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

TL;DR

This paper proposes a new mechanism where free-floating planets are captured into wide orbits around stars during cluster dispersal, explaining the origin of planets at very wide orbits (>100 AU).

Contribution

It introduces a novel recapture scenario for wide-orbit planets via N-body simulations, expanding understanding of planetary system formation in dispersing stellar clusters.

Findings

Planets are captured into wide orbits (~100-10^6 AU) with diverse eccentricities.

3-6% of stars can capture a planetary companion depending on free-floating planet abundance.

Capture efficiency depends on cluster size and host mass, and can involve multiple planets or binary systems.

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. 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 ~100-10^6 AU, and have a wide range of eccentricities (thermal distribution). Typically, 3-6 x (f_FFP/1) % of all stars capture a planetary companion with such properties (where 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. 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. More than one planet can be captured around the same host and planets can be captured into binary systems. Planets can also be captured into pre-existing planetary and 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 x (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.