* Astronomy

Title: The Fate of Scattered Planets
Author: Benjamin C. Bromley, Scott J. Kenyon

As gas giant planets evolve, they may scatter other planets far from their original orbits to produce hot Jupiters or rogue planets that are not gravitationally bound to any star. Here, we consider planets cast out to large orbital distances on eccentric, bound orbits through a gaseous disk. With simple numerical models, we show that super-Earths can interact with the gas through dynamical friction to settle in the remote outer regions of a planetary system. Outcomes depend on planet mass, the initial scattered orbit, and the evolution of the time-dependent disk. Efficient orbital damping by dynamical friction requires planets at least as massive as the Earth. More massive, longer-lived disks damp eccentricities more efficiently than less massive, short-lived ones. Transition disks with an expanding inner cavity can circularize orbits at larger distances than disks that experience a global (homologous) decay in surface density. Thus, orbits of remote planets may reveal the evolutionary history of their primordial gas disks. A remote planet with an orbital distance ~100 AU from the Sun is plausible and might explain correlations in the orbital parameters of several distant trans-Neptunian objects.

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Title: On the origin of planets at very wide orbits from re-capture of free floating planets
Authors: Hagai B. Perets, M. B. N. Kouwenhoven

In recent years several planets have been discovered at wide orbits (>100 AU) around their host stars. Theoretical studies encounter difficulties in explaining their formation and origin. Here we propose a novel scenario for the production of planetary systems at such orbits, through the dynamical recapture of free floating planets (FFPs) in dispersing stellar clusters. This process is a natural extension of the recently suggested scenario for the formation of wide stellar binaries. We use N-body simulations of dispersing clusters with 10-1000 and f_FFP=0.5-2 to study this process. We find that planets are captured into wide orbits, ~100-10^6 AU, and a thermal eccentricity distribution. Typically, 3-6x(f_FFP/1) % of all stars capture a planetary companion (f_FFP is the number of FFP per star). The planetary capture efficiency is comparable to that of capture-formed stellar-binaries, and shows a similar dependence on the cluster size and structure. The capture efficiency is almost independent of the specific planetary mass; planets as well as sub-stellar companions of any mass can be captured, where the capture efficiency decreases with increasing cluster size. For a given cluster size the capture efficiency increases with the host/primary mass. More than one planet can be captured around the same host, and planets can be captured into binary systems. We also expect planets to be captured into pre-existing planetary systems as well as around compact objects, if these formed early enough before the cluster dispersal. In particular, stellar black holes have a high capture efficiency (>50 % and 5-10x(f_FFP/1) % for capture of stars and planetary companions, respectively) due to their large mass. Finally, although rare, two FFPs or brown dwarfs can become bound and form a FFP-binary system with no stellar host through this process.

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Title: Planet-Planet Scattering Alone Cannot Explain the Free-Floating Planet Population
Authors: Dimitri Veras, Sean N. Raymond

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.

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Astronomers discover 10 more free-floating planets in the milky way

Astronomers have found a clutch of planets that wander alone through interstellar space. The discovery of the objects, which do not orbit any star, will help scientists better understand how planetary systems form and evolve.
The 10 free-floating planets are thousands of light years in the direction of the central bulge of the Milky Way, towards the constellation of Sagittarius. Their masses and compositions are thought to be equivalent to Jupiter and Saturn - mainly hydrogen and helium with trace amounts of heavier elements.
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