Hot Jupiters, cold kinematics

High phase space densities of host stars reflect an age bias

¹ Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43 221 00 Lund, Sweden
e-mail: alex@astro.lu.se
² Centre for Mathematical Sciences, Lund University, Box 118 221 00 Lund, Sweden

Received: 29 March 2021
Accepted: 24 November 2021

Abstract

Context. The birth environments of planetary systems are thought to influence planet formation and orbital evolution through external photoevaporation and stellar flybys. Recent work has claimed observational support for this, in the form of a correlation between the properties of planetary systems and the local Galactic phase space density of the host star. In particular, hot Jupiters are overwhelmingly present around stars in regions of high phase space density, which may reflect a formation environment with high stellar density.

Aims. We aim to investigate whether the high phase space density may have a Galactic kinematic origin: hot Jupiter hosts may be biased towards being young and therefore kinematically cold, because tidal inspiral leads to the destruction of the planets on gigayear timescales, and the velocity dispersion of stars in the Galaxy increases on similar timescales.

Methods. We used 6D positions and kinematics from Gaia for the hot Jupiter hosts and their neighbours, and we constructed distributions of the phase space density. We investigated correlations between the stars’ local phase space density and peculiar velocity.

Results. We find a strong anti-correlation between the phase space density and the host star’s peculiar velocity with respect to the Local Standard of Rest. Therefore, most stars in ‘high-density’ regions are kinematically cold, which may be caused by the aforementioned bias towards detecting hot Jupiters around young stars before the planets’ tidal destruction.

Conclusions. We do not find evidence in the data for hot Jupiter hosts preferentially being in phase space overdensities compared to other stars of similar kinematics, nor therefore for their originating in birth environments of high stellar density.