Hubble Space Telescope observations of a 7 Earth-mass planet find an unusual water-rich world swathed in a thick, steamy atmosphere.

GJ 1214b was first discovered by the ground-based MEarth Project led by scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) in 2009. Orbiting a red dwarf star in the constellation of Ophiuchus just 40 light years from Earth, GJ 1214b circles its star at a distance of two million kilometres. With an average temperature of approximately 230 degrees Celsius, follow-up observations in 2010 suggested that the planet’s atmosphere was likely composed of water vapour.

But, says CfA's Zachory Berta, "GJ1214b probably does not have liquid water anywhere on it – it's just too hot for that."

Berta is part of a team who recently used the NASA/ESA Hubble Space Telescope to examine the composition of the planet’s atmosphere as it passed in front of its host star, allowing the star’s light to filter through its atmosphere to reveal its constituent parts.

“We’re using Hubble to measure the infrared colour of sunset on this world,” says Berta. “GJ 1214b is like no planet we know of. The Hubble measurements really tip the balance in favour of a steamy atmosphere.” 

With a diameter 2.7 times that of Earth and weighing in at seven Earth masses, GJ 1214b’s density is just two grams per cubic centimetre. For comparison, Earth’s average density is 5.5 grams per cubic centimetre, suggesting that GJ 1214b has significantly more water but much less rock than Earth.

"The outer part of the planet – the atmosphere – probably has a lot of steam in it," Berta tells Astronomy Now. "But as you go deeper into the planet, it's not like oceans we know of, it's water in very strange forms. It's still made out of molecules of water, but because of the very high pressure that you could reach deep inside this planet, the water could be in a solid form, even if it's hotter than the normal boiling point of water."

GJ 1214b most probably began life far from its star where ice was plentiful in the outer reaches of its solar system. At some point on its path towards the star it would have passed through the habitable zone where surface temperatures may have temporarily mirrored those on Earth.

"If liquid water had been stable on the planet's "surface" it would have evaporated very quickly," adds Berta. "More observations should give us a much clearer picture of what's going on in GJ1214b's atmosphere. Right now, what we have still only a rough sketch."

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