从四次JWST NIRISS/SOSS凌星观测中未在LHS 1140 b中检测到氦
No Helium Detected in LHS 1140 b from Four JWST NIRISS/SOSS Transits
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中文总结 AI 辅助
本研究通过四次JWST NIRISS/SOSS凌星观测，未在LHS 1140 b中检测到氦，拒绝了地面观测的最佳拟合模型，其大气性质仍待后续分析确认。
中文摘要 AI 辅助
为确定哪些低质量系外行星拥有大气层，LHS 1140 b仍是最有利的目标之一。其质量为5.6 M⊕、半径为1.7 R⊕，体积较大，轨道周期为24.7天，相对较长，这意味着它很可能拥有大气层；值得注意的是，近期的内部模型支持它要么是氢主导的“迷你海王星”，要么是“水世界”，而非真正的类地行星。另一种可能性是它拥有富氦大气层，这一假设得到了近期地面观测的支持，这些观测在凌星期间检测到了亚稳态氦三重线。这些观测表明，该行星高层大气可能存在当前的氦逃逸现象，但在后续的一次观测中未检测到该信号，说明逃逸具有时间变异性。在此，我们展示了2023年至2026年间用JWST NIRISS/SOSS对LHS 1140 b进行的四次观测，这些观测覆盖了亚稳态氦三重线。这些观测跨越了地面测量的时间节点，尽管没有一次与地面凌星观测同期，但四次观测对之前报道的氦吸收水平均具有敏感性。然而，我们在所有观测中均未检测到氦吸收。我们在每次观测中都以>3σ的置信度拒绝了地面观测的最佳拟合模型，且未发现质量损失随时间变化的明确趋势。我们的结果表明，之前报道的地面检测可能是虚假的，不过如果可检测到的氦吸收发生在<50%的凌星事件中，也不能排除变异性的可能。因此，在未来完成透射光谱和发射光谱分析之前，LHS 1140 b的性质仍然是个谜。
英文摘要
In the effort to determine which low-mass exoplanets have atmospheres, LHS 1140 b remains one of the most favorable targets. Its large size (5.6 $\rm M_{\oplus}$ and 1.7 $\rm R_{\oplus}$) and relatively long orbital period (24.7 days) imply an atmosphere may be likely, and notably, recent interior models favor either a hydrogen-dominated "mini-Neptune" or a "water world" over a true terrestrial planet. Another possibility is that it has a helium-rich atmosphere. This hypothesis is supported by recent ground-based observations that detected the metastable helium triplet during transit. These observations indicated there may be current helium escape from the planet's upper atmosphere, yet the signal was not detected during a subsequent observation, suggesting time-variable escape. Here we present four observations of LHS 1140 b with JWST NIRISS/SOSS, which covers the metastable helium triplet, obtained between 2023 and 2026. These observations span the epoch of the ground-based measurements, and although none were contemporaneous with the ground-based transits, all four are sensitive to helium absorption at the previously reported level. However, we detect no helium absorption in any visit. We reject the best-fit ground-based model at $>3σ$ in each visit, and find no clear trend in mass-loss with time. Our results suggest the reported ground-based detection may be spurious, although variability cannot be excluded if detectable helium absorption occurs in $\lesssim50\%$ of transits. The nature of LHS 1140 b thus remains a mystery until future transmission and emission analyses are complete.
