetary orbits, MESA modeling of planetary structures, and SPH modeling of planet-planet collisions, and show the effects of these common dynamical processes on the observable structural properties of typical exoplanets.

    Xu Huang (MIT): Linking Giant Planets to Super Earths

    Giant planets and super Earths are often studied as they were two isolated population. The loneliness of hot Jupiters (except for the WASP-47 system) suggests that they may be mutually exclusive with super Earths in the formation path. In this talk, I will examine systems which cohabiting giant-planets (outside of 1 AU) and super-Earths, and discuss how these giant planets shape up the architectures of their planetary systems.

    System like this: cold Jupiters with close-in super-Earth companions have been discovered in Radial velocity surveys. Using N-body simulations, we show the formation of these cold Jupiters, through scattering events, would excite the super-Earths in the same system, leaving them dynamically hot. The recent discoveries of eccentric and inclined Kepler single-transiting planets is consistent with the predicted dynamically hot super Earths from this mechanism.

    Richard Alexander (U Leicester): Spontaneous Breaking of Mean-motion Resonances during Type I Migration

    It has long been recognized that slow migration through a gas-rich protoplanetary disc leads to pairs of planets being captured in mean-motion resonances, but such resonances are rarely observed in real exoplanetary systems. Here we present a new mechanism for breaking mean-motion resonances between planets undergoing Type I migration. We use 2-D hydrodynamic simulations to follow the migration of pairs of super-Earth-mass planets through the inner regions of protoplanetary discs, where convergent migration naturally causes the planets to become trapped in mean-motion resonances. Once in resonance the planet-planet interaction increases the orbital eccentricity, and this eccentricity qualitatively alters the planet-disc interaction. Our simulations show that this change in the planet-disc torque results in the planets migrating out of resonance, typically on a time-scale of a few thousand orbits. This mechanism appears to operate over a broad range of disc conditions, and may have wide applicability. We discuss the implications of these results for the formation of tightly-packed planetary systems. If mean-motion resonances are readily broken in this manner, disc-driven migration from beyond the snow-line is a highly plausible formation channel for these close-packed systems.




    Free Evening

    (Public Lecture by Josh Winn at the Wheeler Opera House, starting at 5:30 PM)



THURSDAY Morning — Circumbinary Planets  [Chair: Elisa Quintana]

    Nader Haghighipour (U Hawaii): A Comprehensive Study of the Existence and Frequency of Inclined and Non-Transiting
          Circumbinary Planets

    A survey of the currently known circumbinary planets indicates that the orbits of many of these objects are slightly inclined and they process with rates that place them out of transit for the majority of time. We have carried out a comprehensive 3D study of the post-formation evolution of CBPs (including planet-disk interaction, planet migration, and planet-planet scattering) and a detailed analysis of the orbital dynamics of these objects. We have paid specific attention to the orbital evolution of those on inclined orbits. Results point to the existence of (3D) islands of stability where CBPs maintain stable orbits and never transit. We have determined the frequency of transiting and non-transiting CBPs for different parameters of the binary and orbital inclination of the planet, and have also derived, for the first time, empirical formulae to calculate the locations of the boundary of stability for inclined CBPs. We have examined the possibility of the accumulation of CBPs near these (3D) stability boundaries and found that no such preference exists. We show that, consistent with observational results, majority of CBPs are on inclined, and even non-transiting orbits and that these orbits are the natural results of the post-formation evolution of these objects.

    William Welsh (San Diego State) : KIC 8610483: A Non-transiting Circumbinary Planet

    Kepler has revealed roughly a dozen transiting circumbinary planets. In addition to these, there are three binary systems that show very good evidence for hosting a non-transiting planet. In this talk I will present our investigation of one of these, KIC 8610483, a 49-day eclipsing binary system with a ~50 Earth-mass non-transiting planet. The planet's presence is made known by its perturbation of the binary orbit, in particular, the eclipse-timing variations seen in an O-C diagram. The rate of apsidal motion allows us to estimate the planet's mass, and the periodic ripples allows us to measure the planet's ~385-day orbital period. A very interesting dynamical aspect of this planet's orbit is that in addition to a solution with a low mutual inclination, a high-inclination orbit (roughly 45 degrees from the binary plane) also agrees well with the observations. This is unlike all the other known circumbinary planets. Such a high-inclination orbit is stable, and over a period of roughly 150 years its precession causes the inclination to vary between ~40-75 degrees from the binary plane. In this scenario, the planet never comes closer than ~40 degrees to being co-planar.

    Gongjie Li (Harvard): Uncovering Circumbinary Planetary Architectural Properties from Selection Biases

    Studying newly discovered circumbinary planetary systems improves our understanding of planetary system formation. Learning the architectural properties of these systems is essential for constraining the different formation mechanisms. We first revisit the stability limit of circumbinary planets. Next, we focus on eclipsing stellar binaries and obtain an analytical expression for the transit probability in a realistic setting, where a finite observation period and planetary orbital precession are included. Our understanding of the architectural properties of the currently observed transiting systems is then refined, based on Bayesian analysis and a series of tested hypotheses. We find that (1) it is not a selection bias that the innermost planets reside near the stability limit for eight of the nine observed systems assuming a uniform distribution in the planetary semimajor axis, but this pile-up is consistent with a log-uniform distribution of the planetary semimajor axis; (2) it is not a selection bias that the planetary and stellar orbits are nearly coplanar (≤3º), and this along with previous studies may imply an occurrence rate of circumbinary planets similar to that of single star systems; (3) the dominance of observed circumbinary systems with only one transiting planet may be caused by selection effects; (4) formation mechanisms involving Lidov-Kozai oscillations, which may produce misalignment and large separation between planets and stellar binaries, are consistent with the lack of transiting circumbinary planets around short-period stellar binaries, in agreement with previous studies. As a consequence of (4), eclipse timing variations may better suit the detection of planets in such configurations.

