We’ve seen helium baked off a rocky exoplanet’s atmosphere

我们观测到一颗岩石系外行星的大气层中正在流失氦气

Most of the gas in the Universe is a mixture of hydrogen and helium. It’s thought that the initial atmospheres of most planets also start out that way. However, over billions of years, as planets evolve, the composition of their atmospheres may shift. Hydrogen can react with other chemicals, and both it and helium can be lost to space. 宇宙中大部分气体都是氢和氦的混合物。人们认为，大多数行星的初始大气层也是如此。然而，随着行星在数十亿年间的演化，其大气成分可能会发生变化。氢气可以与其他化学物质发生反应，而氢和氦都有可能流失到太空中。

Venus, Earth, and Mars are thought to have second atmospheres, with their original hydrogen/helium envelopes having been lost and/or transformed. The dynamics of loss are complicated. Lighter elements are lost more easily, but hydrogen can be protected by being incorporated into molecules like methane and ammonia. The gravity of the body can help retain some molecules, and a magnetic field can limit radiation’s ability to blast material out of the atmosphere. Proximity to a star will matter too, both because of the radiation it produces and because it can heat the atmosphere and expand it to where gravity’s influence is less substantial. 金星、地球和火星被认为拥有“次生大气层”，它们原始的氢/氦包层已经流失或发生了转化。流失的动力学过程非常复杂。较轻的元素更容易流失，但氢可以通过结合成甲烷和氨等分子而得到保护。天体的引力有助于留住部分分子，而磁场则能限制辐射将物质从大气层中剥离的能力。行星与恒星的距离也很重要，这不仅是因为恒星产生的辐射，还因为辐射会加热大气层并使其膨胀，从而削弱引力的束缚作用。

Given all these complications, it can be difficult to know what to expect to find on exoplanets. But a study in Wednesday’s issue of Nature describes observations of helium being lost from the atmosphere of an exoplanet orbiting the star LHS 1140, about 50 light-years away. Based on the rate at which the helium is being lost, we can infer something about the remaining atmosphere. 考虑到所有这些复杂因素，很难预知在系外行星上会发现什么。但周三出版的《自然》杂志上的一项研究描述了对一颗系外行星大气层中氦气流失的观测，该行星围绕约 50 光年外的 LHS 1140 恒星运行。根据氦气的流失速率，我们可以推断出其剩余大气层的一些情况。

Maybe an atmosphere?

可能存在大气层？

LHS 1140a is a red dwarf star with two known planets orbiting it. One of them, LHS 1140c, is close to the star, completing an orbit in a bit under four days and receiving about five times more radiation from its host star than the Earth receives from the Sun. Also, a second planet, LHS 1140b, is considerably farther out. It takes nearly 25 days to complete an orbit. That places it significantly closer to its host star than Mercury is to the Sun. Because LHS 1140a is such a dim star, this means it receives less than half as much light from its star as the Earth does. If all of that light ended up heating the planet, it would be warm enough to have liquid water on its surface. LHS 1140a 是一颗红矮星，已知有两颗行星围绕其运行。其中一颗 LHS 1140c 距离恒星很近，公转周期不到四天，从主星接收到的辐射量约为地球从太阳接收到的五倍。另一颗行星 LHS 1140b 则远得多，公转周期接近 25 天。这使得它比水星距离太阳更近。由于 LHS 1140a 是一颗非常暗淡的恒星，这意味着它从恒星接收到的光照不到地球接收到的光照的一半。如果这些光能全部转化为行星的热量，其表面温度将足以维持液态水的存在。

We’ve tracked LHS 1140b through its gravitational influence on its host star and by watching it transit across the front of the star from Earth’s perspective. This data indicates it’s about five-and-a-half Earth masses and has a radius that’s 1.7 times Earth’s. That’s consistent with a roughly Earth-like composition, with a lot of rocky material and either a significant amount of water and/or atmosphere (with the exact amount depending on how much iron). 我们通过 LHS 1140b 对主星的引力影响，以及从地球视角观测其凌日现象，追踪到了这颗行星。数据显示，它的质量约为地球的 5.5 倍，半径是地球的 1.7 倍。这与大致类似地球的成分相符，即含有大量的岩石物质，以及相当数量的水和/或大气层（具体含量取决于铁的含量）。

We’re unsure if LHS 1140b has an atmosphere, much less know anything about its composition. We know that the system appears to be at least 3 billion years old, which has given the atmosphere ample time to evolve. Red dwarfs are also prone to outbursts where they emit lots of energetic radiation, and the planets have had lots of time to experience those, as well. That was the backdrop against which a team of US-based researchers searched for helium as LHS 1140b orbited its host star. 我们不确定 LHS 1140b 是否拥有大气层，更不用说了解其成分了。我们知道该系统至少有 30 亿年的历史，这给了大气层充足的演化时间。红矮星也容易发生爆发，释放出大量高能辐射，而这些行星也有足够长的时间来经历这些过程。这就是美国研究团队在 LHS 1140b 绕主星运行时搜寻氦气的背景。

