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With a mass in the Neptune regime and a radius of Jupiter, WASP-107b presentsa challenge to planet formation theories. Meanwhile, the planet's low surfacegravity and the star's brightness also make it one of the most favorabletargets for atmospheric characterization. Here, we present the results of anextensive 4-year Keck/HIRES radial-velocity (RV) follow-up program of theWASP-107 system and provide a detailed study of the physics governing theaccretion of its gas envelope. We reveal that WASP-107b's mass is only 1.8Neptune masses ($M_b = 30.5 \pm 1.7$ $M_\oplus$). The resulting extraordinarilylow 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_\oplus$ at 3$\sigma$ is significantly lower than what is traditionallyassumed to be necessary to trigger massive gas envelope accretion. Wedemonstrate that this large gas-to-core mass ratio most plausibly results fromthe onset of accretion at $\gtrsim 1$ AU onto a low-opacity, dust-freeatmosphere and subsequent migration to the present-day $a_b = 0.0566 \pm0.0017$ AU. Beyond WASP-107b, we also detect a second more massive planet ($M_c\sin i = 0.36 \pm 0.04$ $M_{J}$) on a wide eccentric orbit ($e_c = 0.28 \pm0.07$) which may have influenced the orbital migration and spin-orbitmisalignment of WASP-107b. Overall, our new RV observations and envelopeaccretion modeling provide crucial insights into the intriguing nature ofWASP-107b and the system's formation history. Looking ahead, WASP-107b will bea keystone planet to understand the physics of gas envelope accretion.

WASP-107b's density is even lower: a case study for the physics of planetary gas envelope accretion and orbital migration