Using mass–radius composition models, small planets (R ≲ 2 R _⊕) are typically classified into three types: iron-rich, nominally Earth-like, and those with solid/liquid water and/or atmosphere. These classes are generally expected to be variations within a compositional continuum. Recently, however, Luque & Pallé observed that potentially Earth-like planets around M dwarfs are separated from a lower-density population by a density gap. Meanwhile, the results of Adibekyan et al. hint that iron-rich planets around FGK stars are also a distinct population. It therefore remains unclear whether small planets represent a continuum or multiple distinct populations. Differentiating the nature of these populations will help constrain potential formation mechanisms. We present the RhoPop software for identifying small-planet populations. RhoPop employs mixture models in a hierarchical framework and a nested sampler for parameter and evidence estimates. Using RhoPop, we confirm the two populations of Luque & Pallé with >4σ significance. The intrinsic scatter in the Earth-like subpopulation is roughly half that expected based on stellar abundance variations in local FGK stars, perhaps implying M dwarfs have a smaller spread in the major rock-building elements (Fe, Mg, Si) than FGK stars. We apply RhoPop to the Adibekyan et al. sample and find no evidence of more than one population. We estimate the sample size required to resolve a population of planets with Mercury-like compositions from those with Earth-like compositions for various mass–radius precisions. Only 16 planets are needed when σMp=5% and σRp=1% . At σMp=10% and σRp=2.5% , however, over 154 planets are needed, an order of magnitude increase.

中文翻译：

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绕 M 矮星运行的小行星的密度差距：使用开源代码 RhoPop 进行严格的统计确认

使用质量半径组成模型，小行星（右≲ 2右 _⊕）通常分为三种类型：富含铁的、名义上类地的、以及具有固态/液态水和/或大气的。这些类别通常被认为是组合连续体内的变体。然而最近，卢克和帕莱观察到，M型矮星周围的潜在类地行星与密度较低的行星之间存在着一定的密度差距。与此同时，Adibekyan 等人的结果。暗示 FGK 恒星周围的富铁行星也是一个独特的群体。因此，目前尚不清楚小行星是否代表一个连续体或多个不同的种群。区分这些群体的性质将有助于限制潜在的形成机制。我们提出了罗波普用于识别小行星种群的软件。罗波普采用分层框架中的混合模型和嵌套采样器来进行参数和证据估计。使用罗波普，我们确认 Luque 和 Pallé 的两个种群 >4σ意义。类地亚群的固有散射大约是根据当地 FGK 恒星的恒星丰度变化所预期的一半，这可能意味着 M 矮星在主要造岩元素（Fe、Mg、Si）方面的分布比 FGK 恒星要小。我们申请罗波普阿迪贝基安等人。抽样并没有发现超过一个群体的证据。我们估计了从具有类地成分的行星中分辨出具有类水星成分的行星群体所需的样本量，以实现不同的质量半径精度。仅需要 16 个行星 σ中号p=5% 和 σ右p=1% 。在 σ中号p=10% 和 σ右p=2.5% 然而，需要超过 154 个行星，数量级增加。