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

Abstract

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 ⊕). 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 <4.6 M ⊕ at 3σ 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 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 i M sin 0.36 0.04 c 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.

Metastable Helium Reveals an Extended Atmosphere for the Gas Giant HAT-P-18b

“…This is noticeably smaller than the 7.26% ± 0.24% depth measured by CARMENES and Keck/ NIRSPEC for WASP-107b (Allart et al 2019;Kirk et al 2020). The difference may be due to the smaller gravitational potential of WASP-107b (the mass of WASP-107b has recently been suggested to be even lower by Piaulet et al 2021), or differences in the EUV spectra of the two stars (WASP-107 is a K6 star while HAT-P-18 is a K2). Detailed comparative modeling of these two planets may make clear the primary control on the metastable helium signal.…”

The TESS–Keck Survey. IV. A Retrograde, Polar Orbit for the Ultra-low-density, Hot Super-Neptune WASP-107b

“…Originally discovered via the transit method by WASP-South, WASP-107b was later observed by K2 in Campaign 10 (Howell et al 2014). These transits revealed a radius close to that of Jupiter, R b = 10.8 ± 0.34 R ⊕ = 0.96 ± 0.03 R J (Dai & Winn 2017;Močnik et al 2017;Piaulet et al 2021). However, follow-up radial velocity (RV) measurements with the CORALIE spectrograph demonstrated a mass of just 38 ± 3 M ⊕ (Anderson et al 2017), meaning this Jupiter-sized planet has just one-tenth its density.…”

“…However, follow-up radial velocity (RV) measurements with the CORALIE spectrograph demonstrated a mass of just 38 ± 3 M ⊕ (Anderson et al 2017), meaning this Jupiter-sized planet has just one-tenth its density. Higherprecision RVs from Keck/High Resolution Echelle Spectrometer (HIRES) suggested an even lower mass of 30.5 ± 1.7 M ⊕ (Piaulet et al 2021). This low density challenges the standard core-accretion model of planet formation.…”

“…) planetary companion, which may be responsible for this present day misaligned orbit through its gravitational influence on WASP-107b (Piaulet et al 2021).…”

“…This low density challenges the standard core-accretion model of planet formation. If runaway accretion brought WASP-107b to a gas-to-core mass ratio of ∼3 but was stopped prematurely before growing to gas giant size, orbital dynamics and/or migration may have played a significant role in this system (Piaulet et al 2021). Alternatively WASP-107b's radius may be inflated from tidal heating, which would allow a lower gas-to-core ratio consistent with core accretion (Millholland et al 2020).…”