    Rebecca Martin (UNLV): Polar Alignment of a Protoplanetary Disk around an Eccentric Binary

    We show that an initially mildly misaligned circumbinary accretion disk around an eccentric binary can evolve to an orientation that is perpendicular to the orbital plane of the binary (polar alignment). The process operates above a critical initial misalignment angle that depends upon the eccentricity of the binary and the mass of the disk. This evolution has important implications for planet formation around eccentric binary star systems.

    Eric Jensen (Swarthmore): Alignment of Stellar and Planetary Orbits in Young Binary Systems

    The dynamical evolution of multi-body systems can depend strongly on the relative inclinations of the orbits in the system. Our own solar system (with a single star) hosts planets with nearly coplanar orbits, but the dynamics in multiple-star systems are likely to be more complex.

    The observable spatial orientation and rotation of protoplanetary disks provides an opportunity to probe the orientation of planetary orbits at the time of formation, revealing information both about formation mechanisms and about conditions for future orbital evolution. When such disks are in binary systems (either as circumstellar disks around the individual stars, or as circumbinary disks outside the binary orbit), their orientations can be compared with information about the binary system in order to determine relative orientations of orbits. We have mapped circumstellar and circumbinary disks in a number of young binaries with ALMA. Results so far are consistent with a picture in which the circumstellar disks in wide binaries are misaligned with each other, and thus not closely aligned with the binary orbit, providing the necessary conditions for Kozai-Lidov oscillations. In contrast, observations to date suggest that circumbinary disks are aligned with stellar binary orbits to within a few degrees.

    Adam Kraus (UT Austin): The Ruinous Influence of Close Binary Companions on Planetary Systems

    The majority of solar-type stars are found in binary systems, and the dynamical influence of binary companions is expected to profoundly influence planetary systems. However, the difficulty of identifying planets in binary systems has left the magnitude of this effect uncertain; despite numerous theoretical hurdles to their formation and survival, at least some binary systems clearly host planets. We present high-resolution imaging of nearly 500 Kepler Objects of Interest (KOIs) obtained using adaptive-optics imaging and nonredundant aperture-mask interferometry on the Keck II telescope. We super-resolve some binary systems to projected separations of under 5 AU, showing that planets might form in these dynamically active environments. However, the full distribution of projected separations for our planet-host sample more broadly reveals a deep paucity of binary companions at solar-system scales, matching similar results for protoplanetary disk survival in star-forming regions. Our results demonstrate that a fifth of all solar-type stars in the Milky Way are disallowed from hosting planetary systems due to the influence of a binary companion.

    Laetitia Rodet (U Grenoble): On the Origin of the Wide-orbit Circumbinary Giant Planet HD106906: A Dynamical Scenario and its Impact
          on the Disk

    HD 106906 b is a 11 MJ exoplanet identified in 2014 by direct imaging and located at a projected separation of 730 AU from its host star. An asymmetric belt of debris was recently resolved around the central star, and complementary observations revealed that the latter is in fact a pair of intermediate-mass stars. Given its present configuration and the multiple observations gathered so far, the HD 106906 system represents a unique test-bed for the formation and dynamical evolution models of planetary systems!

    I will present the dynamical simulations we have conducted to explain the architecture of the system. We studied a tentative scenario where the planet first forms close-in before being ejected via resonant interaction with the central binary, and furthermore stabilizes at large distance possibly thanks to a fly-by. Our approach is both semi-analytical and numerical. This allowed us to derive probability estimates of the proposed scenario. We demonstrated that the current shape of the disk does not constraint drastically the formation scenario, but suggests that the planet orbit is eccentric. We will finally give an insight of the planet-related processes that could have caused the observed asymmetries within the disk.

    Veselin Kostov (NASA Goddard): Tatooine’s Future: The Eccentric Response of Kepler’s Circumbinary Planets to Common-Envelope
          Evolution of their Host Stars

    Inspired by the recent Kepler discoveries of circumbinary planets (CBPs) orbiting nine close binary stars, we explore the fate of the former as the latter evolve off the main sequence. We combine binary star evolution models with dynamical simulations to study the orbital evolution of these planets as their hosts undergo common-envelope stages. Five of the systems experience at least one Roche-lobe overflow and common-envelope stages; two systems trigger a double-degenerate supernova explosion. Kepler’s circumbinary planets predominantly remain gravitationally bound at the end of the common-envelope phase, migrate to larger orbits, and may gain significant eccentricity; their orbital expansion can be more than an order of magnitude and can occur over the course of a single planetary orbit. The orbits these planets can reach are qualitatively consistent with those of the currently known post-common-envelope, eclipse-time variations circumbinary candidates. Additionally, unlike Mercury orbiting the Sun, a circumbinary planet with the same semi-major axis can survive the common envelope evolution of a close binary star with a total mass of 1 MSun.


