《Nature》报道，在太阳系外的行星中发现二氧化碳，而二氧化碳被认为是4个生命标志之一，其他三个是水、甲烷和氧气

Carbon dioxide discovered on distant planet

Gassy signature of habitability spied in the atmosphere of a 'hot Jupiter'.

Katharine Sanderson

Carbon dioxide, one of the telltale signs that a planet may be able to support life, has been spotted in the atmosphere of a gas giant orbiting a star 63 light years from Earth.

Although there's no way that this particular planet could support life, being able to spot carbon dioxide in its atmosphere offers hope for probing the atmospheres of planets more like Earth — and so bolstering the search for life outside the Solar System.

HD 189733b is a hot Jupiter — a planet similar in mass to Jupiter in our Solar System, but one that orbits much closer to its star, and so is much hotter. Giovanna Tinetti from University College London, UK and her colleagues have managed to measure the spectrum of the light coming from the day side of the planet using a technique called 'secondary transit'.

This involves recording the light spectrum of the planet and its star, and then measuring the spectrum of the star alone while the planet is hidden behind it. The difference of the two spectra is the spectrum of the light coming directly from the planet. Tinetti used the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) onboard the Hubble Space Telescope.

What emerged was a spectrum that, after its absorption and emission part were disentangled and it was compared with theoretical models, revealed the presence of carbon dioxide in the planet's atmosphere.

"This is an exciting result," says Tinetti. "This is the first near-infrared spectrum of a planet. Even from a technical point of view it's a nice result."

Spectacular finding

"Carbon dioxide is one of the big four biomarkers for a habitable, if not inhabited planet," says Alan Boss from the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, who was not involved with the work. The other three are water, methane and oxygen – and now oxygen is the only one left to be observed in the atmosphere of a planet outside the Solar System. "They really have nailed it," Boss says of the result.

The NICMOS instrument looks at the near-infrared part of the spectrum whereas previous studies of this planet using the Spitzer Space Telescope have been looking at different areas of the spectrum where a carbon dioxide signature wouldn't show up, such as the mid-infrared region. These different instruments also probe different layers of the atmosphere so that ultimately a picture of the chemistry going on between them can be gleaned.

在遥远的行星上发现二氧化碳

———在“热木星”的大气层中检测到可居住的空气特征

凯萨琳·桑德森

二氧化碳，被认为是一颗行星可能适合生命生存的标志信号，最近在一颗距离地球63光年的行星的巨大环绕大气层中被检测出来。

尽管这颗行星是否有生命的存在还没办法知道，但是从该行星的大气中可以发现二氧化碳带给了人们去寻找大气层更加接近地球的行星的希望，也给人类在太阳系外寻找生命迹象起到了支撑作用。

HD 189733b 是一个炽热的木星 —— 一颗和太阳系中的木星极为相似的行星，但是它绕行的轨道更接近与它的恒星，因此比木星更为炎热。英国伦敦大学的乔烦娜·迪奈蒂和他的同事曾试图利用“二级转变”的方法来测量这颗行星白天反射出来的光谱。

这个方法包括首先记录下这个行星和它的恒星的光谱，然后在行星在恒星背面时单独测量恒星的光谱。两次所测光谱的有差异的部分就是直接来之于这颗行星的光谱。迪奈蒂利用装备在哈勃望远镜上的近红外摄影及多目标光谱仪 (NICMOS) 进行这项研究。

一个光谱经过吸收和散发进行分解，再和理论模型进行比较就可以揭露隐藏的信息。这些信息揭示了这个行星的大气中含有二氧化碳。

迪奈蒂说：“这是个令人兴奋的结果，这是行星上发现的第一个近红外光谱。即便是从技术角度上看这也是无可挑剔的结果。”

伟大的发现

“二氧化碳是可居住环境的四个生命标志之一，如果没有就是不能居住的星球。”华盛顿卡耐基学院地磁研究部的艾伦·波斯如是说，但是他并没有参与这项工作。另外三个生命标志是水、甲烷和氧气，然而现在只有氧气还未在太阳系之外的行星中被发现。波斯说：“他们确实证实了这个行星中有二氧化碳。”

近红外摄影及多目标光谱仪 (NICMOS) 可以观察到光谱中的近红外光线，然而先前利用斯皮策太空望远镜研究这颗行星可以看到的是光谱中的不同区域比如说中红外线，二氧化碳就不能被发现了。不同的仪器可以探测到大气层中不同层次的成分，因此最终的化学成分图要待到他们收集了所有的信息才能出来。