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-year 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 its gas envelope. We reveal that WASP-107b's mass is only 1.8 Neptune masses ($M_b = 30.5 \pm 1.7$ $M_\oplus$). 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 $<4.6$ $M_\oplus$ 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 $\gtrsim 1$ AU onto a low-opacity, dust-free atmosphere and subsequent migration to the present-day $a_b = 0.0566 \pm 0.0017$ 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 ($e_c = 0.28 \pm 0.07$) which 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.

中文翻译：

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WASP-107b 的密度更低：行星气体包层吸积和轨道迁移物理学的案例研究

WASP-107b 的质量在海王星范围内，半径为木星，对行星形成理论提出了挑战。同时，这颗行星的低表面重力和恒星的亮度也使其成为大气表征最有利的目标之一。在这里，我们展示了 WASP-107 系统的广泛的 4 年 Keck/HIRES 径向速度 (RV) 后续计划的结果，并详细研究了控制其气体包层吸积的物理学。我们发现 WASP-107b 的质量只有 1.8 海王星质量（$M_b = 30.5 \pm 1.7$ $M_\oplus$）。由此产生的极低的密度表明 WASP-107b 的 H/He 包膜质量分数大于 85%%，除非它被大幅充气。对应的核心质量$<4。3$\sigma$ 处的 6$ $M_\oplus$ 明显低于传统上假设的触发大量气体包层吸积所必需的值。我们证明，这种大的气核质量比最有可能是由于在 $\gtrsim 1 $ AU 处开始吸积到低不透明度、无尘的大气中，然后迁移到今天的 $a_b = 0.0566 \下午 0.0017 澳元。除了 WASP-107b，我们还在宽的偏心轨道 ($e_c = 0.28 \pm 0.07$) 上探测到第二颗质量更大的行星 ($M_c \sin i = 0.36 \pm 0.04$ $M_{J}$)，它可能有影响了 WASP-107b 的轨道迁移和自旋轨道错位。总的来说，我们新的 RV 观测和包络吸积模型为 WASP-107b 的有趣性质和系统的形成历史提供了重要的见解。展望未来，