Sunday 4 December 2011

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.