Occurrence Rates from Direct Imaging Surveys

TL;DR

Direct imaging surveys suggest about 1% of young stars host massive planets at wide orbits, with emerging evidence linking planet occurrence to debris disks and a continuous substellar companion mass function.

Contribution

This paper synthesizes recent findings from direct imaging surveys, highlighting statistical properties and potential formation mechanisms of wide-orbit giant planets.

Findings

Approximately 1% occurrence rate of massive planets at wide separations.

Possible correlation between debris disks and planet occurrence.

Substellar companion mass function appears smooth and continuous.

Abstract

The occurrence rate of young giant planets from direct imaging surveys is a fundamental tracer of the efficiency with which planets form and migrate at wide orbital distances. These measurements have progressively converged to a value of about 1% for the most massive planets ($\approx$5-13 $M_\mathrm{Jup}$) averaged over all stellar masses at separations spanning a few tens to a few hundreds of AU. The subtler statistical properties of this population are beginning to emerge with ever-increasing sample sizes: there is tentative evidence that planets on wide orbits are more frequent around stars that possess debris disks; brown dwarf companions exist at comparable (or perhaps slightly higher) rates as their counterparts in the planetary-mass regime; and the substellar companion mass function appears to be smooth and may extend down to the opacity limit for fragmentation. Within a few years, the conclusion of second-generation direct imaging surveys will enable more definitive interpretations with the ultimate goal of identifying the dominant origin of this population and uncovering its relationship to planets at smaller separations.