Upper Limits on Planet Occurrence around Ultracool Dwarfs with K2

TL;DR

This study used K2 data to search for transiting planets around 827 ultracool dwarfs, found none, and established upper limits on the occurrence rates of such planets based on simulations and injection-recovery tests.

Contribution

It provides the first comprehensive upper limits on planet occurrence rates around ultracool dwarfs using K2 data, extending previous studies on early- and mid-type M dwarfs.

Findings

Short period mini-Neptune- and Jupiter-sized planets are rare around ultracool dwarfs.

Upper limits on planet occurrence rates were established for planets 0.5-10 R⊕ with periods 1-26.3 days.

No transiting planets were detected in the sample.

Abstract

The occurrence of planets orbiting ultracool dwarfs is poorly constrained. We present results from a Guest Observer program on NASA's K2 spacecraft to search for transiting planets orbiting a sample of 827 ultracool dwarfs. Having found no transiting planets in our sample, we determined an upper limit on the occurrence of planets. We simulated planets orbiting our sample for a range of orbital periods and sizes. For the simulated planets that transit their host, we injected the transit light curve into the real K2 light curves, then attempted to recover the injected planets. For a given occurrence rate, we calculated the probability of seeing no planets, and use the results to place an upper limit on planet occurrence as a function of planet radius and orbital period. We find that short period, mini-Neptune- and Jupiter-sized planets are rare around ultracool dwarfs, consistent with results for early- and mid-type M dwarf stars. We constrain the occurrence rate $\eta$ for planets between $0.5$ and $10$ R$_{\oplus}$ with orbital periods between 1 and 26.3 days.