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Free-floating planets
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Title: Close encounters involving free-floating planets in star clusters
Author: Long Wang, M. B. N. Kouwenhoven, Xiaochen Zheng, Ross P. Church, Melvyn B. Davies

Instabilities in planetary systems can result in the ejection of planets from their host system, resulting in free-floating planets (FFPs). If this occurs in a star cluster, the FFP may remain bound to the star cluster for some time and interact with the other cluster members until it is ejected. Here, we use N-body simulations to characterise close star-planet and planet-planet encounters and the dynamical fate of the FFP population in star clusters containing 500-2000 single or binary star members. We find that FFPs ejected from their planetary system at low velocities typically leave the star cluster 40% earlier than their host stars, and experience tens of close (<1000 AU) encounters with other stars and planets before they escape. The fraction of FFPs that experiences a close encounter depends on both the stellar density and the initial velocity distribution of the FFPs. Approximately half of the close encounters occur within the first 30 Myr, and only 10% occur after 100 Myr. The periastron velocity distribution for all encounters is well-described by a modified Maxwell-Bolzmann distribution, and the periastron distance distribution is linear over almost the entire range of distances considered, and flattens off for very close encounters due to strong gravitational focusing. Close encounters with FFPs can perturb existing planetary systems and their debris structures, and they can result in re-capture of FFPs. In addition, these FFP populations may be observed in young star clusters in imaging surveys; a comparison between observations and dynamical predictions may provide clues to the early phases of stellar and planetary dynamics in star clusters.

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Scattered Planets
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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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RE: Starless planets
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  Some Stars Capture Rogue Planets

New research suggests that billions of stars in our galaxy have captured rogue planets that once roamed interstellar space. The nomad worlds, which were kicked out of the star systems in which they formed, occasionally find a new home with a different sun. This finding could explain the existence of some planets that orbit surprisingly far from their stars, and even the existence of a double-planet system.
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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: Nomads of the Galaxy
Authors: Louis E. Strigari, Matteo Barnabe, Philip J. Marshall, Roger D. Blandford

We estimate that there may be up to ~10^5 compact objects in the mass range 10^{-8} -10^{-2} solar mass per main sequence star that are unbound to a host star in the Galaxy. We refer to these objects as nomads; in the literature a subset of these are sometimes called free-floating or rogue planets. Our estimate for the number of Galactic nomads is consistent with a smooth extrapolation of the mass function of unbound objects above the Jupiter-mass scale, the stellar mass density limit, and the metallicity of the interstellar medium. We analyse the prospects for detecting nomads via Galactic microlensing. The Wide-Field Infrared Survey Telescope (WFIRST) will measure the number of nomads per main sequence star greater than the mass of Jupiter to ~ 13%, and the corresponding number greater than the mass of Mars to ~25%. All-sky surveys such as GAIA and LSST can identify nomads greater than about the mass of Jupiter. We suggest a dedicated drift scanning telescope that covers approximately 100 square degrees in the Southern hemisphere could identify nomads as small as 10^{-8} solar mass via microlensing of bright stars with characteristic lightcurve timescales of a few seconds.

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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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Title: Unbound or Distant Planetary Mass Population Detected by Gravitational Microlensing
Authors: T. Sumi, K. Kamiya, A. Udalski, D.P. Bennett, I.A. Bond, F. Abe, C.S. Botzler, A. Fukui, K. Furusawa, J.B. Hearnshaw, Y. Itow, P.M. Kilmartin, A. Korpela, W. Lin, C.H. Ling, K. Masuda, Y. Matsubara, N. Miyake, M. Motomura, Y. Muraki, M. Nagaya, S. Nakamura, K. Ohnishi, T. Okumura, Y.C. Perrott, N. Rattenbury, To. Saito, T. Sako, D.J. Sullivan, W.L. Sweatman, P.J. Tristram, P.C.M. Yock, M.K. Szymanski, M. Kubiak, G. Pietrzynski, R. Poleski, I. Soszynski, L. Wyrzykowski, K. Ulaczyk

Since 1995, more than 500 exoplanets have been detected using different techniques, of which 11 were detected with gravitational microlensing. Most of these are gravitationally bound to their host stars. There is some evidence of free-floating planetary mass objects in young star-forming regions, but these objects are limited to massive objects of 3 to 15 Jupiter masses with large uncertainties in photometric mass estimates and their abundance. Here, we report the discovery of a population of unbound or distant Jupiter-mass objects, which are almost twice (1.8_{-0.8}^{+1.7}) as common as main-sequence stars, based on two years of gravitational microlensing survey observations toward the Galactic Bulge. These planetary-mass objects have no host stars that can be detected within about ten astronomical units by gravitational microlensing. However a comparison with constraints from direct imaging suggests that most of these planetary-mass objects are not bound to any host star. An abrupt change in the mass function at about a Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.

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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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Free-Floating Planets May be More Common Than Stars

Astronomers, including a NASA-funded team member, have discovered a new class of Jupiter-sized planets floating alone in the dark of space, away from the light of a star. The team believes these lone worlds were probably ejected from developing planetary systems.
The discovery is based on a joint Japan-New Zealand survey that scanned the center of the Milky Way galaxy during 2006 and 2007, revealing evidence for up to 10 free-floating planets roughly the mass of Jupiter. The isolated orbs, also known as orphan planets, are difficult to spot, and had gone undetected until now. The newfound planets are located at an average approximate distance of 10,000 to 20,000 light-years from Earth.

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Starless planets may be habitable after all

Liquid water may survive on free-floating planets that have no star to warm them. If they also support life, they could act as stepping stones to spread life around the galaxy.
Gravitational tussles with other planets or passing stars can eject planets from their solar systems. But even in the cold of space, these wayward worlds could stay warm, thanks to the decay of radioactive elements in their rocky cores.

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