Astrometric planet search around M8--L2 dwarfs from the ground and with Gaia

TL;DR

This study uses high-precision ground-based and Gaia astrometry to search for planets around ultra-cool dwarfs, establishing that giant planets are rare and demonstrating the potential for combined observational strategies.

Contribution

It presents a ground-based astrometric survey of M8-L2 dwarfs achieving 100 micro-arcsecond accuracy, and discusses synergies with Gaia for planet detection and binary characterization.

Findings

Giant planets are rare around ultra-cool dwarfs at all separations.

Ground-based astrometry achieves precision comparable to Gaia for faint objects.

Gaia can characterize approximately 100 astrometric binaries with ultracool primaries.

Abstract

Ultra-cool dwarfs are very low-mass stars or brown dwarfs and because of their faintness they are difficult targets for radial velocity and transit planet searches. High-precision astrometry is one way to efficiently discover planets around these objects. We are conducting a planet search survey of 20 M8-L2 using ground-based imaging astrometry with FORS2 at VLT. The realised accuracy of 100 micro-arcseconds allows us to set stringent constraints on the presence of planets, to discover astrometric binaries, and to measure parallaxes with an unprecedented precision of 0.1 %. The obtained detection limits firmly establish that giant planets are rare around UCDs at all separations. The astrometric performance of our programme is comparable to what is expected from Gaia observations of single faint objects and we discuss potential synergies for planet searches around ultracool dwarfs. We estimate that Gaia will be able to characterise ~100 astrometric binaries with an ultracool primary.