Planet2018

Last-modified: 2019-04-05 (¶â) 17:11:59 (1518d)

Planet

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Schedule & History †

2017ǯÅÙ 2016ǯÅÙ 2015ǯÅÙ 2014ǯÅÙ

5/30 Dimitri Veras, The growing field of post-main-sequence exoplanetary science, with strong connections to the solar system

The quest for identifying the bulk chemical composition of extrasolar planets and robust observational evidence that between 25% and 50% of all Milky Way white dwarfs host currently dynamically-active planetary systems motivate investigations that link their formation and fate. Here I provide a review of our current knowledge of these systems, including an update on the observational and theoretical aspects of the groundbreaking discovery of at least one disintegrating minor planet transiting white dwarf WD 1145+017. I show how this field incorporates several facets of solar system physics and chemistry, and how its interdisciplinary nature requires input from orbital dynamics, stellar evolution, astrochemistry, atmospheric science and surface processes.

10/31 Jason Woo, The curious case of Mars' formation

Dynamical models of planet formation coupled with cosmochemical data from martian meteorites show that Mars' isotopic composition is distinct from that of Earth. Reconciliation of formation models with meteorite data require that Mars grew further from the Sun than its present position. Here, we evaluate this compositional difference in more detail by comparing output from two N-body planet formation models. The first of these planet formation models simulates what is termed the `Classical' case wherein Jupiter and Saturn are kept in their current orbits. We compare these results with another model based on the `Grand Tack', in which Jupiter and Saturn migrate through the primordial asteroid belt. Our estimate of the average fraction of chondrite assembled into Earth and Mars assumes that the initial solid disk consists of only sources of enstatite chondrite composition in the inner region, and ordinary chondrite in the outer region. Results of these analyses show that both models tend to yield Earth and Mars analogues whose accretion zones overlap. The Classical case fares better in forming Mars with its documented composition (29% to 68% enstatite chondrite plus 32% to 67% ordinary chondrite) though the Mars analogues are generally too massive. We also further calculate the isotopic composition of 17O, 50Ti, 54Cr, 142Nd, 64Ni, and 92Mo in the martian mantle from the Grand Tack simulations. We find that it is possible to match the calculated isotopic composition of all the above elements in Mars' mantle with their measured values, but the resulting uncertainties are too large to place good restriction on the early dynamical evolution and birth place of Mars.

11/7 Yuji Matsumoto¡¡The formation and dynamical evolution of super-Earths through in-situ giant impacts around M dwarfs

Recently, Earth-sized exoplanets are observed around TRAPPIST-1, which is M8 dwarf and has ~0.08 solar mass. Several on-going missions are targeting planets around M dwarfs. It is expected that more and more such planetary system around M dwarfs would be observed. There are some theoretical studies considering planetary formation around M dwarfs. However, the formation and dynamical evolution of close-in super-Earths have not understood systematically yet. We investigate the formation of close-in super-Earths through N-body simulations around M¡¡dwarfs. We use the same disk model with changing the stellar mass. At first, the dynamical evolution of isolation mass protoplanets is studied. As Matsumoto & Kokubo (2017) reported, protoplanets collide with their neighboring protoplanets soon after the orbital crossing around 1 solar mass star. Around M dwarfs, dynamical scattering between protoplanets becomes violent and such tournament-like collisional evolution is not seen. We also perform calculations with the systems that initially have protoplanets and planetesimals around M dwarfs. We find that planetesimals are ejected during some close scattering by protoplanets. The growth of protoplanets would be suppressed due to the ejection of planetesimals.

11/14 Adrien Leleu/ Stability and detectability of co-orbital exoplanets

Despite the existence of co-orbital bodies in the solar system (1:1 Mean-motion resonance), and the prediction of the formation of co-orbital planets by planetary system formation models, no co-orbital exoplanets (also called trojans) have been detected thus far. It can be due to the rarity of the configuration, the degeneracy of the co-orbital signature with other configurations, or the observational biases. After a description of various stable co-orbital configurations for a pair of planet, I will discuss the stability of the lagrangian equilibria (L4 and L5) during the migration in the protoplanetary disc for the variables associated to the resonant angle, but also in the direction of the eccentricities and inclination. Finally I will discuss the signature of co-orbitals exoplanets in Transit Timing Variation, transits, and combination of transit and radial velocity measurements.

