ABSTRACT
Recent gravitational microlensing observations predict a vast population of free‐floating giant planets that outnumbers main‐sequence stars almost twofold. A frequently invoked mechanism for ...generating this population is a dynamical instability that incites planet–planet scattering and the ejection of one or more planets in isolated main‐sequence planetary systems. Here, we demonstrate that this process alone probably cannot represent the sole source of these Galactic wanderers. By using straightforward quantitative arguments and N‐body simulations, we argue that the observed number of exoplanets exceeds the plausible number of ejected planets per system from scattering. Thus, other potential sources of free floaters, such as planetary stripping in stellar clusters and post‐main‐sequence ejection, must be considered.
The discovery of over 50 planets around evolved stars and more than 35 debris discs orbiting white dwarfs highlight the increasing need to understand small body evolution around both early and ...asymptotic giant branch (GB) stars. Pebbles and asteroids are susceptible to strong accelerations from the intense luminosity and winds of GB stars. Here, we establish equations that can model time-varying GB stellar radiation, wind drag and mass-loss. We derive the complete three-dimensional equations of motion in orbital elements due to (1) the Epstein and Stokes regimes of stellar wind drag, (2) Poynting–Robertson drag, and (3) the Yarkovsky drift with seasonal and diurnal components. We prove through averaging that the potential secular eccentricity and inclination excitation due to Yarkovsky drift can exceed that from Poynting–Robertson drag and radiation pressure by at least three orders of magnitude, possibly flinging asteroids which survive YORP spin-up into a widely dispersed cloud around the resulting white dwarf. The GB Yarkovsky effect alone may change an asteroid's orbital eccentricity by 10 per cent in just 1 Myr. Damping perturbations from stellar wind drag can be just as extreme, but are strongly dependent on the highly uncertain local gas density and mean free path length. We conclude that GB radiative and wind effects must be considered when modelling the post-main-sequence evolution of bodies smaller than about 1000 km.
'Oumuamua was discovered passing through our solar system on a hyperbolic orbit. It presents an apparent contradiction, with colors similar to those of volatile-rich solar system bodies but with no ...visible outgassing or activity during its close approach to the Sun. Here, we show that this contradiction can be explained by the dynamics of planetesimal ejection by giant planets. We propose that 'Oumuamua is an extinct fragment of a comet-like planetesimal born a planet-forming disk that also formed Neptune- to Jupiter-mass giant planets. On its pathway to ejection 'Oumuamua's parent body underwent a close encounter with a giant planet and was tidally disrupted into small pieces, similar to comet Shoemaker-Levy 9's disruption after passing close to Jupiter. We use dynamical simulations to show that 0.1%-1% of cometary planetesimals undergo disruptive encounters prior to ejection. Rocky asteroidal planetesimals are unlikely to disrupt due to their higher densities. After disruption, the bulk of fragments undergo enough close passages to their host stars to lose their surface volatiles and become extinct. Planetesimal fragments such as 'Oumuamua contain little of the mass in the population of interstellar objects but dominate by number. Our model makes predictions that will be tested in the coming decade by the Large Synoptic Survey Telescope.
ABSTRACT
White dwarfs containing orbiting planetesimals or their debris represent crucial benchmarks by which theoretical investigations of post-main-sequence planetary systems may be calibrated. The ...photometric transit signatures of likely planetary debris in the ZTF J0139+5245 white dwarf system have an orbital period of about 110 d. An asteroid which breaks up to produce this debris may spin itself to destruction through repeated close encounters with the star without entering its Roche radius and without influence from the white dwarf’s luminosity. Here, we place coupled constraints on the orbital pericentre (q) and the ratio (β) of the middle to longest semiaxes of a triaxial asteroid which disrupts outside of this white dwarf’s Roche radius (rRoche) soon after attaining its 110-d orbit. We find that disruption within tens of years is likely when β ≲ 0.6 and q ≈ 1.0–2.0rRoche, and when β ≲ 0.2 out to q ≈ 2.5rRoche. Analysing the longer time-scale disruption of triaxial asteroids around ZTF J0139+5245 is desirable but may require either an analytical approach relying on ergodic theory or novel numerical techniques.