Coming and going

来来去去

Imaging exoplanet atmospheres is difficult because the amount of the host star’s light that goes through them is tiny relative to the amount that comes to Earth directly from the star. Looking at a red dwarf improves matters, since there’s less light overall, and planets are relatively larger compared to these small stars. So, a red dwarf with two known planets made for an appealing target. 对系外行星大气层进行成像非常困难，因为穿过大气层的恒星光线相对于直接到达地球的恒星光线来说微乎其微。观测红矮星可以改善这一情况，因为红矮星的总光量较少，且行星相对于这些小型恒星来说相对较大。因此，一颗拥有两颗已知行星的红矮星成为了一个理想的观测目标。

To image it, the researchers used near-infrared imaging hardware attached to a telescope at the Las Campanas Observatory in the Atacama Desert. The images included before, during, and after LHS 1140b and LHS 1140c were undergoing a transit, as well as images taken when they were elsewhere in their orbit. Helium was not apparent during LHS 1140c’s transit but showed up both before and after the transit of LHS 1140b. 为了进行成像，研究人员使用了安装在阿塔卡马沙漠拉斯坎帕纳斯天文台望远镜上的近红外成像设备。图像涵盖了 LHS 1140b 和 LHS 1140c 凌日过程的前、中、后阶段，以及它们在轨道其他位置时的图像。在 LHS 1140c 凌日期间没有发现明显的氦气，但在 LHS 1140b 凌日的前后都观测到了氦气。

The helium signal extended well beyond the planet’s radius, which the researchers suggest is a sign that LHS 1140b has both a leading and a trailing tail of helium. (While a tail drifting off behind the orbit of the planet makes intuitive sense, it’s possible that magnetic interactions and the stellar wind have been observed driving the creation of leading tails in other systems.) 氦气信号延伸到了行星半径之外很远的地方，研究人员认为这表明 LHS 1140b 同时拥有前导尾和拖尾。（虽然在行星轨道后方漂移的尾巴在直觉上说得通，但在其他系统中，磁相互作用和恒星风也被观察到会驱动前导尾的形成。）

The researchers interpret the tails as a sign that helium is currently being driven out of the atmosphere by the high-energy radiation the star produces. They confirmed this is possible using observations of the star made with the X-ray imaging satellite XMM-Newton. They estimate that the current escape of helium amounts to about 100,000 kilograms a second, relatively close to a rate that would have eliminated an atmosphere that was initially 1.5 percent of the total mass of the planet. As red dwarfs are even more active in their early years, it’s likely that the loss rate was even higher at some points in the past. That said, helium couldn’t be detected when they repeated the observations a year later (this doesn’t mean it wasn’t there; it was just below the detection limit). So, there’s lots of variability in the atmosphere loss, which may explain why we’re still seeing it. 研究人员将这些尾迹解释为氦气正被恒星产生的高能辐射从大气层中驱逐出来的迹象。他们利用 X 射线成像卫星 XMM-牛顿对该恒星的观测证实了这种可能性。他们估计，目前氦气的流失量约为每秒 10 万公斤，这一速率足以在过去消除掉占行星总质量 1.5% 的初始大气层。由于红矮星在早期更为活跃，过去某些时期的流失率可能更高。话虽如此，当他们一年后重复观测时，却未能检测到氦气（这并不意味着它不存在，只是低于检测限）。因此，大气流失存在很大的变异性，这或许可以解释为什么我们现在还能观测到它。

The leftovers

剩余物

The fact that helium is being lost at all actually tells us something about the composition of the atmosphere. If there were still lots of hydrogen present, it would be absorbing a lot of the radiation and acting as a shield that protects the helium. The fact that LHS 1140b is losing this much helium suggests there is little to no hydrogen left in the atmosphere. At the same time, the rate of helium loss provides some indication of the amount of energy available to liberate other atoms. The researchers suggest that nothing with an atomic mass above nine would be able to escape the atmosphere. This means that even atomic versions of oxygen and nitrogen would be retained. Any molecules, including ammonia, methane, a… 氦气正在流失这一事实实际上揭示了大气层的成分。如果大气中仍存在大量氢气，它会吸收大量辐射并起到保护氦气的屏障作用。LHS 1140b 正在流失如此多的氦气，这表明大气中几乎没有剩余氢气。同时，氦气的流失速率也反映了可用于释放其他原子的能量大小。研究人员认为，原子质量大于 9 的物质将无法逃逸出大气层。这意味着即使是氧和氮的原子形式也会被保留下来。任何分子，包括氨、甲烷……