Artist's impression of the molton exoplanet CoRoT-7b. Image: ESO/L. Calçada

What would the atmosphere of an Earth-like planet be like if it was close enough to its parent star to evaporate rock? Astronomers have attempted to answer this question by using computer models to vaporise the Earth.

In The Hitchhiker's Guide to the Galaxy, the Earth was vaporised by the Vogons to make way for a hyperspace bypass. But don't panic! Astronomers have less sadistic intentions as their simulations were run in order to create models that can be used to understand exoplanet composition.

Almost 800 exoplanets have been confirmed to date, and a subset of these have been dubbed 'super-Earths,' as they are usually several times more massive than the Earth. Some of these super-Earths orbit dangerously close to their parent star, where it is meltingly hot - literally. At temperatures between 270 and 1700 degrees Celsius, the crust and mantle of these super-hot super-Earths starts to melt and turn to vapour. The atmospheres produced from this vaporisation are difficult to understand, as there is nothing similar in our own Solar System.

It is possible to measure the average density of terrestrial exoplanets, but there are usually several different combinations of elements that agree with a particular density, resulting in what is known as a degeneracy. For example, exoplanet GJ 1214 b could either have a hydrogen and helium atmosphere, or be a water world with an atmosphere of steam. Spectra of the atmospheres of super-Earths could decipher which is the more likely scenario, but these spectra still need to be compared to a model atmosphere in order to fully comprehend the alien world.

Bruce Fegley at Washington University in St. Louis, along with Laura Schaefer and Katharina Lodders worked with Mark Marley at the NASA Ames Research Center to run the vaporisation simulations. They modelled two different hypothetical planets, one with a composition similar to Earth, and another known as bulk silicate Earth (BSE).

The BSE planet has a composition similar to the Earth's before the continental crust formed. The crust of the Earth has a large amount of granite, which needs water to form. While both models mainly have a composition of silicon and oxygen, known as felsic, the basaltic crust of the BSE planet is richer in iron and magnesium, known as mafic. The vaporisation of the two simulated planets took into account both temperature and pressure.

"The reason for doing this is to predict abundances of spectroscopically observable molecules," explains Fegley. "CoRoT-7b is anhydrous but other hot rocky exoplanets may have had water, like Earth, and thus we needed to model atmospheric chemistry on a hot, wet rocky exoplanet."

The astronomers were able to calculate which elements would be vapour at these hellish temperatures. Both simulations show that the atmospheres would be dominated by steam from vaporising water, along with carbon dioxide from vaporising carbonate rocks. However the BSE atmosphere would contain methane and ammonia below 730 degrees Celsius, and above this temperature, the atmosphere would be dominated by sulphur dioxide. At temperatures greater than 1430 degrees Celsius, both planets would have silicon monoxide present, which could result in the rock-forming elements condensing and falling as pebbles, bringing a new meaning to the term hailstones.

"We can break the degeneracy problem if both the temperature and pressure of the atmosphere are known," Fegley tells Astronomy Now. "Once we have the temperature and pressure of the [spectral] line-forming region, one can compare the predicted and observed gas abundances to distinguish between different compositions (e.g., water-rich vs. water-poor, felsic vs. mafic). But I think that at present the best that can be done is to distinguish between water-rich and water-poor."

The paper was published in the 10 August issue of The Astrophysical Journal, and can be found here: http://arxiv.org/abs/1108.4660