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The smallest-known planet outside our Solar System, Corot-7b, probably began as a Saturn-sized "gas giant" planet, say researchers. They say the planet, which orbits at one-sixtieth the distance from the Earth to the Sun, has had much of its mass boiled away by the star's heat. The team also suggests that, if its orbit is not exactly circular, Corot-7b is a hotbed of volcanic activity. The research was presented at the 215th American Astronomical Society meeting. More than 400 of these "exoplanets" have been discovered in recent years, with every indication that this number will rise dramatically as observation methods are refined. But for now those methods are by their nature best at spotting planets that are large or close to their parent stars. Mass shedding Corot-7b - spotted by the French space-based telescope Corot - has been hailed as the smallest exoplanet, just five times the mass of the Earth. Because it has a diameter only 70% larger than that of the Earth, it is presumed to be rocky. This stands in contrast to the majority of exoplanets that appear to be "gas giants", huge Jupiter-like planets made primarily of gas. The first exoplanets to be spotted were gas giants very near their host stars. "We call them 'hot Jupiters,' and they weren't what astronomers expected to find," said Brian Jackson, one of the researchers, from Nasa's Goddard Space Flight Center.
"Now, we're beginning to see Earth-sized objects in similar orbits. Could there be a connection?" The team suggest that there has been a long interplay in Corot-7b's history between the loss of mass and an effect known as tidal migration. Just as a star's gravity acts on a planet, a planet's gravity pulls up "tides" of matter on the surface of its host star. The mass of that tide, in turn, has an effect on the planet's orbit. In Corot-7b's case, the team reckons that as mass was burned off, the tides on its star changed, with the resulting change bringing the planet ever nearer the star. As it drew in closer, surface temperatures rose, driving off more mass and exacerbating the process. Using a computer model, the team estimated that Corot-7b once orbited at a distance 50% higher than its current one, and had about 100 times as much mass as Earth. Rory Barnes of the University of Washington also suggested that the planet may be volcanically active if the orbit of the planet is even slightly elliptical, rather than circular. The shifting gravity it would experience as it moved nearer to and farther from the star in its 20-hour orbit would stretch and squash the planet mercilessly. That would drive friction in the planet's interior and, as a result, volcanoes on its surface. Calculations at the University of Washington's Virtual Planet Laboratory showed that if the planet deviates from a circular orbit by just 250km - one ten-thousandth of the distance to its star - volcanic activity would be likely. An orbits that is so precisely circular is highly unlikely. "If conditions are what we speculate, then Corot-7b could have multiple volcanoes going off continuously and magma flowing all over the surface," Dr Barnes said.
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