Making BEASTies: dynamical formation of planetary systems around massive stars

TL;DR

This paper proposes that some planets around massive stars may form through capture or theft rather than in-situ formation, supported by N-body simulations showing such events occur early in star-forming regions.

Contribution

It introduces a new formation mechanism for planets around massive stars, emphasizing capture and theft over traditional accretion processes, supported by simulation data.

Findings

Capture of free-floating planets occurs within 10 million years.

Observed wide orbits of BEAST planets are more consistent with capture.

Planet theft from other stars is less likely than capture for these systems.

Abstract

Exoplanets display incredible diversity, from planetary system architectures around Sun-like stars that are very different to our Solar System, to planets orbiting post-main sequence stars or stellar remnants. Recently the B-star Exoplanet Abundance STudy (BEAST) reported the discovery of at least two super-Jovian planets orbiting massive stars in the Sco Cen OB association. Whilst such massive stars do have Keplerian discs, it is hard to envisage gas giant planets being able to form in such hostile environments. We use N-body simulations of star-forming regions to show that these systems can instead form from the capture of a free-floating planet, or the direct theft of a planet from one star to another, more massive star. We find that this occurs on average once in the first 10Myr of an association's evolution, and that the semimajor axes of the hitherto confirmed BEAST planets (290 and 556au) are more consistent with capture than theft. Our results lend further credence to the notion that planets on more distant (>100au) orbits may not be orbiting their parent star.