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.
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