THURSDAY Evening — Highly Inclined Systems and Hierarchical Triples  [Chair: Erika Nesvold]

    Josh Winn (Princeton): Stellar Obliquities: a Synthesis of Results

    The obliquities of planet-hosting stars range widely, from very well-aligned to retrograde orientations. Many explanations have been offered for this wide distribution, including primordial star/disk tilts, Kozai cycles, and planet/planet scattering. I will review the knowledge that has been gained through 6 different measurement techniques (including a new technique I will describe), and evaluate the proposed theories in light of all the data.

    J.J. Zanazzi (Cornell): Primordial Misalignments from Star-Disk-Binary Interactions: Additional Disk Physics

    Gravitational interactions between a protostar, a circumstellar disk, and a highly inclined binary companion have been shown to produce substantial misalignments between the stellar spin and disk angular momentum axis. This excitation occurs when the frequency of disk precession driven by the binary becomes comparable to the frequency of stellar precession driven by the disk, and has been proposed as a possible route to generate spin-orbit misalignment in Hot Jupiter systems while the planet is still embedded in the natal porto-planetary disk. Previous works on star-disk-binary systems typically assume the simplest prescriptions for the disk physics: a flat disk with homologous surface density evolution. Here we explore how disk warping from viscosity effects the dynamical evolution of star-disk-binary systems. We find that the viscous dissipation from disk warping may cause the stellar spin axis to align with the disk angular momentum on timescales less than a few million years, depending on the disk surface density distribution. We study how photo evaporation and a non-constant viscosity effects the resonance crossing between the frequency of disk precession driven by the binary and the frequency of stellar precession driven by the disk. We also examine the coupling between an inclined planet and precessing disk. Last, we comment on mechanisms involving magnetic torques between the star and porto-planetary disk to generate star-disk misalignments.

    Christopher Spalding (Caltech): Spin-Orbit Misalignment as a Driver of the Kepler Dichotomy

    Within the Kepler dataset, the number of single-transiting planetary systems relative to those with multiple transits, is consistent with roughly half of such systems possessing low mutual inclinations between planetary orbits. However, an overabundance of single-transiting systems suggests the presence of a separate population within which planetary orbits pose significant mutual inclinations. The physical origin of this so-called “Kepler Dichotomy” remains elusive. Here we show that the mutual inclinations required to explain the single systems arise as a natural consequence secular interactions with a host star whose spin axes is inclined with respect to the natal disk. Such primordial misalignments arise naturally within the disk-hosting stage by way of gravitational torques from stellar companions, and have been previously invoked as explanations for the commonness of spin-orbit misalignments in hot Jupiter systems. Accordingly, our model places the early dynamical evolution of hot super-Earths and hot Jupiters into a unified theoretical framework.

    Adrian Hamers (IAS): Hints for Hidden Planetary Companions to Hot Jupiters in Stellar Binaries

    Searches for stellar companions to hot Jupiters (HJs) have revealed that planetary systems hosting a HJ are more likely to have a stellar companion with a semimajor axis between 50 and 2000 AU, compared to field stars. This correlation suggests that HJ formation is affected by the stellar binary companion. A previously considered model is high-eccentricity migration, in which the binary companion induces high-eccentricity Lidov– Kozai (LK) oscillations in the proto-HJ orbit, triggering orbital migration driven by tides. A pitfall of this “binary-LK” model is that the observed stellar binaries hosting HJs are typically too wide to produce HJs in sufficient numbers because of suppression of LK oscillations by short-range forces. We propose a modified model in which there is a second giant planet orbiting the proto-HJ at a semimajor axis of several tens of au. Such companions are currently hidden to observations, but their presence could be manifested by a propagation of the perturbation of the stellar binary companion inward to the proto-HJ, thereby overcoming the barrier imposed by short-range forces. Our prediction for planetary companions to HJs in stellar binaries should be testable by future observations.

    Cristobal Petrovich (CITA): Constraining the Dynamical History of Warm Jupiters with GAIA

    The dynamical history of the warm Jupiters (gas giants at ~0.1-1 AU) remains elusive. One intriguing scenario, termed high-eccentricity migration, involves their orbital migration due to the secular interactions with an outer massive companion and tidal dissipation. Recent constraints on this scenario based on the warm Jupiters eccentricities, stability, and apsidal orientations have been put forward. I will discuss a new set of constraints involving the mutual inclinations between the warm Jupiters and outer planetary companions, and argue that high-eccentricity migration predicts a significantly wider distribution compared to the alternative dynamical history scenarios. I will show that the Gaia’s end-of-mission astrometric measurements combined with the current radial velocity data has the potential to measure the mutual inclinations of a few of these systems with uncertainty levels that will provide an important new test-bed to constrain the origin of this class of warm Jupiters. Further constraints will be possible from warm Jupiter discoveries in ongoing and future transit-based planet searches.

    Trent Dupuy (UT Austin): Testing Orbital Alignment in Planet-Hosting Binaries

    Most planetary systems only offer the possibility to measure either the initial conditions of planet formation (e.g., protoplanetary disks) or the final outcome (e.g., demographics of field samples). Planet-hosting binaries offer the rare opportunity to observe both simultaneously. We will present new results from our continuing Keck adaptive optics program to monitor the stellar orbits of Kepler planet hosts that have binary companions at solar-system scales of 20–100 AU. The astrometric orbital arcs that we measure enable a fundamental test: whether or not the stellar orbits are seen edge-on and thus co-aligned with the transiting planets in the system. This orbit-orbit alignment test allows us to critically examine the possible formation pathways for these systems. We will also discuss preliminary results for a subset of our sample for which we have obtained resolved radial velocities with NIRSPAO that allow us to determine additional orbit parameters (eccentricity and semimajor axis). Full orbit determinations will allow us to address whether special conditions, e.g., circular orbits, are needed to promote planet formation in binary systems, perhaps explaining why the systems in our sample survived in the first place while close stellar companions seem to suppress planet formation in most other comparable binary systems.

