JWST maps the weather on a hot gas giant 700 light-years away
JWST maps the weather on a hot gas giant 700 light-years away
韦伯望远镜绘制出 700 光年外一颗热气态巨行星的天气图
WASP-94A b is a hot, tidally locked gas giant orbiting close to one of the stars in a binary system roughly 690 light-years away from Earth. In a new Science study, scientists led by Sagnick Mukherjee, an astrophysicist at Johns Hopkins University, used the James Webb Space Telescope to learn what the weather looks like out there. WASP-94A b 是一颗炽热且被潮汐锁定的气态巨行星，它围绕着约 690 光年外一个双星系统中的一颗恒星运行。在《科学》杂志发表的一项新研究中，由约翰霍普金斯大学天体物理学家 Sagnick Mukherjee 领导的科学家团队利用詹姆斯·韦伯空间望远镜（JWST）探究了该行星的天气状况。
Tidal locking means that you no longer have day- and night-side temperature differences sweeping across the planet. “We wanted to understand the atmospheres of such planets,” Mukherjee says. “Are they static or dynamic? Do they have winds? Do they have clouds?” His team found that, on WASP-94A b, it’s cloudy in the morning, but the skies are clear in the evening. The fact that we didn’t know this already means we might have gotten the chemistry of this and many other exoplanets surprisingly wrong. 潮汐锁定意味着行星表面不再有昼夜温差的交替。Mukherjee 说：“我们想了解这类行星的大气层。它们是静态的还是动态的？它们有风吗？有云吗？”他的团队发现，在 WASP-94A b 上，早晨多云，而傍晚天空晴朗。我们此前对此一无所知，这意味着我们对这颗行星以及许多其他系外行星的化学成分可能存在严重的误判。
Averaged atmospheres
被平均化的大气层
WASP-94A b has a mass slightly below half of Jupiter but has a diameter that’s over 70 percent wider. “This means the planet has low density, and its atmosphere extends further out into space, which makes it easier to observe,” Mukherjee explains. When astronomers study atmospheres like this, they usually rely on transmission spectroscopy. By analyzing the spectrum of light filtering through the planet’s atmosphere as it crosses in front of its star, they can figure out its chemical composition. WASP-94A b 的质量略低于木星的一半，但直径却比木星大 70% 以上。Mukherjee 解释说：“这意味着该行星密度较低，其大气层向太空延伸得更远，这使得观测变得更容易。”当科学家研究此类大气层时，通常依赖透射光谱法。通过分析行星凌日时穿过其大气层的星光光谱，他们可以推断出其化学成分。
The problem with this approach is that the light filtering through the entire circumference of the planet’s silhouette was averaged out, as though its atmosphere was one homogenous ball of gas. For tidally locked planets, this was a massive oversimplification. On tidally locked worlds, there are massive temperature swings between day and night sides, which usually lead to differences in atmospheric density between the day side and the night side. 这种方法的问题在于，穿过行星轮廓整个圆周的光线被“平均化”了，就好像其大气层是一个均匀的气体球。对于潮汐锁定的行星来说，这是一种极大的简化。在潮汐锁定的世界里，昼夜两侧存在巨大的温差，这通常会导致昼夜两侧大气密度的差异。
These differences, combined with the Coriolis effect that stems from the planet’s slow rotation, cause a phenomenon called equatorial super-rotation. This is where winds on the equator blow eastward faster than the planet is spinning. Circulation models predicted this is exactly what’s happening on WASP-94B a. The leading edge of the planet’s disk, called the morning limb, is the region where the local atmosphere is rotating out of the colder night side and into the hot day side. The trailing edge at the evening limb is where the heated daytime gases are crossing over into the dark side. To catch this process in motion, Mukherjee and his colleagues employed a technique called limb-resolved spectroscopy. 这些差异，加上行星缓慢自转产生的科里奥利力，导致了一种被称为“赤道超旋转”的现象。在这种现象中，赤道上的风向东吹的速度比行星自转的速度还要快。环流模型预测，WASP-94A b 上正是这种情况。行星圆盘的前缘被称为“晨昏线（晨侧）”，是局部大气从寒冷的夜侧旋转进入炎热昼侧的区域。后缘的“昏侧”则是受热的白天气体跨越进入黑夜一侧的地方。为了捕捉这一动态过程，Mukherjee 和他的同事们采用了一种称为“边缘分辨光谱法”的技术。
Slicing transits
切片凌日观测
Because it takes a little bit of time for the planet to fully cross the star’s edge during the beginning and end of the transit, the telescope sees the leading morning limb block the starlight slightly before the trailing evening limb does. Using JWST’s Near Infrared Imager and Slitless Spectrograph (NIRISS), the team measured the light curves as WASP-94A b transited and split the signal. This way, they managed to extract two separate chemical transmission spectra for the exoplanet: one for its morning, and one for its evening limb. 由于行星在凌日开始和结束时完全穿过恒星边缘需要一点时间，望远镜会观察到前方的晨侧边缘比后方的昏侧边缘稍早遮挡星光。利用韦伯望远镜的近红外成像仪和无缝隙光谱仪（NIRISS），研究团队测量了 WASP-94A b 凌日时的光变曲线并拆分了信号。通过这种方式，他们成功提取了该系外行星的两组独立化学透射光谱：一组对应晨侧，另一组对应昏侧。
