Astrometric Confirmation and Preliminary Orbital Parameters of the Young Exoplanet 51 Eridani b with the Gemini Planet Imager

TL;DR

This study confirms the physical association of exoplanet 51 Eridani b with its host star using new Gemini Planet Imager data, and provides preliminary orbital parameters suggesting a non-aligned orbit with the star's wide binary companion.

Contribution

First to combine new and existing astrometry to confirm 51 Eridani b's bound status and estimate its orbital parameters with a Monte Carlo approach.

Findings

Probability of unbound status reduced to 2e-7

Estimated orbital semimajor axis of 14 AU

Orbital inclination suggests misalignment with binary companion

Abstract

We present new Gemini Planet Imager observations of the young exoplanet 51 Eridani b which provide further evidence that the companion is physically associated with 51 Eridani. Combining this new astrometric measurement with those reported in the literature, we significantly reduce the posterior probability that 51 Eridani b is an unbound foreground or background T-dwarf in a chance alignment with 51 Eridani to $2\times10^{-7}$, an order of magnitude lower than previously reported. If 51 Eridani b is indeed a bound object, then we have detected orbital motion of the planet between the discovery epoch and the latest epoch. By implementing a computationally efficient Monte Carlo technique, preliminary constraints are placed on the orbital parameters of the system. The current set of astrometric measurements suggest an orbital semimajor axis of $14^{+7}_{-3}$ AU, corresponding to a period of $41^{+35}_{-12}$ years (assuming a mass of $1.75$ $M_{\odot}$ for the central star), and an inclination of $138^{+15}_{-13}$ deg. The remaining orbital elements are only marginally constrained by the current measurements. These preliminary values suggest an orbit which does not share the same inclination as the orbit of the distant M-dwarf binary, GJ 3305, which is a wide physically bound companion to 51 Eridani.