    Alexander Stephan (UCLA): The Fate of Planets in Evolving Binary Systems

    Exoplanets have been observed around a variety of host stars, at all stages of evolution, including main-sequence, sub giant and red giant branch stages. Furthermore, observations of polluted white dwarfs suggest that these have accreted planetary object remnants. Since a significant fraction of sun-like stars reside in binaries, those stellar companions have a significant role in shaping the planetary configurations. We study the long term dynamical and tidal evolution of planets during the different stages of the host stars' main and post-main sequence evolution, in stellar binaries. We find a new population of short lived Jupiter like planets around evolved stars, which we call Transitional Hot Jupiters (THJs). These emerge from the interplay of stellar evolution, tides, and secular dynamics. The results show a consistency with the absence of observed short period eccentric Jupiter-sized planets around evolved stars.

    We also find a population of planets and Kuiper-like belt objects that can survive through the entire post main sequence evolution only to be accreted during the white dwarf phase. We suggest that this mechanism can naturally explain the observed population of white dwarfs polluted with volatile material.



FRIDAY Morning  — Tides, Ultra-short Periods, and Planet–Star Interactions  [Chair: Rebekah Dawson]

    Hubert Klahr (MPIA): Gravo-turbulent Formation of Planetesimals – Why Things are 100 km in Size

    Comets, asteroids and Kuiper Belt objects are left overs of building material for our earth and the other planets in our solar system. At the time of their formation these originally 100 km large objects were called planetesimals, built up from icy and dusty grains. In our current paradigm of solar system formation, it was turbulent flows and metastable flow patterns like zonal flows and vortices that concentrated mm to cm sized grains in sufficient numbers that a streaming instability and gravitational collapse of these particle clumps was triggered. What was missing until recently was a physically motivated prediction on the typical sizes at which planetesimals should form via this process. With the latest series of simulations covering all the length scales down to the physical size of actual planetesimals we were able to obtain values for the turbulent particle diffusion as a function of the particle load in the gas. Thus, we have all necessary data at hand to feed a ’back of the envelope’ calculation that predicts the initial size of planetesimals as result of a competition between gravitational concentration and turbulent diffusion. Using the diffusion values obtained in numerical simulations we predict preferential planetesimal sizes on the order of 100 km, which coincides with the measured data from both asteroids as well from Kuiper Belt objects. Based on these findings we develop a recipe to introduce planetesimal formation into population synthesis models for Exo-Planets.

    Quentin André (CEA Saclay): Layered Semi-convection and Tides in Giant Planet Interiors

    Layered semi-convection is a strong candidate to explain Saturn’s luminosity excess and may partly explain the abnormally large radii of some hot Jupiters. In this talk, I will describe our new analysis of the transmission of internal (gravity or inertial) waves across a region of layered semi-convection and their associated density staircases (consisting of convective layers separated by thin stably stratified interfaces). Then, I will discuss the impact of layered semi-convection upon the rates of tidal dissipation, and show that it can be enhanced over a standard uniformly convective model. Thus, layered semi-convection stands as a good candidate that may help to explain recent observations suggesting higher tidal dissipation rates than previously thought in our Solar system's giant planets.

    Adrian Barker (U Leeds) Nonlinear Tidal Flows in Planets and Stars

    Tidal interactions between short-period planets and their host stars are thought to play an important role in the evolution of the planetary orbit as well as the stellar and planetary spins. However, the mechanisms responsible for tidal dissipation are not well understood theoretically. I will present results from hydrodynamical (and MHD) simulations of nonlinear tidal flows in short-period gaseous planets and stars from first principles. I will discuss the outcome of two fluid instabilities that could be important for tidal dissipation: the elliptical and precessional instabilities. The elliptical instability is shown to be important for the circularization, spin-orbit synchronization and planetary spin-orbit alignment of short-period planets. The precessional instability occurs in planets with nonzero obliquities that undergo axial precession, and is found to be important in driving tidal evolution of the planetary spin-orbit angle for hot Jupiters. These results have implications for the observability of axial precession of extrasolar planets by e.g. transit depth variations.

    Sivan Ginzburg (Hebrew U): Tidal Heating of Super Earths

    Short-period super Earths with voluminous gas envelopes seem to be very common. The accretion rate of these gas atmospheres from the protoplanetary disk is determined by their ability to cool and radiate away their gravitational energy. Here, we demonstrate that heat from the tidal interaction between the star and the young (and therefore inflated) planet can inhibit the gas cooling and accretion, provided that the initial eccentricities are high enough. Thus, tidal heating provides a mechanism that can simultaneously explain why these planets did not become gas giants and account for the deficit of low-density planets closer to the star, where the tides are even stronger. We suggest that tidal heating may be as important as other factors (such as the nebula's lifetime and atmosphere evaporation) in shaping the observed super-Earth population.

