Astronomy in the News
Mercury Spring 2012 Table of Contents
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Hubble Reveals a New Type of Planet
Harvard-Smithsonian Center for Astrophysics
Our solar system contains three types of planets: rocky, terrestrial worlds, gas giants, and ice giants. Planets orbiting distant stars come in an even wider variety, including lava worlds and "hot Jupiters."
Observations by NASA's Hubble Space Telescope have added a new type of planet to the mix. By analyzing the previously discovered world GJ1214b, astronomer Zachory Berta (Harvard-Smithsonian CfA) and colleagues proved that it is a water world enshrouded by a thick, steamy atmosphere. "GJ1214b is like no planet we know of," said Berta. "A huge fraction of its mass is made up of water."
GJ1214b was discovered in 2009 by the ground-based MEarth (pronounced "mirth") Project. This super-Earth is about 2.7 times Earth's diameter and weighs almost 7 times as much. It orbits a red-dwarf star every 38 hours at a distance of 1.3 million miles, giving it an estimated temperature of 450°F.
In 2010, CfA scientist Jacob Bean and colleagues reported that they had measured the atmosphere of GJ1214b, finding it likely that the atmosphere was composed mainly of water. However, their observations could also be explained by the presence of a worldwide haze in GJ1214b's atmosphere.
Berta and his co-authors used Hubble's WFC3 instrument to study GJ1214b when it crossed in front of its host star. During such a transit, the star's light is filtered through the planet's atmosphere, giving clues to the mix of gases. The atmospheric model most consistent with HST data is a dense atmosphere of water vapor.
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