Ground-based telescopes probe super-Earths

Credit: Paul A. Kempton

For many extrasolar planets — such as ‘hot Jupiters’ — measurements of the mass and radius are sufficient to allow the calculation of the planet’s density and infer their bulk composition. But for the rather smaller ‘super-Earths’, things get complicated. For instance, the mass and radius of the recently discovered transiting super-Earth GJ1214b are known, but three distinct models for the planet’s composition are consistent with the data. Additional information on the composition of the atmosphere is required if the uncertainty is to be reduced. Now a method that provides just that has been developed. A team using the Very Large Telescope in Chile’s Atacama Desert has obtained a transmission spectrum for GJ1214b between 780 nanometres and 1,000 nanometres that rules out cloud-free atmospheres composed mainly of hydrogen. If the planet’s atmosphere is hydrogen-dominated, it must be hazy — or there could be a dense water-vapour atmosphere. This new ground-based observing technique will make investigation of extrasolar planets much more practical, so expect more news of super-Earths in the months to come.

Credit: Paul A. Kempton

For many extrasolar planets — such as ‘hot Jupiters’ — measurements of the mass and radius are sufficient to allow the calculation of the planet’s density and infer their bulk composition. But for the rather smaller ‘super-Earths’, things get complicated. For instance, the mass and radius of the recently discovered transiting super-Earth GJ1214b are known, but three distinct models for the planet’s composition are consistent with the data. Additional information on the composition of the atmosphere is required if the uncertainty is to be reduced. Now a method that provides just that has been developed. A team using the Very Large Telescope in Chile’s Atacama Desert has obtained a transmission spectrum for GJ1214b between 780 nanometres and 1,000 nanometres that rules out cloud-free atmospheres composed mainly of hydrogen. If the planet’s atmosphere is hydrogen-dominated, it must be hazy — or there could be a dense water-vapour atmosphere. This new ground-based observing technique will make investigation of extrasolar planets much more practical, so expect more news of super-Earths in the months to come.