    Benjamin Fulton (U Hawaii/Caltech): Small Planets Come in Two Sizes

    We present new observational evidence showing two distinct populations of small planets. This result is born from precise stellar parameters for a large sample of Kepler planet hosts. The distribution of radii for planets smaller then Neptune is bimodal with two peaks that suggest that these planets tend to have radii of either ~2.5 R⊕ or ~1.3 R⊕, with relatively few planets in-between. We interpret this bimodality in the planet radius distribution as the signature of atmospheric stripping caused by extreme stellar irradiance. The two populations are even more distinct when separated by incident flux. Essentially no planets larger than ~1.8 Earth radii orbit in environments with incident fluxes more than 300 times the flux received by the Earth. All of the larger, sub-Neptune size planets are seen to orbit in regimes of lower incident flux. This “photo-evaporation valley” in the planet radius vs. incident flux domain was previously predicted in theoretical work. The location of the valley corresponds precisely to the observed boundary between planets with primarily rocky compositions and those with gaseous envelopes. We present the first empirical evidence of bimodal radius distribution for small planets made possible by increased stellar radius precision derived from high-resolution spectroscopy of 1250 Kepler planet-hosting stars.

    Ben Nelson (Northwestern): Evidence for Two Hot Jupiter Formation Paths

    Disk migration and high-eccentricity migration are two well-studied theories to explain the formation of Hot Jupiters. The former predicts that these planets can migrate up until the planet-star Roche separation (a_{Roche}) and the latter predicts they will tidally circularize at a minimum distance of 2*a_{Roche}. Considering long-running radial velocity and transit surveys have identified a couple hundred Hot Jupiters to date, we can revisit the classic question of Hot Jupiter formation in a data-driven manner.

    We approach this problem using data from several exoplanet surveys (radial velocity, Kepler, HAT, and WASP) allowing for either a single population or a mixture of populations associated with these formation channels, and applying a hierarchical Bayesian mixture model of truncated power laws to constrain the population-level parameters of interest, e.g., location of inner edges, power law indices, mixture fractions.

    Within the limitations of our chosen models, we find the current radial velocity and Kepler sample of Hot Jupiters can be well explained with a single truncated power law distribution with a lower cutoff near 2*a_{Roche}, a result that still holds after a decade. However, the HAT and WASP data are better modeled with a mixture of multiple populations compared to a single population (Bayes factor > 10^21): we find roughly 85% (15%) of Hot Jupiters reside in a component consistent with a high-eccentricity (disk) migration history. We also speculate on how future exoplanet missions (e.g., TESS) can improve upon this Hot Jupiter population inference.

    Eve Lee (UC Berkeley): Ultra-Short Period Planets, Magnetospheric Truncation, and Tidal Inspiral

    Sub-Neptunes around FGKM dwarfs are evenly distributed in log period down to ~10 days but dwindle in number at shorter orbital periods. We demonstrate how magnetospheric truncation of disks and tidal inspiral of planets are responsible for sculpting this orbital architecture. Both the break at ~10 days and the slope of the occurrence rate down to ~1 day can be reproduced if planets form in disks that are truncated by their host star magnetospheres at co-rotation. Asynchronous tides raised in the star can transport planets from the disk inner edge to the shortest orbital periods (<1 day). Tidal migration can explain why these ultra-short period planets (USPs) are more widely spaced than their longer period counterparts. We predict planet occurrence rates around A stars to also break at short periods, but at ~1 day instead of ~10 days.

    Brian Jackson (Boise State) : Tidal Decay and Roche-Lobe Overflow of Short-Period Gaseous Exoplanets

    Discoveries of 100+ Earth-sized planet candidates with short orbital periods, some only a few hours, have challenged theoretical expectations. The proximity of hot Jupiters to Roche-Lobe overflow (RLO) has suggested some small, short-period planets are actually the fossil cores of disrupted gaseous planets, and recent work provides some support: stable RLO (atmospheres lost via a steady outflow and a thin accretion disk) can drive orbital expansion that stops and reverses at a maximum period that depends on the core mass, and some small, short-period planets have periods consistent with this picture. However, the periods of the very closest-in planets are too short. Instead, unstable RLO (atmospheres quickly shed on dynamical timescales) may explain the small, short-period planets. Indeed, as disrupting hot Jupiters transition to hot sub-Neptunes, the accompanying radius evolution, as well as non-conservative angular momentum evolution, may drive the planets from stable to unstable RLO. On the other hand, simple scaling arguments suggest that the very short viscous timescale for the accretion disk may accommodate the increase in overflow rate that results from the radius evolution, allowing the RLO to remain stable after all. In this presentation, we will discuss recent work on planetary RLO and explore outstanding questions.


FRIDAY Evening  — Planet Formation, Disks and Migration I  [Chair: Rebecca Martin]

    Wei Zhu (Ohio State): Detection of a 1 Earth-mass Planet at 1 AU around a Brown Dwarf

    We report the discovery of a 1 Earth-mass planet in a 1 AU orbit around a brown dwarf host using the microlensing parallax method (arXiv:1703.08548). This is the third published planet from the on-going Spitzer microlensing program. I will first explain the principle behind the microlensing parallax method and, in particular, why it can measure the masses in a model independent way. Then I will present some significant and unique discoveries from the Spitzer microlensing program, and discuss our future plans. The status of the K2 microlensing program (K2 campaign 9) and its scientific potential will also be discussed.

