Dimitri Veras

THE FORMATION MECHANISM OF GAS GIANTS ON WIDE ORBITS

The recent discoveries of massive planets on ultra-wide orbits of HR 8799 and Fomalhaut present a new challenge for planet formation theorists. Our goal is to figure out which of three giant planet formation mechanisms— core accretion (with or without migration), scattering from the inner disk, or gravitational instability—could be responsible for Fomalhaut b, HR 8799 b, c and d, and similar planets discovered in the future. This paper presents the results of numerical experiments comparing the long-period planet formation efficiency of each possible mechanism in model A star, G star, and M star disks. First, a simple core accretion simulation shows that planet cores forming beyond 35 AU cannot reach critical mass, even under the most favorable conditions one can construct. Second, a set of N-body simulations demonstrates that planet–planet scattering does not create stable, wide-orbit systems such as HR 8799. Finally, a linear stability analysis verifies previous work showing that global spiral instabilities naturally arise in high-mass disks. We conclude that massive gas giants on stable orbits with semimajor axes a ≳35 AU form by gravitational instability in the disk. We recommend that observers examine the planet detection rate as a function of stellar age, controlling for the planets’ dimming with time. Any age trend would indicate that planets on wide orbits are transient relics of scattering from the inner disk. If planet detection rate is found to be independent of stellar age, it would confirm our prediction that gravitational instability is the dominant mode of producing detectable planets on wide orbits.We also predict that the occurrence ratio of long-period to short-period gas giants should be highest for M dwarfs due to the inefficiency of core accretion and the expected small fragment mass (~10 M_(Jup)) in their disks.

FORMATION, SURVIVAL, AND DETECTABILITY OF PLANETS BEYOND 100 AU

Direct imaging searches have begun to detect planetary and brown dwarf companions and to place constraints on the presence of giant planets at large separations from their host star. This work helps to motivate such planet searches by predicting a population of young giant planets that could be detectable by direct imaging campaigns. Both the classical core accretion and the gravitational instability model for planet formation are hard pressed to form long-period planets in situ. Here, we show that dynamical instabilities among planetary systems that originally formed multiple giant planets much closer to the host star could produce a population of giant planets at large (≈ 10^2-10^5 AU) separations. We estimate the limits within which these planets may survive, quantify the efficiency of gravitational scattering into both stable and unstable wide orbits, and demonstrate that population analyses must take into account the age of the system. We predict that planet scattering creates detectable giant planets on wide orbits that decreases in number on timescales of ~ 10 Myr. We demonstrate that several members of such populations should be detectable with current technology, quantify the prospects for future instruments, and suggest how they could place interesting constraints on planet formation models.

The great escape: how exoplanets and smaller bodies desert dying stars

Mounting discoveries of extrasolar planets orbiting post-main sequence stars motivate studies aimed at understanding the fate of these planets. In the traditional “adiabatic” approximation, a secondary’s eccentricity remains constant during stellar mass loss. Here, we remove this approximation, investigate the full twobody point-mass problem with isotropic mass loss, and illustrate the resulting dynamical evolution. The magnitude and duration of a star’s mass loss combined with a secondary’s initial orbital characteristics might provoke ejection, modest eccentricity pumping, or even circularisation of the orbit. We conclude that Oort clouds and wide-separation planets may be dynamically ejected from 1M⊙ 7M⊙ parent stars during AGB evolution. The vast majority of planetary material which survives a supernova from a 7M⊙ 20M⊙ progenitor will be dynamically ejected from the system, placing limits on the existence of firstgeneration pulsar planets. Planets around > 20M⊙ black hole progenitors may easily survive or readily be ejected depending on the core collapse and superwind models applied. Material ejected during stellar evolution might contribute significantly to the free-floating planetary population.

Simulations of two-planet systems through all phases of stellar evolution: implications for the instability boundary and white dwarf pollution

Exoplanets have been observed at many stages of their host stars life, including the main-sequence (MS), subgiant and red giant branch stages. Also, polluted white dwarfs (WDs) likely represent dynamically active systems at late times. Here, we perform three-body simulations which include realistic post-MS stellar mass-loss and span the entire lifetime of exosystems with two massive planets, from the endpoint of formation to several Gyr into the WD phase of the host star. We find that both MS and WD systems experience ejections and star-planet collisions (Lagrange instability) even if the planet-planet separation well-exceeds the analytical orbit-crossing (Hill instability) boundary. Consequently, MS-stable planets do not need to be closely packed to experience instability during the WD phase. This instability may pollute the WD directly through collisions, or, more likely, indirectly through increased scattering of smaller bodies such as asteroids or comets. Our simulations show that this instability occurs predominately between tens of Myr to a few Gyr of WD cooling. (Less)

Outward migration of extrasolar planets to large orbital radii

Observations of structure in circumstellar debris discs provide circumstantial evidence for the presence of massive planets at large (several tens of astronomical units) orbital radii, where the time-scale for planet formation via core accretion is prohibitively long. Here, we investigate whether a population of distant planets can be produced via outward migration subsequent to formation in the inner disc. Two possibilities for significant outward migration are identified. First, cores that form early at radii a ∼ 10 au can be carried to larger radii via gravitational interaction with the gaseous disc. This process is efficient if there is strong mass loss from the disc - either within a cluster or due to photoevaporation from a star more massive than the Sun - but does not require the extremely destructive environment found, for example, in the core of the Orion nebula. We find that, depending upon the disc model, gas disc migration can yield massive planets (several Jupiter masses) at radii of around 20-50 au. Secondly, interactions within multiple planet systems can drive the outer planet into a large, normally highly eccentric orbit. A series of scattering experiments suggests that this process is most efficient for lower-mass planets within systems of unequal mass ratio. This mechanism is a good candidate for explaining the origin of relatively low-mass giant planets in eccentric orbits at large radii.

