Strongly irradiated exoplanets develop extended atmospheres that can be utilized to probe the deeper planet layers. This connection is particularly useful in the study of small exoplanets, whose bulk atmospheres are challenging to characterize directly. Here, we report the 3.4σ detection of C ii ions during a single transit of the super-Earth π Men c in front of its Sun-like host star. The transit depth and Doppler velocities are consistent with the ions filling the planet’s Roche lobe and moving preferentially away from the star, an indication that they are escaping the planet. We argue that π Men c possesses a thick atmosphere with abundant heavy volatiles (≳ 50% by mass of atmosphere) but that needs not be carbon rich. Our reasoning relies upon cumulative evidence from the reported C ii detection, the nondetection of H i atoms in a past transit, modeling of the planet’s interior, and the assumption that the atmosphere, having survived the most active phases of its Sun-like host star, will survive another 0.2–2 Gyr. Depending on the current mass of atmosphere, π Men c may still transition into a bare rocky core. Our findings confirm the hypothesized compositional diversity of small exoplanets, and represent a milestone toward understanding the planets’ formation and evolution paths through the investigation of their extended atmospheres.

受到强烈辐射的系外行星会形成扩展的大气层，可用于探测更深的行星层。这种联系在小型系外行星的研究中特别有用，因为这些系外大气很难直接表征。在这里，我们报告了超地球πMen c在其类似太阳的主恒星前的单次穿越过程中对C ii离子的3.4σ检测。传输深度和多普勒速度与填充行星罗氏波瓣并优先远离恒星的离子一致，这表明它们正在逃离行星。我们认为π男人c拥有浓厚的大气层和大量的挥发物（按质量计约为50％的大气层），但不必富含碳。我们的推理依赖于所报告的C ii探测，过去的过境中未探测到H i原子，行星内部建模以及大气（在类似太阳的主恒星最活跃的阶段中幸存下来）的假设的累积证据。，将在另一个0.2–2 Gyr中存活。根据当前的大气质量，πMen c可能仍会转变成裸露的岩心。我们的发现证实了小系外行星的假设组成多样性，并代表了通过研究行星的扩展大气来理解行星形成和演化路径的里程碑。