The radius distribution of small, close-in exoplanets has recently been shown to be bimodal. The photoevaporation model predicted this bimodality. In the photoevaporation scenario, some planets are completely stripped of their primordial H/He atmospheres, whereas others retain them. Comparisons between the photoevaporation model and observed planetary populations have the power to unveil details of the planet population inaccessible by standard observations, such as the core mass distribution and core composition. In this work, we present a hierarchical inference analysis on the distribution of close-in exoplanets using forward models of photoevaporation evolution. We use this model to constrain the planetary distributions for core composition, core mass, and initial atmospheric mass fraction. We find that the core-mass distribution is peaked, with a peak-mass of ∼4M⊕. The bulk core-composition is consistent with a rock/iron mixture that is ice-poor and ‘Earth-like’; the spread in core-composition is found to be narrow ($\lesssim 16{{\ \rm per\ cent}}$ variation in iron-mass fraction at the 2σ level) and consistent with zero. This result favours core formation in a water/ice poor environment. We find the majority of planets accreted a H/He envelope with a typical mass fraction of $\sim 4{{\ \rm per\ cent}}$; only a small fraction did not accrete large amounts of H/He and were ‘born-rocky’. We find four times as many super-Earths were formed through photoevaporation, as formed without a large H/He atmosphere. Finally, we find core-accretion theory overpredicts the amount of H/He cores would have accreted by a factor of ∼5, pointing to additional mass-loss mechanisms (e.g. ‘boil-off’) or modifications to core-accretion theory.

中文翻译：

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揭示出生时的地球人口

最近已证明小的、靠近的系外行星的半径分布是双峰的。光蒸发模型预测了这种双峰。在光蒸发情景中，一些行星完全失去了原始 H/He 大气，而另一些则保留了它们。光蒸发模型和观测到的行星种群之间的比较能够揭示标准观测无法获得的行星种群的细节，例如核心质量分布和核心组成。在这项工作中，我们使用光蒸发演化的前向模型对近距离系外行星的分布进行了分层推理分析。我们使用这个模型来约束核心成分、核心质量和初始大气质量分数的行星分布。我们发现核心质量分布达到峰值，峰值质量为~4M⊕。大块核心成分与岩石/铁混合物相一致，冰贫且“类地”；发现核心成分的分布很窄（$\lesssim 16{{\ \rm per\ cent}}$ 铁质量分数在 2σ 水平上的变化）并且与零一致。该结果有利于在水/冰贫乏的环境中形成核心。我们发现大多数行星吸积了一个典型质量分数为 $\sim 4{{\ \rm per\ cent}}$ 的 H/He 包络；只有一小部分没有积累大量的 H / He 并且是“天生的岩石”。我们发现通过光蒸发形成的超级地球是没有大 H/He 大气时形成的四倍。最后，我们发现核心吸积理论高估了 H/He 核心的数量会增加约 5 倍，这表明存在额外的质量损失机制（例如