WASP-107b's Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration

With a mass in the Neptune regime and a radius of Jupiter, WASP-107b presents a challenge to planet formation theories. Meanwhile, the planet's low surface gravity and the star's brightness also make it one of the most favorable targets for atmospheric characterization. Here, we present the results of an extensive 4 yr Keck/HIRES radial-velocity (RV) follow-up program of the WASP-107 system and provide a detailed study of the physics governing the accretion of the gas envelope of WASP-107b. We reveal that WASP-107b's mass is only 1.8 Neptune masses (M-b = 30.5 1.7 M-circle plus). The resulting extraordinarily low density suggests that WASP-107b has a H/He envelope mass fraction of >85% unless it is substantially inflated. The corresponding core mass of M-circle plus at 3 sigma is significantly lower than what is traditionally assumed to be necessary to trigger massive gas envelope accretion. We demonstrate that this large gas-to-core mass ratio most plausibly results from the onset of accretion at greater than or similar to 1 au onto a low-opacity, dust-free atmosphere and subsequent migration to the present-day a(b) = 0.0566 0.0017 au. Beyond WASP-107b, we also detect a second, more massive planet ((M-c sin i = 0.36 +/- 0.04M(J)) on a wide eccentric orbit (e(c) = 0.28 +/- 0.07) that may have influenced the orbital migration and spin-orbit misalignment of WASP-107b. Overall, our new RV observations and envelope accretion modeling provide crucial insights into the intriguing nature of WASP-107b and the system's formation history. Looking ahead, WASP-107b will be a keystone planet to understand the physics of gas envelope accretion.

Automated summary

WASP-107b, with its mass comparable to Neptune but radius similar to Jupiter, challenges planet formation theories. Its low surface gravity and the star's brightness make it an ideal target for atmospheric characterization. The research presents findings from a 4-year RV follow-up program and detailed study on gas envelope accretion, revealing insights into WASP-107b's nature and the system's formation history. Additionally, a second, more massive planet in the system is detected, potentially impacting the orbital migration and spin-orbit misalignment of WASP-107b.