12/19 Sho Shibata/ Metallicity Enhancement of Gas Giants by Late Accretion of Solids - the Effect of Orbital Evolution

Recent studies of internal structure of gas giants suggest that their envelope is enriched with heavier elements than hydrogen and helium, relative to their central star composition. It is considered that these enrichment in heavy elements are caused by the capture of planetesimals during the late formation stage of gas giants. Zhou & Lin 2007 and Shiraishi & Ida 2008 performed orbital integrations of planetesimals around a growing protoplanet in a constant protoplanetary disk, and estimated the total captured mass of planetesimals for Jupiter and Saturn. However, they assumed a simple formation models and neglected some physically important effects. In this study, we investigated the effect of a planet migration on the capture of planetesimals, in a single-planetary system and a multi-planetary system, respectively. We find that planetesimals swept by a migrating planet are trapped at the mean-motion resonances (MMRs), which works as a barrier and prevents planetesimals from approaching the planet. Thus, planetary migration itself has little contribution to increasing the amount of captured planetesimals. However, in a multi-planetary system, we find an interesting process that breaks the resonance trap and increases the amount of planetesimals captured by the planets. This effect is important to consider the capture of planetesimals in a multi-planetary system.

2/13 Ayumu Kuwahara/¡¡Gas flow around an embedded planet in a protoplanetary disk: the implications for planet formation

The three-dimensional structure of the gas flow around a planet is thought to influence the accretion of both gas and solid materials. In particular, the outflow in the mid-plane region may prevent the accretion of the solid materials and delay the formation of super-Earths' cores. However, it is not yet understood how the nature of the flow field and outflow speed change as a function of the planetary mass. In this study, we investigate the dependence of gas flow around a planet embedded in a protoplanetary disc on the planetary mass. Assuming an isothermal, inviscid gas disc, we perform three-dimensional hydrodynamical simulations on the spherical polar grid, which has a planet located at its centre. We find that gas enters the Bondi or Hill sphere at high latitudes and exits through the mid-plane region of the disc regardless of the assumed planetary mass. The altitude from where gas predominantly enters the envelope varies with the planetary mass. We also derived the analytic solution of the outflow which is consistent of the results of simulations. The planet-induced gas flow may reduce the accretion of dust and pebbles onto the planet when planetary mass is larger than the square root of Stokes number of the particle. Our results suggest that the flow around proto-cores of super-Earths may delay their growth and, consequently, help them to avoid runaway gas accretion within the lifetime of the gas disc. In this talk, I will also introduce a few latest results of test simulations related to the pebble accretion in the realistic three-dimensional gas flow field.

3/7 Sanemichi Takahashi/ Structure Formation in a Young Protoplanetary Disk by a Magnetic Disk Wind

Recent observations with ALMA found that a ring–hole structure may be formed in young protoplanetary disks, even when the disk is embedded in the envelope. We present a one-dimensional model for the formation of a protoplanetary disk from a molecular cloud core and its subsequent long-term evolution within a single framework. Such long-term evolution has not been explored by numerical simulations due to the limitations of computational power. In our model, we calculate the time evolution of the surface density of the gas and dust with the wind mass loss and the radial drift of the dust in the disk. We find that the MHD disk wind is a viable mechanism for the formation of a ring–hole structure in young disks. We perform a parameter study of our model and derive conditions for the formation of ring–hole structures within ~10^6 yr after the start of the collapse of the molecular cloud core. The final outcome of the disk shows five types of morphology; this can be understood by comparing the timescales of the viscous diffusion, the mass loss by MHD disk wind, and the radial drift of the dust. We discuss the implication of the model for the WL17 system, which is suspected to be an embedded, yet transitional, disk.

3/20 Sota Arakawa¡¡The formation and tidal evolution of satellites around large trans-Neptunian objects

Recent studies have revealed that all large (over 1000 km in diameter) trans-Neptunian objects (TNOs) form satellite systems. Although the largest Plutonian satellite, Charon, is thought to be an intact fragment of an impactor directly formed via a giant impact, whether giant impacts can explain the variations in secondary-to-primary mass ratios and spin/orbital periods among all large TNOs remains to be determined. Here we systematically perform hydrodynamic simulations to investigate satellite formation via giant impacts. We find that the simulated secondary-to-primary mass ratio varies over a wide range, which overlaps with observed mass ratios. We also reveal that the satellite systems¡Ç current distribution of spin/orbital periods and small eccentricity can be explained only when their spins and orbits tidally evolve: initially as fluid-like bodies, but finally as rigid bodies. These results suggest that all satellites of large TNOs were formed via giant impacts in the early stage of solar system formation, before the outward migration of Neptune, and that they were fully or partially molten during the giant impact era.

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