And there was quite a difference between the two. The morning limb’s spectrum was just a sloped line, rising at shorter wavelengths, which indicated high-altitude aerosols blocking the light from deeper in the atmosphere. “You would see a lot of dust and cloud particles at very high altitudes,” Mukherjee says. “Going deeper, the clouds likely clear up, and you would probably find water vapor and these kinds of gases.” 两者之间存在显著差异。晨侧的光谱只是一条斜线，在较短波长处上升，这表明高空的气溶胶阻挡了来自大气层深处的光线。Mukherjee 说：“你会看到高空中有大量的尘埃和云颗粒。深入大气层后，云层可能会消散，你可能会发现水蒸气和类似的气体。”
On the evening limb, the spectrum showed no substantial evidence of aerosols and revealed spikes of gaseous water vapor. “This would be a different view where you do not encounter many clouds through your journey, but what you see is just gas—water vapor mostly and other gases, maybe like carbon dioxide,” Mukherjee suggests. By feeding the JWST data into computer models, the team could also predict what the weather engine on WASP-94 b looks like in motion. 在昏侧，光谱没有显示出气溶胶的实质性证据，却揭示了气态水蒸气的峰值。Mukherjee 认为：“这是一种不同的景象，你在观测过程中不会遇到太多云层，看到的只是气体——主要是水蒸气和其他气体，可能还有二氧化碳。”通过将韦伯望远镜的数据输入计算机模型，研究团队还能够预测 WASP-94 b 的天气引擎是如何运作的。
Equatorial winds
赤道风
The average temperature on WASP-94A b exceeds 1,500 Kelvin, and Mukherjee’s team confirmed the evening limb is around 450 Kelvin hotter than the morning limb—hot enough to evaporate potential aerosol materials like iron or magnesium silicate. This temperature difference dictates the weather dynamics on the planet. On the permanent night side, gases in the atmosphere condense into droplets due to lower temperature, forming clouds. “These cloud particles are then dragged by the equatorial wind towards the morning side,” Mukherjee says. WASP-94A b 的平均温度超过 1500 开尔文，Mukherjee 的团队证实，昏侧比晨侧高出约 450 开尔文——这足以蒸发铁或硅酸镁等潜在的气溶胶物质。这种温差决定了该行星的天气动力学。在永久的夜侧，大气中的气体因温度较低而凝结成液滴，形成云层。Mukherjee 说：“这些云颗粒随后被赤道风拖向晨侧。”
As the clouds are pushed into the heat of the day side, most of these droplets evaporate. By the time the winds reach the evening limb again, the clouds are almost completely gone, leaving the skies clear. Based on this day-side/night-side aerosol distribution, the team determined WASP-94 b has actual clouds rather than hazes. The latter are basically photochemical smog created when intense radiation breaks the molecules down. Because hazes are produced by ultraviolet light, they should preferentially appear on the planet’s permanent day side. Global jet streams would then blow them into the evening limb, making the sunset hazy and the morning relatively clear—the exact reverse of what showed up in the data. 当云层被推入炎热的昼侧时，大部分液滴会蒸发。当风再次到达昏侧时，云层几乎完全消失，天空变得晴朗。基于这种昼夜侧的气溶胶分布，研究团队确定 WASP-94 b 拥有的是真正的云，而不是霾。后者本质上是强辐射分解分子时产生的光化学烟雾。由于霾是由紫外线产生的，它们应该优先出现在行星的永久昼侧。全球喷流会将它们吹向昏侧，使日落时分变得朦胧，而早晨相对晴朗——这与数据中显示的结果恰好相反。
The team even managed to calculate how the atmosphere keeps the clouds aloft. The equatorial wind is apparently strong enough to push the heavy mineral droplets through the night side faster than gravity can pull them down. Finally, the researchers ran an experiment where they took their precise JWST data and reanalyzed it without splitting it into two to resolve the limbs. “This had a huge effect on our understanding of the composition of this planet,” Mukherjee says. The results the researchers got when they averaged the atmosphere in a traditional model turned out a bit alarming for exoplanet science in general. 研究团队甚至计算出了大气层如何让云层保持在高空。显然，赤道风的强度足以推动沉重的矿物液滴穿过夜侧，其速度超过了重力将其拉下的速度。最后，研究人员进行了一项实验，他们将精确的韦伯望远镜数据在不进行边缘拆分的情况下重新分析。Mukherjee 说：“这对我们理解这颗行星的成分产生了巨大的影响。”研究人员在传统模型中对大气进行平均化处理后得到的结果，对于整个系外行星科学来说，显得有些令人担忧。