Wednesday 3 July 2013

The clear skies of GJ3470b.

GJ3470b is a roughly Uranus mass planet (14.1 times the Earth's mass, as opposed to Uranus's 14.5 Earth masses), 82 light years from Earth in the constellation of Cancer, orbiting a Red Dwarf star with roughly 54% of the Sun's mass. It orbits its star at approximately 3.6% of the distance at which the Earth orbits the Sun, with a year of only eighty hours, for which reason it has been described as a Hot Uranus.

An artists impression of GJ3470b. National Astronomical Observatory of Japan.

GJ3470b is a transiting exoplanet seen from Earth; it passes in front of its star from out viewpoint, potentially allowing us to calculate its density and study its atmosphere. In a paper published in the Astrophysical Journal on 29 May 2013, and on the online arXiv database at Cornell University Library on 30 May 2013, a team of scientists led by Akihiko Fukui of the Okayama Astrophysical Observatory discuss the results of a study of GJ3470b at optical and near-infrared wavelengths.

Fukui et al. conclude that GJ2470b has an approximate average density of 0.94 g cm¯¹, compared to 1.27 g cm¯¹ for Uranus or 5.515 g cm¯¹ for Earth (or 1.0 for water). This is an average figure, and includes the atmosphere, plus any liquids or solids on the planet, it does not imply the planet is the same throughout. Fukui et al. suggest that the best fit for this density would be a planet with a small rocky core, surrounded by a thick layer of water or ice and a hydrogen rich atmosphere. Furthermore they conclude the atmosphere can be no more than 10% of the total mass (any more and it would boil away) and that the water/ice layer has approximately three times the mass of the rocky part.

They were also able to measure some of the optical properties of the atmosphere. If GJ3470b had a thick cloudy atmosphere it would appear as a solid disk, with little variation in the light it blocked from its parent star, whereas it is possible to detect two dips in stellar output as it crosses the disc of the star, which Fukui et al. conclude represent partial blocking of the star's light by the atmosphere and complete blocking by the planetary core; suggesting that GJ3470b has a reasonably clear atmosphere, lacking a thick cloudy layer. From this they suggest future studies my be able to examine the molecular composition of this atmosphere.

Diagram showing how an observer can detect a non-opaque atmosphere on a transiting planet. National Astronomical Observatory of Japan.

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