    Rebekah Dawson (PSU): The Critical Role of Residual Gas in Establishing Super-Earths' Compositions and Orbital Architectures

    At the end of the gas disk stage, residual gas in disks depleted by photo-evaporation can play a critical role in establishing super-Earths’ orbits and compositions. Dynamical friction from residual disk gas regulates mergers and damps inclinations and eccentricities. Systems with moderate gas damping in disks with high solid surface density spawn gas-enveloped super-Earths with dynamically-cold orbital properties (tight spacings, small eccentricities and inclinations), whereas formation primarily after residual gas spawns rocky, dynamically-hot planets. Previously we argued that a plausible range of disk properties can account for the diversity in super-Earths’ orbital and compositional properties (Dawson et al. 2015, 2016). Here we more deeply explore the interplay between a disk’s solid surface density and gas damping conditions and conclude that a range of disk solid surface densities – rather than differences in the amount of residual gas in the slow stage of gas dispersal – better accounts for the observed diversity by regulating how much growth super-Earths undergo in the presence of residual gas. We also present predictions for the contribution of formation conditions to scatter in the mass-radius relationship and assess the importance of formation conditions relative to atmospheric removal processes.

    Henry Ngo (Caltech): Giant Planet Formation and Migration in Multi-stellar Systems

    The influence of stellar companions on giant planet formation and migration remains an active research question. I will present two adaptive optics imaging surveys for stellar companions around giant planet host stars. In our first survey, we focus on the role of stellar companions in the dynamics of transiting hot Jupiter systems. With a study of 77 systems, we show that less than 20% of hot Jupiters have stellar companions capable of inducing migration via Kozai-Lidov oscillations. We also find that hot Jupiter hosts are three times more likely to have a wide stellar companion (50-2000 AU) than field stars, suggesting that binary star systems may be favorable environments for giant planet formation. Our observations place constraints on alternative hot Jupiter formation theories, such as those involving planet-planet interactions. I will present new results from a recently completed survey of 144 stars hosting giant planets detected by the RV method. This is the largest survey of its kind to date and examines giant planet formation from 0.1-5 AU, in addition to the closer-in giant planetary systems. We test for differences in planet properties with semimajor axes from 0.1-5 AU in single vs multi-stellar systems. We find no evidence that the presence of stellar companions alters the distribution of planetary properties in these systems. Using four years of astrometry for hierarchical triple star systems with giant planets, we fit the stellar orbits to characterize the orbital architectures of these systems.

    Adam Dempsey (Northwestern): Forming the Chaotic GJ876 System Through Smooth Disk Migration

    Over the past 20 years many exoplanetary systems have been discovered which contain a chain of planets in (or near) orbital commensurablities. A natural and well explored mechanism for bringing these planets into their observed period ratios is disk migration.

    The present day orbital architectures of multi-planet, resonant systems provide us with the best testbeds for probing the properties of their nascent protoplanetary disks. Convergence through disk migration has long been the most natural mechanism for creating chains of planets near mean-motion resonances. Using disk migration theory and N-body simulations, we can determine what types of disks give rise to the systems we see today. Our work is focused on constraining the disk progenitor of the Gilese 876 system, an M-dwarf hosting three planets orbiting in a chaotic 4:2:1 Laplace resonance. We show through a suite of dissipative N-body simulations that the large libration amplitude of the Laplace angle together with the short chaotic timescale and well measured planet eccentricities provide a stringent constraint on the possible mass and thickness of the now dissipated disk. In particular, we find that a sizable amount of gas contained in a thin inner disk is necessary for constructing the observed system. By characterizing more resonant systems, in addition to GJ876, we hope to compare our "dynamical" fits to the growing population of observed protoplanetary disks as seen by ALMA.

    Elisa Quintana (NASA Goddard): Dynamical Constraints on the Habitability of Planets Around M Dwarfs

    M dwarfs, stars with 10 - 50% the mass of the Sun, are by far the most abundant type of stars in our galaxy. M dwarfs are cool and intrinsically dim, thus the habitable zone (HZ) resides much closer-in compared to G stars. For planets forming in the HZ of M dwarfs, accretion is much more rapid because orbital periods are shorter and impact velocities are higher. We will present numerical simulations of terrestrial planet formation around a range of M dwarf masses using an integration package that tracks collisional fragmentation which allows us to better estimate the rates and energies of impacts onto the growing planets. We will discuss what types of M dwarfs are capable of forming Earth-like planets, and under what circumstances can they accrete and retain water and atmospheres during the giant impact phase of formation. M dwarfs will be prime targets for NASA’s upcoming missions like TESS and JWST, thus these results can help support observing strategies.

    Hans Baehr (MPIA): Modeling Planet Formation in Young Self-Gravitating Disks

    Last year gave us new observations of young disks around Elias 2-27 and L1448 IRS3B with indications of large scale spiral arms and fragments. These features are signatures of gravitational instability (GI), a possible way of forming planets at wide separations, a region where other models have difficulty forming a planet in a reasonable timescale. Systems like HR 8799, with Jovian size planet beyond 30 AU, indicate that there is the potential for planets to be formed at wide orbits and a need to describe this potential in situ formation. I present local 3D models of disk fragmentation showing formation of gas giant or brown dwarf precursors with a lower critical Toomre value than the common 2D models. This indicates that even more massive disks are required and the resulting fragments are even larger as well, constraining the expected size of objects formed by GI to perhaps exclude low Jupiter masses.

