A ‘super-earth’ with a thick atmosphere possibly made of water vapour has been detected orbiting a red dwarf star 42 light years away. It is the first rocky exoplanet to be found to possess an atmosphere, and it is also only the second super-earth to have had both its mass and radius accurately determined.

The planet, known as GJ 1214b, was discovered transiting its star (i.e. its silhouette periodically passes in front of its star, blocking some of the starlight so the star seems fainter) by the MEarth project (pronounced ‘mirth’), which uses eight off-the-shelf 400mm (16-inch) Ritchey–Chrétien telescopes on top of Mount Hopkins in Arizona to stare at 2,000 red dwarfs looking for signs of planets.

“Since we found the new super-earth using a small ground-based telescope, this means that anyone else with a similar telescope and a good CCD camera can detect it too,” says David Charbonneau of the Harvard–Smithsonian Center for Astrophysics, who led the research which is published in this week’s issue of the journal Nature.

Super-earths are terrestrial planets with a mass somewhere between that of our own planet and icy giants like Uranus. From the size and period of its transit, astronomers deduced that GJ 1214b is 34,441.7 kilometres across, which is 2.7 times the diameter of Earth, and orbits its star every 38 hours at a distance of only two million kilometres (Earth is about 149.6 million kilometres from the Sun). Despite its host star being a cool red dwarf with a temperature of just 2,700 degrees Celsius, at such proximity GJ 1214b will still roast under a surface temperature of 200 degrees Celsius.

To obtain the planet’s mass the astronomers then utilised the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph on the 3.6-metre telescope at the European Southern Observatory to measure the gravitational pull the planet has on the star, and hence how massive it must be. They found that this planet was six times more massive than Earth, and hence gravity on this world would be eight times that of Earth. Based on its mass and radius, the astronomers reckon that it must be primarily made from water-ice, with some rocky material such as silicates and iron thrown into the mix. However, this would is no balmy ocean world – the heat would vapourise any surface water, leading to the creation of a thick, streamy atmosphere. Indeed, the observed measurement of the planet’s radius is at odds with what theoretical models predict it should be – during the transits the planet was measured to be 400 kilometres wider than it should. The only explanation is an atmosphere 200 kilometres thick that blankets the planet. Underneath the clouds of water vapour, the powerful gravity would crush the water inside the planet into a weird form known as ‘ice seven’, which is crystalline in nature and can exist at pressures greater than 20,000 times Earth’s surface pressure.

The only previous planet to have had its mass and radius accurately measured was CoRoT-7b, discovered in a transit by the French CoRoT probe and its mass confirmed by HARPS. It’s a rocky planet with a diameter of just 22,900 kilometres, 1.8 times that of Earth, and a mass five times greater than Earth, orbiting just 2.57 million kilometres from its star; however, there is no evidence of a thick atmosphere like GJ 1214b.

Over the next few years, NASA’s Kepler spacecraft will observe the transits of thousands of planets, including a hundred or so small planets. By following up with HARPS, the ranks of solid planets with known diameters, masses and densities should soon swell substantially.