Formation of planetary debris discs around white dwarfs – I. Tidal disruption of an extremely eccentric asteroid

25–50 per cent of all white dwarfs (WDs) host observable and dynamically active remnant planetary systems based on the presence of close-in circumstellar dust and gas and photospheric metal pollution. Currently accepted theoretical explanations for the origin of this matter include asteroids that survive the stars giant branch evolution at au-scale distances and are subsequently perturbed on to WD-grazing orbits following stellar mass-loss. In this work, we investigate the tidal disruption of these highly eccentric (e > 0.98) asteroids as they approach and tidally disrupt around the WD. We analytically compute the disruption time-scale and compare the result with fully self-consistent numerical simulations of rubble piles by using the N-body code PKDGRAV. We find that this time-scale is highly dependent on the orbits pericentre and largely independent of its semimajor axis. We establish that spherical asteroids readily break up and form highly eccentric collisionless rings, which do not accrete on to the WD without additional forces such as radiation or sublimation. This finding highlights the critical importance of such forces in the physics of WD planetary systems.

Characterizing the Orbital Eccentricities of Transiting Extrasolar Planets with Photometric Observations

The discovery of over 200 extrasolar planets with the radial velocity (RV) technique has revealed that many giant planets have large eccentricities, in striking contrast with most of the planets in the solar system and prior theories of planet formation. The realization that many giant planets have large eccentricities raises a fundamental question: Do terrestrial-size planets of other stars typically have significantly eccentric orbits or nearly circular orbits like the Earth? Here we demonstrate that photometric observations of transiting planets could be used to characterize the orbital eccentricities for individual transiting planets, as well as the eccentricity distribution for various populations of transiting planets (e.g., those with a certain range of orbital periods or physical sizes). Such characterizations can provide valuable constraints on theories for the excitation of eccentricities and tidal dissipation. We outline the future prospects of the technique given the exciting prospects for future transit searches, such as those to be carried out by the COROT and Kepler missions.

The dynamics of two massive planets on inclined orbits

Abstract The significant orbital eccentricities of most giant extrasolar planets may have their origin in the gravitational dynamics of initially unstable multiple planet systems. In this work, we explore the dynamics of two close planets on inclined orbits through both analytical techniques and extensive numerical scattering experiments. We derive a criterion for two equal mass planets on circular inclined orbits to achieve Hill stability, and conclude that significant radial migration and eccentricity pumping of both planets occurs predominantly by 2:1 and 5:3 mean motion resonant interactions. Using Laplace–Lagrange secular theory, we obtain analytical secular solutions for the orbital inclinations and longitudes of ascending nodes, and use those solutions to distinguish between the secular and resonant dynamics which arise in numerical simulations. We also illustrate how encounter maps, typically used to trace the motion of massless particles, may be modified to reproduce the gross instability seen by the numerical integrations. Such a correlation suggests promising future use of such maps to model the dynamics of more coplanar massive planet systems.

Planet-Planet Scattering Leads to Tightly Packed Planetary Systems

The known extrasolar multiple-planet systems share a surprising dynamical attribute: they cluster just beyond the Hill stability boundary. Here we show that the planet-planet scattering model, which naturally explains the observed exoplanet eccentricity distribution, can reproduce the observed distribution of dynamical configurations. We calculated how each of our scattered systems would appear over an appropriate range of viewing geometries; as Hill stability is weakly dependent on the masses, the mass-inclination degeneracy does not significantly affect our results. We consider a wide range of initial planetary mass distributions and find that some are poor fits to the observed systems. In fact, many of our scattering experiments overproduce systems very close to the stability boundary. The distribution of dynamical configurations of two-planet systems may provide better discrimination between scattering models than the distribution of eccentricity. Our results imply that, at least in their inner regions which are weakly affected by gas or planetesimal disks, planetary systems should be packed, with no large gaps between planets.

Detectable close-in planets around white dwarfs through late unpacking

Although 25%-50% of white dwarfs (WDs) display evidence for remnant planetary systems, their orbital architectures and overall sizes remain unknown. Vibrant close-in (~1 Solar radius) circumstellar activity is detected at WDs spanning many Gyrs in age, suggestive of planets further away. Here we demonstrate how systems with 4 and 10 closely-packed planets that remain stable and ordered on the main sequence can become unpacked when the star evolves into a WD and experience pervasive inward planetary incursions throughout WD cooling. Our full-lifetime simulations run for the age of the Universe and adopt main sequence stellar masses of 1.5, 2.0 and 2.5 Solar masses, which correspond to the mass range occupied by the progenitors of typical present-day WDs. These results provide (i) a natural way to generate an ever-changing dynamical architecture in post-main-sequence planetary systems, (ii) an avenue for planets to achieve temporary close-in orbits that are potentially detectable by transit photometry, and (iii) a dynamical explanation for how residual asteroids might pollute particularly old WDs.