Making BEASTies: dynamical formation of planetary systems around massive stars

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.