K2-25b is a Neptune-sized exoplanet (3.45R _⊕) that orbits its M4.5 host with a period of 3.48 days. Due to its membership in the Hyades Cluster, the system has a known age (727 75 Myr). K2-25b’s youth and its similarities with Gl 436b suggested that K2-25b could be undergoing strong atmospheric escape. We observed two transits of K2-25b at Lyα using HST/STIS in order to search for escaping neutral hydrogen. We were unable to detect an exospheric signature, but placed an upper limit of (R _p /R _⋆)∣ _{Ly α} < 0.56 at 95% confidence by fitting the light curve of the Lyα red wing, or < 1.20 in the blue wing. We reconstructed the intrinsic Lyα profile of K2-25 to determine its Lyα flux, and analyzed XMM-Newton observations to determined its X-ray flux. Based on the total X-ray and extreme ultraviolet irradiation of the planet (8763 1049 erg s⁻¹ cm⁻²), we estimated the maximum energy-limited mass-loss rate of K2-25b to be g s⁻¹ (0.56M _⊕ per 1 Gyr), five times larger than the similarly estimated mass-loss rate of Gl 436b (2.2 10¹⁰ g s⁻¹). The photoionization time is about 3 hr, significantly shorter than Gl 436b’s 14 hr. A nondetection of a Lyα transit could suggest K2-25b is not significantly losing its atmosphere, or factors of the system are resulting in the mass loss being unobservable (e.g., atmosphere composition or the system’s large high-energy flux). Further observations could provide more stringent constraints.

K2-25b 是一颗海王星大小的系外行星 (3.45 R _⊕ )，以 3.48 天的周期绕其 M4.5 宿主运行。由于它是 Hyades 星团的成员，该系统的年龄是已知的（727 75 Myr）。K2-25b 的年轻及其与 Gl 436b 的相似之处表明 K2-25b 可能正在经历强烈的大气逃逸。我们使用 HST/STIS观察到 K2-25b 在 Ly α 的两次凌日，以寻找逃逸的中性氢。我们无法检测到外层特征，但通过拟合 Ly α红翼的光变曲线或蓝翼中的 < 1.20，在 95% 的置信度下设置了 ( R _p / R _⋆ )∣ _{Ly α} < 0.56 的上限. 我们重建了内在的 Ly α的K2-25简档来确定其Ly的α通量，并分析XMM牛顿观测以确定其X射线通量。根据行星的总 X 射线和极紫外辐射 (8763 1049 erg s ^-1 cm ^-2 )，我们估计 K2-25b 的最大能量限制质量损失率为gs ^-1 (0.56 M _⊕每 1 Gyr)，比类似估计的 GL 436b 质量损失率 (2.2 10 ¹⁰ gs ^-1 )大五倍。光电离时间约为 3 小时，明显短于 GI 436b 的 14 小时。未检测到 Ly α凌日可能表明 K2-25b 没有显着失去其大气，或者系统的因素导致质量损失无法观察（例如，大气成分或系统的大高能通量）。进一步的观察可以提供更严格的限制。