At least five Neptune-like planets have been spotted orbiting the star HD 10180—and there's evidence of two more worlds, one farther from the star and another closer in.
If the latter observations can be confirmed, the innermost planet may hold the record for the lowest-mass extrasolar planet, or exoplanet, seen to date.
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The five established planets are between 12 and 25 times the mass of Earth and are all roughly around the sizes of Uranus or Neptune, meaning the newfound worlds are most likely icy gas giants.
The five planets are huddled close to their star, with orbits ranging from 0.06 to 1.4 times the distance between Earth and the sun. A sixth, yet to be confirmed planet that's 65 times the mass of Earth is thought to be orbiting farther beyond the group, at about 3.4 times the distance between Earth and the sun.
But it's the seventh planet that has astronomers most excited.
At only 1.4 times the mass of our home world, this exoplanet is what astronomers call a super-Earth. The planet hugs its star at just 0.02 times the distance between Earth and the sun, likely giving the world—if it exists—a hellish environment.
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The team is right to be guarded in announcing the discovery of a super-Earth, said planet hunter Jaymie Matthews, principal investigator for Canada's Microvariability and Oscillations of Stars, or MOST, space telescope.
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Lovis and colleagues "are being cautious, as they should be and as they must be after their public comments on Goldilocks world, which is a similarly weak signal," Matthews said.
"But if [HD 10180's super-Earth] is real, it's almost certainly a terrestrial world with a metal core and rocky mantle," based on the estimated size and mass, he said.
The implications for the formation of planetary systems are also significant.
Planets are thought to form from disks of debris that surround young stars. In this case, the planets probably didn't form where they are now, Lovis said, because there wouldn't have been enough solid material in the inner regions of HD 10180's protoplanetary disk.
"More likely, they originated from the outer, colder regions of the disk, where they could accumulate large quantities of ice and rocks to grow," and then the planets migrated inward, Lovis added.
The big question is how did seven planets migrate together in an orderly fashion without colliding with or ejecting each other?