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 Talk about a sauna. Data gathered by the Hubble Space Telescope suggest that a planet tightly circling a star 40 light-years from Earth is a water world swaddled in a thick, steamy atmosphere. Previous observations of GJ1214b (artist's concept shown), which was first discovered in 2009 and has an estimated surface temperature of about 230°C, couldn't eliminate the possibility that the planet's atmosphere was simply hazy rather than steamy. But the new observations, detailed online in The Astrophysical Journal, reveal that the atmosphere transmits light consistently across a broad range of wavelengths—which tips the balance strongly toward an atmosphere mostly composed of water vapor, the researchers say, because a hazy atmosphere would be more transparent to infrared light than to visible wavelengths. While our solar system contains rocky worlds such as Venus and Earth, gas giants such as Jupiter and Saturn, and icy orbs such as Pluto—and other solar systems sport "lava worlds" and "hot Jupiters"—GJ1214b is the first planet discovered to be composed largely of water rather than rock. Scientists estimate that GJ1214b is about 2.7 times Earth's diameter and weighs about seven times as much, giving the planet a density of about 2 grams per cubic centimeter compared with water's density of 1 gram per cubic centimeter and Earth's average density of 5.5 grams per cubic centimeter.

 講一講類似桑拿浴的情況。從哈勃太空望遠(yuǎn)鏡搜集來的資料顯示：一顆行星在近圍圍繞著一顆恒星旋轉(zhuǎn)，該恒星距離地球40光年，該行星是一顆水星球，裹著一層厚厚的充滿水汽的大氣。行星GJ1214b（見藝術(shù)家提供的概念圖）最初是2009年被發(fā)現(xiàn)的，據(jù)估計(jì)表面溫度大約為230°C。以前的觀察沒有排除這種可能性：這顆行星上的大氣只是霧氣而不是水蒸氣。但是，《天體物理學(xué)雜志》在線版上詳細(xì)闡述的新一輪觀察顯示：該行星大氣在寬泛的波長范圍內(nèi)都能夠一致地傳播光線，因而有力地傾向于其大氣主要是由水蒸氣構(gòu)成的。研究人員稱，原因是大氣中的霧氣對于紅外光比對于可見光更加透明。盡管我們太陽系包含巖石星球像金星和地球，也包含氣態(tài)巨星像木星和土星，還包含冰封的球體像冥王星，盡管別的恒星系統(tǒng)擁有“熔巖星球”和“熱木星”，但是像GJ1214b這樣主要由水而不是巖石構(gòu)成的行星還是第一次發(fā)現(xiàn)的。科學(xué)家們估計(jì)，GJ1214b大約是地球直徑的2.7倍，大約是地球質(zhì)量的7倍，其密度大約為每立方厘米2克。相比之下，水的密度為每立方厘米1克，地球的平均密度為每立方厘米5.5克。