The Great Planetary Heist: theft and capture in star-forming regions

Abstract

Gravitational interactions in star-forming regions are capable of disrupting and destroying planetary systems, as well as creating new ones. In particular, a planet can be stolen, where it is directly exchanged between passing stars during an interaction; or captured, where a planet is first ejected from its birth system and is free-floating for a period of time, before being captured by a passing star. We perform sets of direct N-body simulations of young, substructured star-forming regions, and follow their evolution for 10 Myr in order to determine how many planets are stolen and captured, and their respective orbital properties. We show that in high density star-forming regions, stolen and captured planets have distinct properties. The semimajor axis distribution of captured planets is significantly skewed to wider orbits compared to the semimajor axis distribution of stolen planets and planets that are still orbiting their parent star (preserved planets). However, the eccentricity and inclination distributions of captured and stolen planets are similar, but in turn very different to the inclination and eccentricity distributions of preserved planets. In low-density star-forming regions these differences are not as distinct but could still, in principle, be used to determine whether observed exoplanets have likely formed in situ or have been stolen or captured. We find that the initial degree of spatial and kinematic substructure in a star-forming region is as important a factor as the stellar density in determining whether a planetary system will be altered, disrupted, captured or stolen.

Making BEASTies: dynamical formation of planetary systems around massive stars

“…Our results are an extension of previous work that shows planets at distances >100 au may not be orbiting their parent star (Parker & Quanz 2012;Perets & Kouwenhoven 2012;Li & Adams 2015;Mustill et al 2016;Daffern-Powell et al 2022).…”

“…We use a subset of the N-body simulations described in Daffern-Powell et al (2022), which contain N ⋆ = 1000 stars, drawn from a Maschberger (2013) IMF with a probability distribution of the form…”

“…We analyse the simulations at an age of 10 Myr. In Daffern-Powell et al (2022), the rate of theft and capture of planets was highest earlier on (between 0.1 -2 Myr), but many of these planets were not stable in the long-term. At 10 Myr, our simulations have evolved past the stage at which captured/stolen planets could be further disrupted by stellar encounters.…”

“…Whilst they may form in situ through disc fragmentation (Boss 1997;Mayer et al 2002), they may have formed closer in to their host stars and subsequently moved via dynamical processes, either with other planets in the system (Davies et al 2013), or through the direct (Laughlin & Adams 1998;Smith & Bonnell 2001;Parker & Quanz 2012) and indirect (Fabrycky & Tremaine ⋆ E-mail: R.Parker@sheffield.ac.uk † Royal Society Dorothy Hodgkin Fellow 1 https://exoplanetarchive.ipac.caltech.edu/docs/counts detail.html 2007; Malmberg et al 2007;Parker & Goodwin 2009) influence of other stars in the stellar birth environment. Furthermore, recent work has shown that planets can be captured (where they are free-floating in the star-forming region, and then become bound to another star following an encounter), or even stolen in a direct exchange between two stars, with the planet ending up on a bound orbit around the other star (Daffern-Powell et al 2022).…”

Evaporation before disruption: comparing timescales for Jovian planets in star-forming regions

“…The relatively high stellar densities, combined with non-★ E-mail: R.Parker@sheffield.ac.uk † Royal Society Dorothy Hodgkin Fellow equilibrium initial conditions in the spatial and kinematic substructure, means that planetary systems can be disrupted in their birth environments. At the highest stellar densities (≥ 10 4 M pc −3 ), direct truncation of protoplanetary discs can occur (Vincke & Pfalzner 2016;Winter et al 2018), and at more modest stellar densities (≥ 100M pc −3 ) direct disruption of planetary orbits occurs (Smith & Bonnell 2001;Adams et al 2006;Parker & Quanz 2012;Daffern-Powell et al 2022). However, if massive stars (> 5 M ) are present in a starforming region, the Far Ultraviolet (FUV) and Extreme Ultraviolet (EUV) radiation emitted by these stars can photovaporate the gas content of protoplanetary discs (Scally & Clarke 2001;Adams et al 2004;Fatuzzo & Adams 2008;Concha-Ramírez et al 2019b;Nicholson et al 2019;Parker et al 2021).…”