Sunday 23 June 2013

The atmosphere of GJ 1214b.

GJ 1214b is a 'Super-Earth' planet (a planet significantly larger than Earth but significantly smaller than Uranus or Neptune) orbiting a Red Dwarf star in the constellation of Ophiuchus, 42 light years from Earth, the discovery of which was announced in a paper in the journal Nature by a team of scientists led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics. It orbits the star at a distance of 0.0143 AU (1.43% of the distance at which the Earth orbits the Sun) with an orbital period of 38 hours. 

Since it passes in front of the star when seen from Earth, it is was possible for the discovering team to determine not just the mass of the planet (by the amount it caused the star to wobble) but also its diameter, enabling the mean density of the planet to be determined; 1870 kg mˉ³, compared to 5150 to kg mˉ³ for the Earth. Since a planet significantly larger than the Earth with a similar composition would be expected to be much denser (due to the higher gravity) it was clear that GJ 1214b was a new class of object. Based upon this the discoverers concluded that GJ 1214b was a water world; a small rocky world covered by an ocean hundreds of kilometers deep, with a thin hydrogen/helium atmosphere (water could exist on a planet this close to the star GJ 1214 because it is a lot cooler than our Sun).

Model of GJ 1214b as a waterworld. John Garrett.

However this is not the only possible interpretation of the density of GJ 1214b; a small, dense, rocky planet with an extensive atmosphere could have a similar overall density.

In a paper published on the online arXiv database at Cornell University Library on 23 May 2013, a team of scientists led by Ernst de Mooij of the Department of Astronomy and Astrophysics at the University of Toronto, describe the results of a spectographic study of the atmosphere using the Very Large Telescope in the Atacama Desert in Chile, the William Hershel Telescope on La Palma in the Canary Islands and the Isaac Newton Telescope at Herstmonceux in Sussex, England.

De Mooij et al. predicted the intensity of light that would be produced by the scattering of light from GJ 1214 through the atmosphere of GJ 1214b if the planet had a thin atmosphere with abundant water, a thick atmosphere dominated by hydrogen or a thick, cloudy, hydrogen dominated atmosphere and then plotted the intensity of light detected by these, and previous, observations against this.

Transmission spectrum of GJ1214b, including all available measurements from the literature. Overplotted are three models for the atmosphere of GJ1214b: a hydrogen dominated atmosphere with solar composition (green line), a hydrogen dominated atmosphere with clouds and low methane abundance (red line) and a water dominated atmosphere (blue line). The dashed curves at the bottom are the transmission curves for the different filters. De Mooij et al. (2012).

These results correlate best with the 'waterworld' model of GJ 1214b, which de Mooij et al. conclusede seems the most likely, given the available data.

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