《Nature》報(bào)道，在太陽(yáng)系外的行星中發(fā)現(xiàn)二氧化碳，而二氧化碳被認(rèn)為是4個(gè)生命標(biāo)志之一，其他三個(gè)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(yuǎn)的行星上發(fā)現(xiàn)二氧化碳

———在“熱木星”的大氣層中檢測(cè)到可居住的空氣特征

凱薩琳·桑德森

二氧化碳，被認(rèn)為是一顆行星可能適合生命生存的標(biāo)志信號(hào)，最近在一顆距離地球63光年的行星的巨大環(huán)繞大氣層中被檢測(cè)出來(lái)。

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

HD 189733b 是一個(gè)熾熱的木星 —— 一顆和太陽(yáng)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(guó)倫敦大學(xué)的喬煩娜·迪奈蒂和他的同事曾試圖利用“二級(jí)轉(zhuǎn)變”的方法來(lái)測(cè)量這顆行星白天反射出來(lái)的光譜。

這個(gè)方法包括首先記錄下這個(gè)行星和它的恒星的光譜，然后在行星在恒星背面時(shí)單獨(dú)測(cè)量恒星的光譜。兩次所測(cè)光譜的有差異的部分就是直接來(lái)之于這顆行星的光譜。迪奈蒂利用裝備在哈勃望遠(yuǎn)鏡上的近紅外攝影及多目標(biāo)光譜儀 (NICMOS) 進(jìn)行這項(xiàng)研究。

一個(gè)光譜經(jīng)過(guò)吸收和散發(fā)進(jìn)行分解，再和理論模型進(jìn)行比較就可以揭露隱藏的信息。這些信息揭示了這個(gè)行星的大氣中含有二氧化碳。

迪奈蒂說(shuō)：“這是個(gè)令人興奮的結(jié)果，這是行星上發(fā)現(xiàn)的第一個(gè)近紅外光譜。即便是從技術(shù)角度上看這也是無(wú)可挑剔的結(jié)果。”

偉大的發(fā)現(xiàn)

“二氧化碳是可居住環(huán)境的四個(gè)生命標(biāo)志之一，如果沒(méi)有就是不能居住的星球。”華盛頓卡耐基學(xué)院地磁研究部的艾倫·波斯如是說(shuō)，但是他并沒(méi)有參與這項(xiàng)工作。另外三個(gè)生命標(biāo)志是水、甲烷和氧氣，然而現(xiàn)在只有氧氣還未在太陽(yáng)系之外的行星中被發(fā)現(xiàn)。波斯說(shuō)：“他們確實(shí)證實(shí)了這個(gè)行星中有二氧化碳。”

近紅外攝影及多目標(biāo)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(yuǎn)鏡研究這顆行星可以看到的是光譜中的不同區(qū)域比如說(shuō)中紅外線，二氧化碳就不能被發(fā)現(xiàn)了。不同的儀器可以探測(cè)到大氣層中不同層次的成分，因此最終的化學(xué)成分圖要待到他們收集了所有的信息才能出來(lái)。