David A. Aguilar/Harvard-Smithsonian Center for Astrophysics

ScienceShot: Water World Is New Type of Planet

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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