First VLTI/GRAVITY Observations of HIP 65426 b: Evidence for a Low or Moderate Orbital Eccentricity

TL;DR

This study presents high-precision astrometric and spectral data of the exoplanet HIP 65426 b, suggesting it has a low or moderate orbital eccentricity and providing insights into its atmospheric composition and formation history.

Contribution

The paper provides the first VLTI/GRAVITY astrometric observations of HIP 65426 b and refines its orbital eccentricity and atmospheric parameters, challenging existing formation theories.

Findings

Eccentricity posterior disfavors large eccentricities.

Best-fit atmospheric model has solar metallicity and C/O=0.6.

Data suggests formation not consistent with post-disk scattering.

Abstract

Giant exoplanets have been directly imaged over orders of magnitude of orbital separations, prompting theoretical and observational investigations of their formation pathways. In this paper, we present new VLTI/GRAVITY astrometric data of HIP 65426 b, a cold, giant exoplanet which is a particular challenge for most formation theories at a projected separation of 92 au from its primary. Leveraging GRAVITY's astrometric precision, we present an updated eccentricity posterior that disfavors large eccentricities. The eccentricity posterior is still prior-dependent, and we extensively interpret and discuss the limits of the posterior constraints presented here. We also perform updated spectral comparisons with self-consistent forward-modeled spectra, finding a best fit ExoREM model with solar metallicity and C/O=0.6. An important caveat is that it is difficult to estimate robust errors on these values, which are subject to interpolation errors as well as potentially missing model physics. Taken together, the orbital and atmospheric constraints paint a preliminary picture of formation inconsistent with scattering after disk dispersal. Further work is needed to validate this interpretation. Analysis code used to perform this work is available at https://github.com/sblunt/hip65426.