The First Dynamical Mass Measurement in the HR 8799 System

TL;DR

This paper presents the first dynamical mass measurement of HR 8799 e, using astrometric acceleration data, confirming its mass below the deuterium-fusing limit and refining the system's age and planetary constraints.

Contribution

It introduces a novel dynamical mass measurement method for HR 8799 planets using Gaia astrometry, providing the first direct mass estimate for HR 8799 e.

Findings

HR 8799 e has a mass of approximately 9.6 Jupiter masses.

HR 8799 e's mass is below 13 Jupiter masses with 95% confidence.

No additional planets larger than 5 Jupiter masses exist interior to HR 8799 e.

Abstract

HR 8799 hosts four directly imaged giant planets, but none has a mass measured from first principles. We present the first dynamical mass measurement in this planetary system, finding that the innermost planet HR~8799~e has a mass of $9.6^{+1.9}_{-1.8} \, M_{\rm Jup}$. This mass results from combining the well-characterized orbits of all four planets with a new astrometric acceleration detection (5$\sigma$) from the Gaia EDR3 version of the Hipparcos-Gaia Catalog of Accelerations. We find with 95\% confidence that HR~8799~e is below $13\, M_{\rm Jup}$, the deuterium-fusing mass limit. We derive a hot-start cooling age of $42^{+24}_{-16}$\,Myr for HR~8799~e that agrees well with its hypothesized membership in the Columba association but is also consistent with an alternative suggested membership in the $\beta$~Pictoris moving group. We exclude the presence of any additional $\gtrsim$5-$M_{\rm Jup}$ planets interior to HR~8799~e with semi-major axes between $\approx$3-16\,au. We provide proper motion anomalies and a matrix equation to solve for the mass of any of the planets of HR~8799 using only mass ratios between the planets.