Helium escaping from hot gas giants Many gas giant exoplanets orbit so close to their host star that they are heated to high temperatures, causing atmospheric gases to escape. Gas giant atmospheres are mostly hydrogen and helium, which are difficult to observe. Two papers have now observed escaping helium in the near-infrared (see the Perspective by Brogi). Allart et al. observed helium in a Neptune-mass exoplanet and performed detailed simulations of its atmosphere, which put constraints on the escape rate. Nortmann et al. found that helium is escaping a Saturn-mass planet, trailing behind it in its orbit. They combined this with observations of several other exoplanets to show that atmospheres are being lost more quickly by exoplanets that are more strongly heated. Science, this issue p. 1384, p. 1388; see also p. 1360 Helium is observed in the atmosphere of a warm Neptune-mass exoplanet, constraining the atmospheric loss rate. Stellar heating causes atmospheres of close-in exoplanets to expand and escape. These extended atmospheres are difficult to observe because their main spectral signature—neutral hydrogen at ultraviolet wavelengths—is strongly absorbed by interstellar medium. We report the detection of the near-infrared triplet of neutral helium in the transiting warm Neptune-mass exoplanet HAT-P-11b by using ground-based, high-resolution observations. The helium feature is repeatable over two independent transits, with an average absorption depth of 1.08 ± 0.05%. Interpreting absorption spectra with three-dimensional simulations of the planet’s upper atmosphere suggests that it extends beyond 5 planetary radii, with a large-scale height and a helium mass loss rate of ≲3 × 105 grams per second. A net blue-shift of the absorption might be explained by high-altitude winds flowing at 3 kilometers per second from day to night-side.

中文翻译：

---

从温暖的海王星质量系外行星的扩展大气中光谱解析的氦吸收

氦气从热气态巨行星中逸出 许多气态巨行星系外行星的轨道距离它们的宿主恒星如此之近，以至于它们被加热到高温，从而导致大气气体逸出。气态巨行星大气主要是氢气和氦气，很难观测到。现在有两篇论文观察到了近红外中逸出的氦（见 Brogi 的观点）。阿拉特等人。在海王星质量的系外行星中观察到氦气，并对其大气进行了详细的模拟，这对逃逸率施加了限制。诺特曼等人。发现氦正在逃离土星质量的行星，在其轨道上落后于它。他们将此与对其他几颗系外行星的观察相结合，表明大气被更强烈加热的系外行星以更快的速度消失。科学，这个问题 p。1384 页。第1388章 另见第。1360 在一颗温暖的海王星质量系外行星的大气中观察到氦，限制了大气损失率。恒星加热导致附近系外行星的大气膨胀和逃逸。这些扩展的大气层很难观察到，因为它们的主要光谱特征——紫外线波长的中性氢——被星际介质强烈吸收。我们报告了通过使用基于地面的高分辨率观测在过境的温暖海王星质量系外行星 HAT-P-11b 中检测到中性氦的近红外三重态。氦气特征在两个独立的传输过程中是可重复的，平均吸收深度为 1.08 ± 0.05%。通过对行星高层大气的三维模拟来解释吸收光谱表明它延伸到了 5 个行星半径之外，具有大尺度高度和氦质量损失率≲3×105克/秒。吸收的净蓝移可能是由高空风从白天到夜晚以每秒 3 公里的速度流动来解释的。