Discovered by NASA's Kepler space telescope, the planets orbit a sun-like star about 950 light years away
The smallest exoplanets yet found around a normal star span just 1.03 and 0.87 times the Earth's diameter. The worlds, which are probably rocky like Earth, are too close to their host star to harbour life as we know it, but if they formed farther out as is thought, they may once have been habitable.
Discovered by NASA's Kepler space telescope, the planets orbit a sun-like star about 950 light years away called Kepler-20. They smash the previous record for the smallest exoplanet around a living star, a planet 1.4 times as wide as Earth known as Kepler-10b.
"We've crossed the Earth-sized threshold," says Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The discovery comes just two weeks after the announcement of Kepler's first confirmed planet in the habitable zone around a star of the same type as the sun – though at 2.4 times Earth's width, that planet may be gassy like Neptune. The habitable zone is the region around a star where temperatures are right for liquid water to exist on an object's surface.
That find and the current one "are two big checkmarks" for the Kepler mission, says team member Sara Seager of the Massachusetts Institute of Technology in Cambridge.
Since the only planet known to host life is small and rocky rather than big and gaseous like Jupiter, exoplanet hunters have been searching for such worlds. Kepler's goal is to find true twins of Earth: Earth-sized planets in the habitable zone of their stars. "Now we just have to combine those" two recent finds to find a Goldilocks world like ours, says Seager.
Uncertain mass
It is not clear how massive the two planets are. Kepler finds its quarry by watching the minute dimming of stars as planets cross in front of, or transit, them. But other transiting objects – such as stars or star-like objects known as brown dwarfs – can also dim the target stars.
To distinguish true planets from impostors, the Kepler team uses other telescopes to look for signs that the host star is wobbling due to gravitational tugs from orbiting planets. This wobbling reveals the planets' masses.
But Kepler-20 e, the smaller of the two new planets, and its sibling Kepler-20 f, are too small for their tugs to make a noticeable difference – especially given that three larger planets also whip around the same star on tight orbits. That means e must weigh less than 3 Earth masses and f must be under 14 Earth masses. When theoretical models that include how the planets formed and evolved are taken into account, those upper limits drop to 1.67 and 3.04 Earth masses, respectively.
Those masses, combined with the planets' sizes, suggest the worlds are rocky, says Seager. Both probably have bulk compositions similar to Earth's, with an iron core and a silicate mantle.
But the lack of a measurement of the star's wobble due to these planets means the new finds cannot be officially confirmed as planets. Instead, Fressin and colleagues painstakingly "validated" the finds by modelling all other possible explanations, including brown dwarfs, on NASA's fastest supercomputer. They also checked that no nearby stars could be the culprit and double-checked the transit signal with the infrared Spitzer Space Telescope.
Discovered by NASA's Kepler space telescope, the planets orbit a sun-like star about 950 light years away called Kepler-20. They smash the previous record for the smallest exoplanet around a living star, a planet 1.4 times as wide as Earth known as Kepler-10b.
"We've crossed the Earth-sized threshold," says Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The discovery comes just two weeks after the announcement of Kepler's first confirmed planet in the habitable zone around a star of the same type as the sun – though at 2.4 times Earth's width, that planet may be gassy like Neptune. The habitable zone is the region around a star where temperatures are right for liquid water to exist on an object's surface.
That find and the current one "are two big checkmarks" for the Kepler mission, says team member Sara Seager of the Massachusetts Institute of Technology in Cambridge.
Since the only planet known to host life is small and rocky rather than big and gaseous like Jupiter, exoplanet hunters have been searching for such worlds. Kepler's goal is to find true twins of Earth: Earth-sized planets in the habitable zone of their stars. "Now we just have to combine those" two recent finds to find a Goldilocks world like ours, says Seager.
Uncertain mass
It is not clear how massive the two planets are. Kepler finds its quarry by watching the minute dimming of stars as planets cross in front of, or transit, them. But other transiting objects – such as stars or star-like objects known as brown dwarfs – can also dim the target stars.
To distinguish true planets from impostors, the Kepler team uses other telescopes to look for signs that the host star is wobbling due to gravitational tugs from orbiting planets. This wobbling reveals the planets' masses.
But Kepler-20 e, the smaller of the two new planets, and its sibling Kepler-20 f, are too small for their tugs to make a noticeable difference – especially given that three larger planets also whip around the same star on tight orbits. That means e must weigh less than 3 Earth masses and f must be under 14 Earth masses. When theoretical models that include how the planets formed and evolved are taken into account, those upper limits drop to 1.67 and 3.04 Earth masses, respectively.
Those masses, combined with the planets' sizes, suggest the worlds are rocky, says Seager. Both probably have bulk compositions similar to Earth's, with an iron core and a silicate mantle.
But the lack of a measurement of the star's wobble due to these planets means the new finds cannot be officially confirmed as planets. Instead, Fressin and colleagues painstakingly "validated" the finds by modelling all other possible explanations, including brown dwarfs, on NASA's fastest supercomputer. They also checked that no nearby stars could be the culprit and double-checked the transit signal with the infrared Spitzer Space Telescope.
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