Hot Earths have atmospheres of rock

Hot rocky exoplanets may have atmospheres of vapourised rock, says Brian Jackson of NASA’s Goddard Space Flight Centre, USA at the Exoclimes conference at the University of Exeter. He adds that these planets, which are likened to the planet Mercury, do not completely expose their rocky surface to the Sun thanks to the planet’s exosphere – the uppermost, thin layer of vaporised rocky material which is composed of elements from the surface of the exoplanet. It is these exospheres which provide clues to the planet’s composition.

An artist’s conception of CoRoT-7b. Image: ESO/L Calcada. “Exactly how you would go about searching for these atmospheres is by using the transit method,” says Jackson. The current detection biases lead to many of the currently known exoplanets being on very close orbits to their host stars. These exoplanets are therefore very hot. One exoplanet, CoRoT-7b, which was the first exoplanet to have its radius measured as slightly larger than our home planet, orbits a Sun-like star. “Rocky planets that orbit Sun-like stars get very hot, reaching temperatures of up to about 3000 Kelvin (2,730 degrees Celsius),” says Jackson. However, CoRoT-7b has left some scientists puzzled by the planet’s composition. “This planet could be made of solid material, but what type is unclear.”
“We are entering the era of hot rocky planets,” states Jackson, “and the majority of planets found that are rocky are going to be very hot, so it might be very hard for them to retain their atmosphere.” The heat from the host star of a rocky planet could possibly lead to a hot melted surface and therefore a deep magma ocean extending tens of kilometres down. “The exosphere can be quite dense, and if the surface is hot enough and if the rocky material is volatile enough you can produce an atmosphere that is comparable to Mars’ atmosphere, which is a few tens of a hundredths of a bar.”