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What may be the smallest extrasolar planet, measuring less than twice the size of Earth, has been discovered orbiting a sun-like star.
The world is far hotter than ours, however. And controversy over the size claim has heated up, too.
Astronomers used the COROT space telescope (a mission led by the French Space Agency, and also involving the European Space Agency and others) to detect the new planet as it transited its parent star, dimming the light from the star as it passed in front of it.
The host star is located 457 light-years from Earth, where one light-year is the distance light will travel in a year, or about 6 trillion miles (10 trillion kilometers).
"For the first time, we have unambiguously detected a planet that is 'rocky' in the same sense as our own Earth," said Malcolm Fridlund, ESA's COROT Project Scientist. "We now have to understand this object further to put it into context, and continue our search for smaller, more Earth-like objects with COROT."
He added, "This discovery is a very important step on the road to understanding the formation and evolution of our planet."
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Sara Seager, an astrophysicist at MIT who was not involved in the discovery said, "My first thought is that it's extremely exciting because we've been waiting to find a planet that we can really call rocky. I would just caution that more information, more data, is needed."
For instance, the discovery has yet to be published in a peer-reviewed journal, and not much information about the planet has been released by COROT scientists.
Seager says in order to confirm an exoplanet is rocky, scientists need to nail down its mass and radius (or the combination of size and density, or mass and density).
"It looks like the mass is not well-determined and so that's why they're saying they're not sure what the density is," Seager told SPACE.com. "They think it is terrestrial-like. It might have water ice, or it might have rocks, but it's certainly not a gas giant."
COROT scientists estimate the planet ranges from 5.7 to 11 Earth masses.
One big difference in the newfound planet compared to Earth: COROT-Exo-7b is located very close to its star, orbiting once every 20 hours.
Its temperature is so high, ranging from 1,832 to 2,732 degrees Fahrenheit (1,000 to 1,500 degrees Celsius) that the researchers say the exoplanet could be covered in lava or water vapor.
The density of the planet is still under investigation, though scientists say it may be rocky like Earth and covered in liquid lava.
COROT-Exo-7b may also belong to a class of planets that are thought to be made up of water and rock in almost equal amounts. Given the high temperatures measured, the planet would likely be a very hot and humid place.
"Finding such a small planet was not a complete surprise," said Daniel Rouan, researcher at the Observatoire de Paris Lesia, who coordinates the project with Alain Leger, from Institut d'Astrophysique Spatiale. "COROT-Exo-7b belongs to a class of objects whose existence had been predicted for some time."
Small and odd
Very few of the more than 300 exoplanets found so far have a mass comparable to that of Earth and the other terrestrial planets — Venus, Mars and Mercury. That's because terrestrial planets are extremely difficult to detect.
Of the Earth-like planets detected, this is the first one spotted using the so-called transit method, which can yield both the planet's mass and radius. Other methods just reveal the planet's mass, Seager said.
The newfound planet's size status is also questioned. When astronomers study planets, they're interested in both mass and diameter.
"The claim that it is the 'smallest exoplanet' found to date is not correct," said planet-formation theorist Alan Boss of the Carnegie Institution of Washington. "It is the smallest mass exoplanet found to date that transits, but other hot super-Earths have been found that do not transit but have lower masses."
Boss was not involved in the current discovery.
For instance, he adds Gliese 876 d has "a minimum mass of 5.9 Earth masses and a best estimate for the true mass of 7.5 Earth masses."
Most of the methods used so far are indirect and sensitive to the mass of the planet, which is why bigger worlds are easier to detect.
COROT can directly measure the size of a planet's surface, which is an advantage to astronomers. In addition, because the probe is in space, it has longer periods of uninterrupted observation than from the ground.
The internal structure of COROT-Exo-7b particularly puzzles scientists, as they are unsure whether it is an "ocean planet," a kind of planet whose existence has never been proved so far.
In theory, such planets would initially be covered partially in ice, and they would later drift toward their star, with the ice melting to cover it in liquid.