    Giovanni Dipierro (U Leicester): An Opening Criterion for Dust Gaps in Protoplanetary Disks

    Recent spectacular spatially resolved observations of gaps and ring-like structures in nearby dusty protoplanetary discs have revived interest in studying gap-opening mechanisms. In this talk I’ll describe the two distinct physical mechanisms for dust gap opening by embedded planets in protoplanetary discs: I) A mechanism where low mass planets, that do not disturb the gas, open gaps in dust by tidal torques assisted by drag in the inner disc, but resisted by drag in the outer disc; and II) The usual, drag assisted, mechanism where higher mass planets create pressure maxima in the gas disc which the drag torque then acts to evacuate further in the dust.

    Starting from these numerical evidences, we derive a grain size-dependent criterion for dust gap opening in viscous protoplanetary discs by revisiting the theory of dust drift to include disc-planet tidal interactions and viscous forces.

    Åke Nordlund (U Copenhagen): Chondrule Accretion in Global Disk Models

    We use zoom-in models of accretion disks (Nordlund et al, IAUS 299, 2014: Kuffmeier et al ApJ 826, 2015 and arXiv:1611.10360) as a background for simulating the accretion of chondrules onto planet embryos surrounded by early protoatmospheres, which greatly enhance the efficiency of chondrule accretion. Recent cosmochemical evidence (Bollard et al 2017, submitted to Science Advances) show that chondrules (as found in chondritic meteorites) were produced in large quantities in the early phases of the solar system, and that an efficient transport mechanism was able to distribute them radially across the solar system, with a significant fraction being recycled. The results are based on accurate lead-lead absolute dating of chondrules, which also yields irrefutable evidence for chondrule recycling. We have identified turbulent disk wind outflows as a working transport mechanism, and have verified that the mechanism is operative in our realistic zoom-in simulations of accretion disks. We are now extending the range of the zoom simulations to a smallest grid scale of about 20 km, resolving the hot, optically thick protoplanetary atmospheres around planetary embryos of different mass with hierarchically nested grids with the new DISPATCH code, using a total of about 20 billion cells, extending out to and merging with the adaptive mesh resolution representation of the accretion disk, which again is immersed in a 40 pc cubed RAMSES model of a giant molecular cloud. With this method, we are able to carry forward the type of 3-D modeling of planet embryo Hill spheres pioneered by Ormel et al (MNRAS 447, 2015), now with much improved numerical resolution. We conclude that chondrule accretion, aided by horseshoe orbit motions and Hill sphere dynamics provides an efficient path for rapid planet growth, consistent with the latest cosmochemical evidence.



SATURDAY Morning  — Planet Formation, Disks and Migration II  [Chair: Re’em Sari]

    Steve Lubow (STScI): Evolution of Non-coplanar Planets and Disks in Binaries

    We have been analyzing the tilt and eccentricity evolution of a planet and a disk that orbit a member of a binary star system. The planet and disk are initially mutually coplanar but tilted with respect to the binary. A giant planet orbit generally evolves to become noncoplanar with the disk once the planet opens a gap. The planet and disk can undergo tilt and eccentricity (Kozai-Lidov) oscillations as a result of 3 body interactions. In addition, 4 body secular interactions, related to the nu16 resonance in the solar system, play a role in enhancing planet-disk misalignment. Small mass planets generally remain coplanar with a more massive disk.

    Jeff Jennings (U Munich): The Effects of Photo-evaporation on Gas Giant Migration in the Nascent Planetary Disk

    The formation and early (≤10 Myr) orbital evolution of planets occurs in a setting of viscous disc evolution as well as disc mass loss due to high energy irradiance from the host star. This photoevaporative wind may be driven by FUV, EUV and X-ray radiation or a combination thereof, each energetic regime a result of distinct physical processes. Their intensities and radial profiles in the nascent disc thus differ, with implications for the torques felt by an inwardly migrating gas giant. We investigate the effects of photoevaporation on gas giant parking radii across energetic regimes in a comparison with the observed gas giant semimajor axis distribution, finding the results disfavor a scenario of FUV-dominated photoevaporation and support the X-ray-dominant case. We place our work in the larger context of planetary system architecture, proposing this process is integral to the ‘initial conditions’ set for the dynamical evolution of a planetary system once the gas disc is dispersed.

    Yasuhiro Hasegawa (NASA JPL): Super-Earths as Failed Cores in Orbital Migration Traps

    We explore whether close-in super-Earths were formed as rocky bodies that failed to grow fast enough to become the cores of gas giants before the natal protostellar disk dispersed. We model the failed cores' inward orbital migration in the type I regime, to stopping points at distances where the tidal interaction with the protostellar disk applies zero net torque. The three kinds of migration traps considered are those due to the dead zone's outer edge, the ice line, and the transition from accretion to starlight as the disk's main heat source. As the disk disperses, the traps move toward final positions of > 1~au. Planets at this location exceeding about 3~M⊕ open a gap, decouple from their host trap, and migrate inward in the type II regime to reach the vicinity of the star. We synthesize the population of planets formed in this scenario, finding that some fraction of the observed super-Earths can be failed cores. Most super-Earths formed this way have more than 4~M⊕, so their orbits when the disk disperses are governed by type II migration. The failed core scenario suggests a division of the observed super-Earth mass-radius diagram into five zones according to the inferred formation history.

