Planets intermediate in size between the Earth and Neptune, and orbiting closer to their host stars than Mercury does the Sun, are the most common type of planet revealed by exoplanet surveys over the last quarter century. Results from NASA's Kepler mission have revealed a bimodality in the radius distribution of these objects, with a relative underabundance of planets between 1.5 and 2.0 . This bimodality suggests that sub‐Neptunes are mostly rocky planets that were born with primary atmospheres a few percent by mass accreted from the protoplanetary nebula. Planets above the radius gap were able to retain their atmospheres (“gas‐rich super‐Earths”), while planets below the radius gap lost their atmospheres and are stripped cores (“true super‐Earths”). The mechanism that drives atmospheric loss for these planets remains an outstanding question, with photoevaporation and core‐powered mass loss being the prime candidates. As with the mass‐loss mechanism, there are two contenders for the origins of the solids in sub‐Neptune planets: the migration model involves the growth and migration of embryos from beyond the ice line, while the drift model involves inward‐drifting pebbles that coagulate to form planets close‐in. Atmospheric studies have the potential to break degeneracies in interior structure models and place additional constraints on the origins of these planets. However, most atmospheric characterization efforts have been confounded by aerosols. Observations with upcoming facilities are expected to finally reveal the atmospheric compositions of these worlds, which are arguably the first fundamentally new type of planetary object identified from the study of exoplanets.

中文翻译：

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海王星以下大小行星的性质和起源


大小介于地球和海王星之间的行星，其轨道距离其主恒星的距离比水星距离太阳的距离更近，是过去四分之一个世纪的系外行星调查所揭示的最常见的行星类型。 NASA开普勒任务的结果揭示了这些天体半径分布的双峰性，行星相对数量不足在 1.5 到 2.0 之间 。这种双峰性表明，亚海王星大多是岩石行星，它们诞生时的主要大气层质量有百分之几是从原行星状星云吸积的。半径间隙以上的行星能够保留其大气层（“富含气体的超级地球”），而半径间隙以下的行星则失去了大气层并被剥离了核心（“真正的超级地球”）。驱动这些行星大气层损失的机制仍然是一个悬而未决的问题，其中光蒸发和核心驱动的质量损失是主要的候选者。与质量损失机制一样，关于海王星以下行星中固体的起源有两个争论者：迁移模型涉及冰线以外的胚胎的生长和迁移，而漂移模型涉及向内漂移的卵石，凝结形成靠近的行星。大气研究有可能打破内部结构模型的简并性，并对这些行星的起源施加额外的限制。然而，大多数大气特征描述工作都被气溶胶所困扰。即将推出的设施的观测预计将最终揭示这些世界的大气成分，这可以说是通过系外行星研究发现的第一个全新类型的行星物体。