Among exoplanets, the small-size population constitutes the dominant one, with a diversity of properties and compositions ranging from rocky to gas dominated envelope. While a large fraction of them have masses and radii similar to or smaller than Neptune, yet none share common properties in term of orbital period and insulation with our ice giants. These exoplanets belong to multi-planet systems where planets are closely packed within the first tenth of AU and often exposed to strong irradiation from their host star. Their formation process, subsequent evolution, and fate are still debated and trigger new developments of planet formation models. This paper reviews the characteristics and properties of this extended sample of planets with radii between ∼1.6 and 4.0 $R_{\oplus }$ . Even though we still lack real Neptune/Uranus analogues, these exoplanets provide us with key observational constraints that allow the formation of our ice giants to be placed in a more general framework than the sole example of our solar system.

中文翻译：

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海王星级系外行星形成和演化的观测限制

在系外行星中，小型族群占主导地位，其性质和成分多种多样，从岩石到以气体为主的包层。虽然其中很大一部分的质量和半径类似于或小于海王星，但在轨道周期和绝缘方面，它们与我们的冰巨星没有共同的特性。这些系外行星属于多行星系统，其中行星在 AU 的前十分之一内紧密排列，并且经常暴露于来自其主恒星的强烈辐射。它们的形成过程、随后的演化和命运仍然存在争议，并引发了行星形成模型的新发展。本文回顾了这个半径在 ∼1.6 到 4.0 $R_{\oplus }$ 之间的行星扩展样本的特征和性质。尽管我们仍然缺乏真正的海王星/天王星类似物，