Re ected spectroscopy of small exoplanets I: determining the atmospheric composition of sub-Neptunes planets

Direct imaging of widely separated exoplanets from space will obtain their reected light spectra and7 measure their atmospheric properties, and small and temperate planets will be the focus of the next8 generation telescopes. In this work, we used our Bayesian retrieval algorithm ExoReL< to determine9 the constraints on the atmospheric properties of sub-Neptune planets from observations taken with a10 HabEx-like telescope. Small and temperate planets may have a non-H2-dominated atmosphere, and11 therefore, we introduced the compositional analysis technique in our framework to explore the bulk12 atmospheric chemistry composition without any prior knowledge about it. We have developed a novel13 set of prior functions for the compositional analysis free parameters. We compared the performances of14 the framework with the at prior and the novel prior and we reported a better performance when using15 the novel priors set. We found that the retrieval algorithm can not only identify the dominant gas of16 the atmosphere but also to constrain other less abundant gases with high statistical condence without17 any prior information on the composition. The results presented here demonstrates that reected light18 spectroscopy can characterize small exoplanets with diverse atmospheric composition. The Bayesian19 framework should be applied to design the instrument and the observation plan of exoplanet direct20 imaging experiments in the future.

从太空对大间距系外行星进行直接成像，可获取其反射光光谱并测定大气特性，而小型温带行星将是下一代望远镜的研究重点。本研究中，我们利用自主研发的贝叶斯反演算法（Bayesian retrieval algorithm）ExoReL<，基于类HabEx望远镜的观测数据，确定了亚海王星行星大气特性的约束条件。小型温带行星的大气可能并非以H₂为主，因此我们在框架中引入成分分析技术（compositional analysis technique），在无任何先验知识的情况下探究其整体大气化学成分。我们针对成分分析的自由参数开发了一套全新的先验函数（prior functions）。我们对比了框架在平坦先验和新先验下的性能，结果表明使用新先验集时性能更优。我们发现，该反演算法不仅能识别大气中的主导气体，还能在无成分先验信息的情况下，以高统计置信度（statistical confidence）约束其他低丰度气体。本文结果表明，反射光光谱技术（reflected light spectroscopy）可对具有多样大气成分的小型系外行星进行表征。未来，该贝叶斯框架应被应用于系外行星直接成像实验的仪器设计与观测方案制定中。