《Nature》報道，在太陽系外的行星中發現二氧化碳，而二氧化碳被認為是4個生命標志之一，其他三個是水、甲烷和氧氣

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.

在遙遠的行星上發現二氧化碳

———在“熱木星”的大氣層中檢測到可居住的空氣特征

凱薩琳·桑德森

二氧化碳，被認為是一顆行星可能適合生命生存的標志信號，最近在一顆距離地球63光年的行星的巨大環繞大氣層中被檢測出來。

盡管這顆行星是否有生命的存在還沒辦法知道，但是從該行星的大氣中可以發現二氧化碳帶給了人們去尋找大氣層更加接近地球的行星的希望，也給人類在太陽系外尋找生命跡象起到了支撐作用。

HD 189733b 是一個熾熱的木星 —— 一顆和太陽系中的木星極為相似的行星，但是它繞行的軌道更接近與它的恒星，因此比木星更為炎熱。英國倫敦大學的喬煩娜·迪奈蒂和他的同事曾試圖利用“二級轉變”的方法來測量這顆行星白天反射出來的光譜。

這個方法包括首先記錄下這個行星和它的恒星的光譜，然后在行星在恒星背面時單獨測量恒星的光譜。兩次所測光譜的有差異的部分就是直接來之于這顆行星的光譜。迪奈蒂利用裝備在哈勃望遠鏡上的近紅外攝影及多目標光譜儀 (NICMOS) 進行這項研究。

一個光譜經過吸收和散發進行分解，再和理論模型進行比較就可以揭露隱藏的信息。這些信息揭示了這個行星的大氣中含有二氧化碳。

迪奈蒂說：“這是個令人興奮的結果，這是行星上發現的第一個近紅外光譜。即便是從技術角度上看這也是無可挑剔的結果。”

偉大的發現

“二氧化碳是可居住環境的四個生命標志之一，如果沒有就是不能居住的星球。”華盛頓卡耐基學院地磁研究部的艾倫·波斯如是說，但是他并沒有參與這項工作。另外三個生命標志是水、甲烷和氧氣，然而現在只有氧氣還未在太陽系之外的行星中被發現。波斯說：“他們確實證實了這個行星中有二氧化碳。”

近紅外攝影及多目標光譜儀 (NICMOS) 可以觀察到光譜中的近紅外光線，然而先前利用斯皮策太空望遠鏡研究這顆行星可以看到的是光譜中的不同區域比如說中紅外線，二氧化碳就不能被發現了。不同的儀器可以探測到大氣層中不同層次的成分，因此最終的化學成分圖要待到他們收集了所有的信息才能出來。