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.

See also Kepler 69c; a Super Venus rather than a Super Earth? Questioning the possible planet of Alpha Centauri, Two new more planets discovered in the Gliese 676 system, The peculiar planets of Kepler-36 and The atmosphere of GJ1214b.

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The atmosphere of GJ1214b.

The planet GJ1214b was discovered in December 2009 by the MEarth Project at Fred Lawrence Whipple Observatory in Arizona and described in a paper in the journal Nature. It orbits the star GJ1214A (A implies the first body in a system, B the second; stars are capitalized, planets are not), approximately 40 light years from Earth in the constellation of Ophiuchus. GJ1214A is a cool Red Dwarf star, only one fifth the size of our sun and 0.3% as bright. GJ1214b orbits this star every 38 hours, at 1.4% the distance at which our planet orbits the sun.

An artists impression of GJ1214b and it's star.

The planet was discovered by the tidal effect it exerts upon its star; a large planet orbiting close to a small star exerts a considerable gravitational pull, causing the star to wobble back and forth as the planet orbits it. This can be detected by sensitive telescopes using the Doppler effect, as the star moves towards us the light waves it emits are slightly compressed, making it appear slightly blueish (to very sensitive spectrometers, not human astronomers), as it moves away the light waves are expanded, making it appear slightly reddish.

Although GJ1214b was discovered by the Doppler method, it also transits (passes in front of) its star, enabling scientists to estimate both the the mass of the planet (by the Doppler method) and its radius (by the dimming it causes when it passes in front of its star) and therefore its density (by multiplying the two). This reveals a planet with some rather unusual properties. GJ1214b is 6.55 times as massive as the Earth, and has a radius 2.678 that of the Earth. This implies a planet with a much lower density than the Earth, which is surprising for a planet this size.

It was initially suggested that the planet could be made up largely of ice, which is far less dense than rock. But the surface temperature of GJ1214b is estimated to be between 399 and 555 K, or 126-282 °C. Another proposal is that GJ1214b might be an ocean planet entirely covered by an ocean hundreds of kilometers deep, with an icy core kept solid by the pressure of the water above it. Water can remain a liquid at temperatures above 100 °C if the pressure is high enough, but the although GJ1214b is big, its low density gives it an estimated gravity of 0.91 that of the Earth, so it is highly unlikely that water could remain a liquid at its surface. It has also been suggested that GJ1214b might be a relatively small (still bigger than the Earth) rocky planet with a thick, dense, atmosphere. But no mechanism has been suggested by which a small, hot, planet could retain such an atmosphere so close to a star, so if this is the case then the planet must be very young, unlikely orbiting a Red Dwarf star estimated to be 6 billion years old, or have moved recently into its current position, which is even harder to explain.

On 28 November 2011 a team led by Zachory Berta of the Harvard-Smithsonian Centre for Astrophysics published a paper on the arXiv online database at Cornell University Library, detailing the results of a spectroscopic study of the atmosphere of GJ1214b using the Wild Field Camera 3 on the Hubble Space Telescope. This found that the upper atmosphere of GJ1214b probably contains a thick layer of cloud, made up of some form of largish molecules, rather than a simple gas such as hydrogen; Berta suggest that this may be water (H₂O).

Any gas made up of molecules containing more than one type of atoms tends to have a greenhouse effect; it absorbs light (energy) at a variety of wavelengths, but emits it in the infra-red part of the spectrum. The obvious model for this in our solar system is Venus, which has an atmosphere containing large amounts of Carbon Dioxide (CO₂) and Sulphuric Acid (H₂SO₄), and a dramatic runaway greenhouse effect. This would suggest that GJ1214b is likely to be hotter, and less Earthlike, than has previously been suggested. The term 'super-Earth' has been used to describe planets of sizes intermediate between Earth and Neptune, but it would appear that in the case of GJ1214b it would appear that 'super-Venus' might be more appropriate.

See also A new study of the HAT-P-13 planetary system, The Keper-18 planetary system, Just how big is Iota Draconis b? and Exoplanets on Sciency Thoughts Youtube.