SCYON Abstract

Received on March 14 2012

On the origin of planets at very wide orbits from the re-capture of free floating planets

Authors	Hagai B. Perets (^1,2) and M.B.N. Kouwenhoven (³)
Affiliation	(¹) Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, USA 02138 (²) Technion - Israel Institute of Technology, Haifa, Israel (³) Kavli Institute for Astronomy and Astrophysics at Peking University, Yi He Yuan Lu 5, Hai Dian District, Beijing 100871, China
Accepted by	Astrophysical Journal
Contact	hperets@physics.technion.ac.il; thijskouwenhoven@gmail.com
URL	http://arxiv.org/abs/1202.2362
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Abstract

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 and stellar associations. 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 stars and comparable numbers of FFPs to study this process. We find that planets are captured into wide orbits in the typical range ~few times 100-10⁶ AU, and have a wide range of eccentricities (thermal distribution). Typically, 3-6 times (f_FFP/1)% of all stars capture a planetary companion with such properties (where f_FFP is the number of FFP per star in the birth clusters). The planetary capture efficiency is comparable to that of capture-formed stellar-binaries, and shows a similar dependence on the cluster size and structure. It is almost independent of the specific planetary mass; planets as well as sub-stellar companions of any mass can be captured. The capture efficiency decreases with increasing cluster size, and for a given cluster size the it increases with the host/primary mass. We also find that more than one planet can be captured around the same host through independent consecutive captures; similarly planets can be captured into binary systems, both in circumstellar and circumbinary orbits. We also expect planets to be captured into pre-existing planetary (and protoplanetary systems) as well as into orbits around black holes and massive white dwarfs, if these formed early enough before the cluster dispersal. In particular, stellar black holes have a high capture efficiency (>50% and 5-10 times (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.