    Daniel Carrera (PSU): Planetesimal Formation by the Streaming Instability in a Photo-evaporating Disk

    Recent years have seen growing interest in the streaming instability as a candidate mechanism to produce planetesimals. However, these investigations have been limited to small-scale simulations. In this talk I will present the results of a global protoplanetary disk evolution model that incorporates planetesimal formation by the streaming instability, along with photoevaporation by EUV, FUV, and X-ray photons, dust evolution, the water ice line, and stratified turbulence. Our simulations produce 70-140 Earth masses of planetesimals in the outer disk. Our most comprehensive model produces 8 Earth masses of planetesimals inside 3 au, where they can give rise to terrestrial planets. The planetesimal mass inside 3 au depends critically on the timing of the formation of an inner disk cavity by high-energy photons. We find that the combination of the streaming instability and photoevaporation is efficient at converting solids into planetesimals. Our model, however, does not form enough early planetesimals near the water ice line to give rise to giant planets and super-Earths with gaseous envelopes. Additional processes such as particle pileups, and possibly mass loss driven by other mechanisms such as MHD disk winds, are needed to explain the early formation of planetesimals in the inner disk.

    Erika Nesvold (Carnegie Inst): HD 106906: A Case Study for External Perturbations of a Debris Disk

    Models of debris disk morphology are often focused on the effects of a planet orbiting interior to or within the disk. Nonetheless, an exterior planetary-mass perturber can also excite eccentricities in a debris disk, via Laplace-Lagrange secular perturbations in the coplanar case or Kozai-Lidov perturbations for mutually inclined companions and disks. HD 106906 is an ideal example of such a a system, as it harbors a confirmed exterior 11 Jupiter-mass companion at a projected separation of 650 au outside a resolved, asymmetric disk. We use collisional and dynamical simulations to investigate the interactions between the disk and the companion, and to use the disk’s observed morphology to place constraints on the companion’s orbit. We conclude that the observed exterior companion could be responsible for the disk's asymmetries, suggesting that exterior perturbers, as well as interior planets, should be considered when investigating the cause of observed asymmetries in a debris disk.

    Kaustubh Hakim (U Amsterdam): A Laboratory Approach to Probe the Mineralogy of Carbon-enriched Rocky Exoplanets

    Spectroscopic observations of newly born stars with high C/O ratios indicate the likely formation of carbon-enriched rocky exoplanets, but the mineralogical make up of such planets is poorly constrained. We prepared two types of bulk chemical compositions enriched in carbon in the Fe-Ca-Mg-Al-Si-C-S-O system, carbon-deficient and carbon-enriched, based on recent chemical modeling of condensates in protoplanetary disks. To probe the mineralogy of carbon-enriched rocky exoplanets, we performed high-pressure high-temperature laboratory experiments on an end-loaded piston cylinder apparatus and subjected these compositions to the pressures of 1-2 GPa and temperatures between 1250-1550 °C.

    In both types run products, we observed a clear segregation of the starting composition into silicate and iron-rich mineral phases. Graphite was found in all carbon-enriched runs. In iron-rich melts of our carbon-deficient runs, we found two-liquid immiscibility but not in our carbon-enriched runs. In both types of run products, a variety of mantle-forming silicate minerals such as olivine, orthopyroxene, clinopyroxene and spinel were present, implying their similarity to those of Earth and Mars. However, silicate melts in carbon-enriched runs contained higher amounts of volatile elements like carbon and sulfur.

    Our results imply that graphite is the dominant carbon-bearing phase in carbon-enriched rocky exoplanets. Carbon is soluble in iron-rich melts and silicate melts. Iron carbides and iron silicides can form under reducing conditions. But silicon carbide (often assumed to be the dominant mineral in carbon-enriched exoplanets) is unlikely to be stable in the chemical conditions of a planetary interior.

    Since the density of graphite is lower than that of the silicate and iron-rich metallic melts/minerals, a differentiated carbon-enriched rocky exoplanet might have a graphite crust covering its silicate mantle and iron-rich core. If the graphite crust is thick enough, these exoplanets might harbor layers of diamond at the bottom of graphite layers due to higher pressures in their interior. Our interior structure models of Kepler-102b, a Mars-sized exoplanet, show that about 10 wt% graphite crust would decrease its mass by 8%, suggesting future astronomy missions that determine both radius and mass of rocky exoplanets could provide quantitative limits on their carbon content.

    Ana-Maria Piso (UCLA): The Role of Ice Compositions and Disk Dynamics for Snowlines and the C/N/O Ratios in Active Disks

    The elemental compositions of planets define their chemistry, and could potentially be used as beacons for their formation location if the elemental gas and grain ratios of planet birth environments, i.e. protoplanetary disks, are well understood. In disks, the ratios of volatile elements, such as C/O and N/O, are regulated by the abundance of the main C, N, O carriers, their ice binding environment, and the presence of snowlines of major volatiles at different distances from the central star. We explore the effects of disk dynamical processes and ice compositions on the snowline locations of the main C, O and N carriers, and the C/N/O ratios in gas and dust throughout the disk. The gas-phase N/O ratio enhancement in the outer disk (exterior to the H2O snowline) exceeds the C/O ratio enhancement for all reasonable volatile compositions. Ice compositions and disk dynamics individually change the snowline locations of CO and N2 by a factor of 2-3, and when considered together the range of possible CO and N2 snowline locations is ~10 - ~70 AU in a standard disk model. Observations that anchor snowline locations at different stages of planet formation are therefore key to develop C/N/O ratios as a probe of planet formation zones.