系外星球上的岩石雨（圖）

On Earth, strange things, including frogs and fish, sometimes fall from the sky, but on a distant extrasolar planet, the weather could be even weirder: When a front moves in, small rocks rain down on the surface, a new study suggests.

地球上存在奇怪的事情——有時包括青蛙和魚在內的動物從天而降。然而一項新研究表明：在一顆遙遠的系外行星上，天氣狀況更加不可思議——當鋒面到來時，小塊岩石就會降落到行星表面上。

The exoplanet, COROT-7b, was discovered in February by the COROT space telescope launched by the French and European space agencies. Last month it became the first planet outside our solar system to be confirmed as a rocky body — most other known exoplanets are gas giants.

2月份，法國和歐洲宇航局聯合發射的COROT太空望遠鏡發現了系外行星COROT-7b。上個月，該行星成為太陽系以外第一顆有待證實的岩石行星，多數已知的其他系外行星都是氣態的巨型行星。

The planet is nearly twice the size of Earth and about five times the mass of our world. Calculations have indicated it has a density about that of Earth"s, which means it is likely made up of silicate rocks, just as Earth"s crust is.

這顆行星的大小是地球的將近兩倍，質量大約是我們地球的5倍。計算顯示，該行星的密度大約跟地球相當，這意味著它可能是由硅酸鹽岩石構成的，恰似地殼中的成分。

The planet is likely much less hospitable to life though, as it is only about 1.6 million miles (2.6 million km) away from its parent star — 23 times closer than Mercury sits to the sun.

然而，這顆行星不大可能適於生命生存，因為它距其主恆星僅約160萬英里（260萬公里），比水星距太陽近23倍。

Locked and scorching

引力束縛與灼熱的溫度

Because the planet is so close to the star, it is gravitationally locked to it in the same way the Moon is locked to Earth. One side of the planet always faces its star, just as one side of the Moon always faces Earth.

由於這顆行星距其恆星太近，引力把它束縛在恆星周圍，就如同月球被束縛在地球周圍一樣。這顆行星總是由相同的一面朝向其主恆星，正如月球總是由相同的一面朝向地球一樣。

This star-facing side has a temperature of about 4,220 degrees Fahrenheit (2,326 degrees Celsius) — hot enough to vaporize rock. So unlike the much cooler Earth, COROT-7b has no volatile gases (carbon dioxide, water vapor, nitrogen) in its atmosphere. Instead its atmosphere consists of what might be called vaporized rock.

朝向恆星的那一面溫度大約為4,220華氏度（2,326攝氏度），足以將岩石蒸發掉。地球的溫度要低得多，因此COROT-7b跟地球不同，它的大氣中沒有漂浮的氣體（如：二氧化碳、水汽、氮等），其大氣是由可以稱之為「汽化的岩石」構成的。

"The only atmosphere this object has is produced from vapor arising from hot molten silicates in a lava lake or lava ocean," said Bruce Fegley Jr., of Washington University in St. Louis.

「這顆天體上唯一的一種大氣產生於熾熱的液態硅酸鹽散發出的蒸氣，這種硅酸鹽存在於熔岩湖或熔岩海里，」華盛頓大學聖路易斯分校的布魯斯·費格雷說。

Rocky weather forecast

岩石天氣預測

To find out what COROT-7b"s atmosphere might be like, Fegley and his colleagues modeled it. They found that COROT-7b"s atmosphere is made up of the ingredients of rocks and when "a front moves in," pebbles condense out of the air and rain into lakes of molten lava below.

為了搞清COROT-7b大氣的情況，費格雷及同事對此進行了模擬。他們發現，COROT-7b的大氣是由岩石成分構成的，當鋒面來臨時，小石子從空氣中冷凝出來，降落到下面的熔岩湖中。

"Sodium, potassium, silicon monoxide and then oxygen — either atomic or molecular oxygen — make up most of the atmosphere," Fegley said. But there are also smaller amounts of the other elements found in silicate rock, such as magnesium, aluminum, calcium and iron.

「鈉、鉀、一氧化硅和氧（原子氧或分子氧）構成了大氣的主要成分，」費格雷說。但是，在硅酸鹽岩石中還有少量的其他成分，如：鎂、鋁、鈣和鐵。

The rock rains form similarly to Earth"s watery weather: "As you go higher the atmosphere gets cooler and eventually you get saturated with different types of "rock" the way you get saturated with water in the atmosphere of Earth," Fegley explained. "But instead of a water cloud forming and then raining water droplets, you get a "rock cloud" forming and it starts raining out little pebbles of different types of rock."

岩石雨的形成類似於地球上的雨水天氣。「越是在高處，大氣的溫度就越低，最終大氣中的各種岩石處於飽和狀態，就跟地球大氣中的水分處於飽和狀態時一樣，」費格雷解釋說，「但是，那裡的大氣飽和時形成的不是水雲，因而降落的也不是雨水；那裡的大氣飽和之後形成的是『岩石雲』，然後開始降落各種不同的小石子。」

Menagerie of minerals

五花八門的礦物

The exoplanet"s atmosphere condenses out minerals such as enstatite, corundum, spinel, and wollastonite.

這顆系外行星的大氣冷凝後產生礦物，如：頑火輝石、剛玉、尖晶石和硅灰石。

Elemental sodium and potassium, which have very low boiling points in comparison with rocks, do not rain out but would instead stay in the atmosphere, where they would form high gas clouds buffeted by the stellar wind from COROT-7. These large clouds may be detectable by Earth-based telescopes. The sodium, for example, should glow in the orange part of the spectrum, like a giant but very faint sodium vapor streetlamp.

跟岩石相比，鉀和鈉兩種元素的沸點要低得多，因而不會以雨的形式降落下來。相反，兩種元素滯留在大氣中，形成高空氣雲，受到來自恆星COROT-7的星風的衝擊。這些大團的氣雲可以被地面望遠鏡探測到。例如，鈉在光譜中呈現出橙色光，就象一盞巨大而非常暗淡的鈉汽燈。

Observers have recently spotted sodium in the atmospheres of two other exoplanets.

最近，觀察人員在其他的兩顆系外行星上也發現了鈉。

The new modeling finding is detailed in the Oct. 1 issue of The Astrophysical Journal.

這次新模擬所發現的情況詳細地記錄在10月1日出版的《天體物理學雜誌》上。