SubNeptunes - Probing Exoplanetary Atmospheres in the Sub-Neptune Regime

亚海王星行星代表了系外行星科学的新前沿。系外行星的人口统计数据显示，在地球和海王星之间大小的行星（1-4个地球半径）压倒性地统治着已知的系外行星数量。这些行星统称为“亚海王星”，涵盖了从多岩石的超级地球到富含挥发物的迷你海王星的所有行星类型。这些行星在系外行星中占主导地位，但在太阳系中没有类似的行星。这个种群的一个决定性特征是“半径谷”，这是一种半径上的双峰分布，两个峰被一个大约1.8地球半径的山谷隔开。随着关于行星形成、内部和大气的相互竞争的理论努力理解这一奇特的制度，越来越清楚的是，大气观测对于确定它们的性质、物理过程和起源至关重要。我们建议对半径谷附近的十个亚海王星进行一次全面的大气调查。该项目将解决系外行星科学中的三个基本问题：次海王星的大气成分有多多样化？在海王星下的大气中，不平衡过程是如何运作的？亚海王星是如何形成和演化的？这些问题的答案将结合空间（HST和JWST）和地面上最先进的设施，以及理论建模和大气检索技术的关键进展，进行开创性的经批准的观测。这将使我们能够在整个半径谷的不同行星上探测到关键的分子种类，如H2O、CH4、NH3和HCN，并对大气中的物理和化学过程及其形成条件进行关键的限制。除了回答上述转型问题外，我们还将使用经批准的JWST观测结果对海王星下的宜居带进行深入的大气特征描述。

Sub-Neptune planets represent the new frontier in exoplanet science. Exoplanet demographics reveal that planets between Earth and Neptune in size (1-4 Earth radii) overwhelmingly dominate the known exoplanet population. These planets, collectively referred to as "sub-Neptunes", encompass the entire range of planet types from rocky super-Earths to volatile-rich mini-Neptunes. These planets dominate the exoplanet population but have no analog in the solar system. A defining feature of this population is the "Radius Valley", a bimodal distribution in radii with two peaks separated by a valley around 1.8 Earth radii. As competing theories of planet formation, interiors and atmospheres strive to understand this exotic regime, it is becoming clear that atmospheric observations are essential to place definitive constraints on their properties, physical processes and origins. We propose to conduct a comprehensive atmospheric survey of ten nearby sub-Neptunes across the Radius Valley. The project will address three fundamental questions in exoplanetary science: How diverse are the atmospheric compositions of sub-Neptunes? How do disequilibrium processes operate in sub-Neptune atmospheres? How do sub-Neptunes form and evolve? These questions will be answered using a combination of pioneering approved observations with state-of-the-art facilities in space (HST and JWST) and on ground, and key advancements in theoretical modelling and atmospheric retrieval techniques. This will enable new detections of key molecular species such as H2O, CH4, NH3, and HCN in diverse planets across the radius valley and key constraints on physical and chemical processes in the atmospheres and on their formation conditions. In addition to answering the above transformational questions, we will conduct an in-depth atmospheric characterisation of a habitable-zone sub-Neptune using approved JWST observations.