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Inhospitable land

The smallest planet discovered to date outside the Solar System has a rocky surface and extreme temperatures

ESO/L.CALCADANew world: artist’s impression of the planet Corot-7b, in red, and its starESO/L.CALCADA

Astronomers no longer have any doubts. There are other planets with rocky surfaces and dimensions very similar to those of the Earth outside our Solar System. The most robust evidence confirming this old suspicion was published on September 16 by an international team of researchers, which includes the Brazilian astronomer Sylvio Ferraz Mello, from the University of São Paulo (USP). Coordinated by the Swiss astronomer Didier Queloz from the Geneva Observatory, the group calculated the mass of a planet that tracks one of the stars in the Unicorn constellation, in the neighborhood of the Solar System.

This planet, which is 500 light-years away, is the smallest found to date outside the Solar System. Its diameter is 1.8 times greater than that of Earth and it has 5 times more mass, which puts it in the category that astronomers call super-Earths. It was discovered in February this year by Corot, a French-European-Brazilian satellite launched in 2006 with the purpose of identifying planets around other stars (exoplanets) and determining their size. The new planet has been called Corot-7b and was described in an article published in Astronomy and Astrophysics. Its mass was calculated from measurements taken by a piece of extremely high-resolution equipment attached to one of the telescopes of the European Southern Observatory (ESO) in the Atacama Desert in the north of Chile.

By knowing the mass and diameter of Corot-7b, the astronomers could see that its density is very close to that of the Earth: 5.5 grams per cubic centimeter – which means that a 1 cm square cube with the same composition as Earth would contain a mass of 5.5 grams. Based on this information the astronomers concluded that the planet, in the constellation of the Unicorn, can only be rocky and have a solid surface, like Mercury, Venus, Earth and Mars, the closest planets to the Sun. Planets the size of Jupiter, whose diameter is 11 times bigger than Earth’s and the biggest in the Solar System, are generally gaseous.

“It’s the first time that the density of a planet outside the Solar System has been calculated,” celebrates the astrophysicist Eduardo Janot Pacheco, from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) at USP, coordinator of Brazil’s participation in Corot. In April this year, the team of the Swiss astronomer Michel Mayor, which identified the first exoplanet in 1995 and has already discovered 150 others, had announced observation of the planet Gliese 581e, which has double the Earth’s mass. However, there are doubts about its structure. “We know neither the diameter nor the density of Gliese 581e, which may have a liquid surface,” says Pacheco.

The interest in detecting rocky planets around other suns is due to the expectation that they might harbor life. “We’re looking for planets on which life might develop,” says the coordinator of the Brazilian group.

Despite its rocky structure, Corot-7b is likely to be highly inhospitable and very different from Earth. It is very much closer to its star than Mercury (the first planet in our system) is to the Sun. In addition to being probably hotter and drier, it suffers from extreme temperatures. As it revolves at the same rate at which it completes a circuit around its sun, Corot-7b only exposes one of its surfaces to the light, where the temperature comes close to 1,000 degrees Celsius. On the dark side, however, temperatures are always below freezing. “Generally speaking, this planet is not suitable for life,” says Pacheco. “But there must be regions that have intermediate temperatures that might allow extremophile microorganisms to survive in conditions in which others cannot,” bets the researcher from the IAG, who believes that there is life at other points in the Universe, but not necessarily the same as forms that we already know. “There are billions and billions of stars. It would be very pretentious of us to think that life only exists here,” he says.

Since Michel Mayor announced the discovery of the first extra-solar planet 14 years ago, orbiting the star Pegasus 51, a further 373 have been identified. Almost all of them are gaseous giants and similar to Jupiter and Uranus. In addition to Corot-7b, there are other candidates for rocky planets, such as Corot-7c, whose mass is eight times greater than the Earth’s, and which is a companion of 7b.

One of the reasons why almost nothing is known about rocky planets is that it is difficult to observe them. Even the Corot satellite, which was developed to detect the subtle reduction in luminosity (eclipse) that the planets cause when they pass in front of the stars, depends on luck. In only 1% of the cases is the planet’s orbit at a plane that favors observation.

In little more than two years of activity, Corot analyzed the light of 60,000 stars and identified at least 5 other exoplanets, in addition to Corot-7b and 7c. “There are still a dozen or so candidates under analysis,” commented Michel Auvergne, an astronomer from the Paris Observatory and the chief researcher on the Corot project, in early September, during a visit to São Paulo. He came to discuss the mission data with the Brazilian team and brought some good news with him: the satellite, that was to have been deactivated in early 2010, will remain active for a further three years and functioning even better. “We’ve improved the program that eliminates the noise from the collected data,” says the engineer Vanderlei Cunha Parro, from the Mauá Institute of Technology (IMT). “This will increase our chances of finding other planets.”

The projects
Corot time analysis center (Cat-Corot) (nº 06/50341-5); Type Regular Line of Aid for Research Projects; Coordinator Eduardo Janot Pacheco – IAG-USP; Investment R$ 37,608.43 (FAPESP).
2. Characterization study of the spreading function of the optical system of the image-capturing channel for investigating exoplanets of the Corot satellite (nº 06/03008-9); Type Regular Line of Aid for Research Projects; Coordinator Vanderlei Cunha Parro – IMT; Investment R$ 30,799.15 (FAPESP).

Scientific article
Queloz, D. et al. The Corot-7 planetary system: two orbiting super-Earths. Astronomy and Astrophysics. 2009. In press.