Three super-Earth planets found in habitable zone of nearby star
Gliese 667C might be the dimmest of a stellar threesome, but it’s got something its two brighter companions don’t: a trio of super-Earths circling around the star’s skinny habitable zone, where the planets could potentially be capable of supporting life.
For astronomers, the record-breaking trio are the crown jewels of a planetary system containing as many as seven possible planets (though researchers say the evidence is currently strongest for just five of them). The three lie inside the habitable zone, a Goldilocks region where the temperature is neither too hot nor too cold to support liquid water -- a necessary ingredient for life.
“It is quite remarkable,” said Mikko Tuomi, an astronomer at Britain’s University of Hertfordshire and coauthor of the study set to be published in the journal Astronomy & Astrophysics.
“This suggests that there might in fact be more habitable-zone planets in the universe than there are stars,” Tuomi said. “This means that we will have to revise the estimated occurrence rates of such planets considerably in the near future -- we might have been underestimating those.”
The system lies just 22 light-years away, in the constellation Scorpius -- a relative stone’s throw compared with the distant stars typically studied with the planet-hunting Kepler space telescope, which was hobbled last month.
The researchers combined data from several powerful telescopes around the world to gather information on the planetary system. Unlike Kepler, which watches for a dip in light as a crossing planet blocks a tiny spot on its star, the researchers relied on a technique known as radial velocity.
As a planet revolves around its star, the star is tugged back and forth, causing it to wobble. When the star moves toward us, its light waves get squeezed into a smaller area, making them bluer. When the star moves away, that light waves stretch out, making them longer, or redder. It’s the same principle behind the Doppler effect, and scientists use it to determine a planet’s mass: the bigger the color shift, the larger the planet that’s tugging on the star.
Two hotter planets reside too close to Gliese 667C’s rays to support liquid water; two cooler planets lie too far away. Because this red dwarf star is just a third of our Sun’s mass and very dim, Gliese 667C’s habitable zone lies very close in -- its inner rim is a tenth of the distance to Earth, and its outer rim still lies more than 5 million miles closer to the Sun than the closest point in Mercury’s orbit.
Our sun’s habitable zone, by contrast, lies much farther out -- Venus and Mars mark its inner and outer edges -- because our Sun is much brighter than Gliese 667C.
Gliese 667C’s super-Earths are thought to be no more than 10 Earth masses or roughly half the size of Neptune, Tuomi said -- making them good candidates for being rocky planets. It will take more data for the planets to be confirmed.
And who knows, Tuomi said, it’s possible researchers will continue to find more low-mass planets outside the habitable zone.
“There is no magic in it,” he said. “The more data we have, the more we will know.”