The potential existence of a distant planet ('Planet Nine') in the Solar system has prompted a re-think about the evolution of planetary systems. As the Sun transitions from a main-sequence star into ...a white dwarf, Jupiter, Saturn, Uranus and Neptune are currently assumed to survive in expanded but otherwise unchanged orbits. However, a sufficiently distant and sufficiently massive extra planet would alter this quiescent end scenario through the combined effects of Solar giant branch mass-loss and Galactic tides. Here, I estimate bounds for the mass and orbit of a distant extra planet that would incite future instability in systems with a Sun-like star and giant planets with masses and orbits equivalent to those of Jupiter, Saturn, Uranus and Neptune. I find that this boundary is diffuse and strongly dependent on each of the distant planet's orbital parameters. Nevertheless, I claim that instability occurs more often than not when the planet is as massive as Jupiter and harbours a semimajor axis exceeding about 300 au, or has a mass of a super-Earth and a semimajor axis exceeding about 3000 au. These results hold for orbital pericentres ranging from 100 to at least 400 au. This instability scenario might represent a common occurrence, as potentially evidenced by the ubiquity of metal pollution in white dwarf atmospheres throughout the Galaxy.
The birth and death of planets may be affected by mass outflows from their parent stars during the T-Tauri or post-main-sequence phases of stellar evolution. These outflows are often modelled to be ...isotropic, but this assumption is not realistic for fast rotators, bipolar jets and supernovae. Here we derive the general equations of motion for the time evolution of a single planet, brown dwarf, comet or asteroid perturbed by anisotropic mass loss in terms of a complete set of planetary orbital elements, the ejecta velocity, and the parent star's co-latitude and longitude. We restrict our application of these equations to (1) rapidly rotating giant stars and (2) arbitrarily directed jet outflows. We conclude that the isotropic mass-loss assumption can safely be used to model planetary motion during giant branch phases of stellar evolution within distances of hundreds of au. In fact, latitudinal mass-loss variations anisotropically affect planetary motion only if the mass loss is asymmetric about the stellar equator. Also, we demonstrate how constant-velocity, asymmetric bipolar outflows in young systems incite orbital inclination changes. Consequently, this phenomenon readily tilts exoplanetary orbits external to a nascent disc of the order of degrees.
ABSTRACT We obtained high-speed photometry of the disintegrating planetesimals orbiting the white dwarf WD 1145+017, spanning a period of four weeks. The light curves show a dramatic evolution of the ...system since the first observations obtained about seven months ago. Multiple transit events are detected in every light curve, which have varying durations ( 3-12 minutes) and depths ( 10%-60%). The time-averaged extinction is 11%, much higher than at the time of the Kepler observations. The shortest-duration transits require that the occulting cloud of debris has a few times the size of the white dwarf, longer events are often resolved into the superposition of several individual transits. The transits evolve on timescales of days, both in shape and in depth, with most of them gradually appearing and disappearing over the course of the observing campaign. Several transits can be tracked across multiple nights, all of them recur on periods of 4.49 hr, indicating multiple planetary debris fragments on nearly identical orbits. Identifying the specific origin of these bodies within this planetary system, and the evolution leading to their current orbits remains a challenging problem.
Abstract
An ice giant planet was recently reported orbiting white dwarf WD J0914+1914 at an approximate distance of 0.07 au. The striking non-detection of rocky pollutants in this white dwarf’s ...photosphere contrasts with the observations of nearly every other known white dwarf planetary system. Here, I analyse the prospects for exterior extant rocky asteroids, boulders, cobbles, and pebbles to radiatively drift inward past the planet due to the relatively high luminosity ($0.1 \, \mathrm{L}_{\odot }$) of this particularly young (13 Myr) white dwarf. Pebbles and cobbles drift too slowly from Poynting–Robertson drag to bypass the planet, but boulders and asteroids are subject to the much stronger Yarkovsky effect. In this paper, I (i) place lower limits on the time-scales for these objects to reach the planet’s orbit, (ii) establish 3 m as the approximate limiting radius above which a boulder drifts too slowly to avoid colliding with the planet, and (iii) compute bounds on the fraction of boulders that succeed in traversing mean motion resonances and the planet’s Hill sphere to eventually pollute the star. Overall, I find that the planet acts as a barrier against rather than a facilitator for radiatively driven rocky pollution, suggesting that future rocky pollutants would most likely originate from distant scattering events.
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