Precise Masses in the WASP-47 System

TL;DR

This study provides the most precise mass measurements for the WASP-47 system, revealing the composition of its small planets and the orbital inclination of its outer giant planet, with implications for system formation.

Contribution

It offers the first precise mass measurements of all planets in WASP-47, constrains their compositions, and analyzes the system's dynamical architecture and transit probabilities.

Findings

WASP-47 e has a volatile-rich envelope, not Earth-like.

WASP-47 c likely orbits close to the plane of inner planets.

Transit probability of WASP-47 c is about 10%.

Abstract

We present precise radial velocity observations of WASP-47, a star known to host a hot Jupiter, a distant Jovian companion, and, uniquely, two additional transiting planets in short-period orbits: a super-Earth in a ~19 hour orbit, and a Neptune in a ~9 day orbit. We analyze our observations from the HARPS-N spectrograph along with previously published data to measure the most precise planet masses yet for this system. When combined with new stellar parameters and reanalyzed transit photometry, our mass measurements place strong constraints on the compositions of the two small planets. We find unlike most other ultra-short-period planets, the inner planet, WASP-47 e, has a mass (6.83 +/- 0.66 Me) and radius (1.810 +/- 0.027 Re) inconsistent with an Earth-like composition. Instead, WASP-47 e likely has a volatile-rich envelope surrounding an Earth-like core and mantle. We also perform a dynamical analysis to constrain the orbital inclination of WASP-47 c, the outer Jovian planet. This planet likely orbits close to the plane of the inner three planets, suggesting a quiet dynamical history for the system. Our dynamical constraints also imply that WASP-47 c is much more likely to transit than a geometric calculation would suggest. We calculate a transit probability for WASP-47 c of about 10%, more than an order of magnitude larger than the geometric transit probability of 0.6%.