A primary science goal for JWST is to detect and characterize the atmospheres of terrestrial planets orbiting M dwarfs (M-Earths). The existence of atmospheres on M-Earths is highly uncertain because their host stars’ extended history of high X-ray and ultraviolet irradiation may act to completely remove their atmospheres. We present two JWST secondary eclipse observations of the M-Earth Gl 486b (also known as GJ 486b) between 5 and 12 μm. We combined these observations with a precise analysis of the host star parameters to derive a planetary dayside temperature of T _p = 865 ± 14 K. We compared this temperature to the maximum expected temperature for a zero albedo, zero heat redistribution bare rock and derived a temperature ratio of R=Tp,daysideTp,max=0.97±0.01 . This value is consistent with an airless body with a slight nonzero albedo or a thin atmosphere with <1% H₂O or <1 ppm CO₂. However, it is inconsistent with an Earth- or Venus-like atmosphere, and the spectrum shows no clear emission or absorption features. Additionally, our observations are inconsistent with the water-rich atmospheric scenario allowed by previous transit observations and suggest the transmission spectrum was instead shaped by stellar contamination. Given the potential for atmospheric escape throughout the system’s ≥6.6 Gyr lifetime, we conclude that the observations are likely best explained by an airless planet. This result is the most precise measurement yet of terrestrial exoplanet thermal emission with JWST, which places a strong constraint on the position of the “cosmic shoreline” between airless bodies and those with atmospheres.

中文翻译：

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类地系外行星 GL 486b 上没有厚厚的大气层


JWST 的主要科学目标是探测和表征围绕 M 矮星（M 地球）运行的类地行星的大气层。M-Earths上大气层的存在是高度不确定的，因为它们的宿主恒星长时间受到高X射线和紫外线照射，可能会完全消除它们的大气层。我们提出了 5 到 12 μm 之间的 M-Earth Gl 486b（也称为 GJ 486b）的两次 JWST 次食观测。我们将这些观测结果与对宿主恒星参数的精确分析相结合，得出了 T_p = 865 ± 14 K 的行星日边温度。我们将该温度与零反照率、零热再分布裸岩的最高预期温度进行了比较，得出的温度比为 R=Tp，daysideTp，max=0.97±0.01。该值与具有轻微非零反照率或稀薄气氛（<1% H₂O 或 <1 ppm CO₂）的无气物体一致。然而，它与类似地球或金星的大气层不一致，光谱没有显示出明显的发射或吸收特征。此外，我们的观测结果与之前的凌日观测所允许的富含水的大气情景不一致，并表明传输光谱反而是由恒星污染形成的。鉴于该系统在 ≥6.6 Gyr 生命周期中可能存在大气逸出的可能性，我们得出结论，这些观测结果可能最好用一颗没有空气的行星来解释。这一结果是迄今为止用 JWST 对类地系外行星热发射进行的最精确测量，它对无气天体和有大气天体之间的“宇宙海岸线”位置施加了强有力的限制。