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In the News this month: first evidence of a solid exoplanet

Artists impression of the exoplanet Corot-7b
Artists impression of the exoplanet Corot-7b CREDIT: ESO/L. Calcada
Using various techniques, astronomers have, over the last decade, discovered many hundreds of planets outside our own solar system. Most of these techniques are indirect because planets are much fainter than the stars they orbit, and so are very hard to detect directly. Because their effects are easier to spot, larger planets are easier to find, but smaller and smaller planets are being discovered as techniques and technology improve. One of the smallest exoplanets known to date is CoRoT-7b, a planet discovered by the CoRoT satellite in February 2009, orbiting an otherwise unremarkable 11th magnitude star catalogued as TYC 4799-1733-1, located almost 500 light years away in the constellation of Monoceros. Most of the known exoplanets are thought to be larger versions of Jupiter, likely to be large gas giants, but new observations of CoRoT-7b suggest that it is far more like our own Earth.

A team of astronomers, led by Didier Queloz at the Geneva Observatory in Switzerland, used the HARPS instrument on ESO's 3.6-metre telescope at the La Silla observatory in Chile, to observe the CoRoT-7 system and try and determine the mass of CoRoT-7b. HARPS, or the High Accuracy Radial velocity Planet Searcher, is a high resolution spectrograph which enables astronomers to measure the tiny changes in velocity of a star as it is gently tugged by the gravitational pull of it's orbiting planets. These velocity shifts are extremely small, so very accurate spectrographs are needed to see the effects.

In the case of CoRoT-7b, the planet is so close to its parent star that it completes one orbit every 20.4 hours, blocking out a tiny fraction of the stars' light for just one hour during each orbit. Because the planet is so small, the team had to obtain more than 70 hours of observations to see the tiny changes in the stars' spectrum that would tell them about the planet. What the team found was that CoRoT-7b is one of the lightest exoplanets known, with a mass of just 4.8 times that of the Earth, putting it in the category of so-called "super-Earths". Since the planet directly transits the star, passing directly between the star and us, astronomers have already been able to determine that the planet's radius is less than twice that of Earth. If you know both the mass and the radius of a planet, you can calculate its density. The team did this and found that CoRoT-7b has a density of 5.5 grams per cubic centimetre, very similar to the density of the Earth. This suggests that CoRoT-7b is a rocky planet, not a gas giant like Jupiter, and is likely to be composed mainly of silicates with a small iron core, the first time such a determination has been made for such a small exoplanet.

As well as determining the mass and density of CoRoT-7b, the team also discovered a new planet, CoRoT-7c, which is slightly larger with a mass of 8.4 times that of Earth. Unfortunately, this planet does not directly transit the star, so its radius, and hence density, cannot be determined. The research will be published in the journal Astronomy and Astrophysics on October 22nd.


Queloz, D., Bouchy, F., Moutou, C., Hatzes, A., Hebrard, G., Alonso, R., Auvergne, M., Baglin, A., Barbieri, M., Barge, P., Benz, W., Bordé, P., Deeg, H., Deleuil, M., Dvorak, R., Erikson, A., Ferraz Mello, S., & M. Fridlund et al., . (2009). The CoRoT-7 planetary system: two orbiting super-Earths Astronomy and Astrophysics DOI: 10.1051/0004-6361/200913096

Posted by Megan on Monday 05th Oct 2009 (10:21 UTC) | Add a comment | Permalink

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