Scientists say they have found a planet seven times more massive than Earth orbiting in a star's habitable zone 42 million light-years away. It could have seas, and perhaps just as important, it could have an Earth-like climate because it has a day-night cycle.
This undated artist rendering shows Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star. So far, astronomers have detected more than 840 planets orbiting other stars, a handful of those are super Earths that fall within their stars' habitable zones.
Courtesy of JPL-Caltech/Ames/NASA/AP
EnlargeAstronomers have uncovered evidence for a super Earth-size planet in the habitable zone of a star 42 light-years away in the southern-hemisphere constellation Pictor.
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If the data truly signal a planet, the object could host liquid water on its surface, the team estimates. Liquid water is seen as a key ingredient for organic life.
So far, astronomers have detected more than 840 planets orbiting other stars. A handful of those are super Earths that fall within their stars' habitable zones. But only two ? including this newly announced planet candidate ? are far enough away from their stars to allow for a day-night cycle. The others orbit so close to their stars that they've become tidally locked, presenting the same face to their stars as they swing about their orbits.
A day-night cycle on the new planet "improves its chances of hosting an Earth-like climate," the team wrote in the formal report of their discovery, which has been accepted for publication in the journal Astronomy and Astrophysics.
The host star, tagged as HD 40307, has about 77 percent of the sun's mass and is a sun-like 4.5 billion years old. Researchers previously had detected three super Earths orbiting the star. The potentially habitable planet is one of three additional planet-candidates the team is reporting around HD 40307.
It's a uniquely crafted system. Five of the six planets orbit within a scant 23 million miles of the star. HD 40307g, the outermost of the six, orbits another 33 million miles out. That brings all six orbits well within the radius of Mercury's orbit around the sun. Mercury is the innermost planet in our solar system, orbiting the sun at an average distance of 58 million miles ? close enough to broil the sun-facing hemisphere of this slowly turning planet to a toasty 801 degrees Fahrenheit.
But HD 40307g's host star is faint enough that the planet, with a year some 200 days long, falls well within the star's habitable zone.
The six planets exhibit minimum masses ranging from 3.5 Earth masses to 9.5 times the Earth's mass. The potentially habitable super Earth comes in at about seven Earth masses.
Based on calculations involving a similar, habitable-zone super Earth discovered by NASA's Kepler mission, Kepler 22b (which is also the other super Earth with a day-night cycle), the team posits that the planet could be a mini-Neptune, with a rocky core and thick atmosphere.
Other astronomers have been trying to confirm the existence of the initial three planets by hunting for the slight dimming a planet imparts to its star's light as it swings in front of the star during its orbit ? and event known as a "transit." But no one has spotted anything yet.
Given that it is easier to spot transits for close-in planets than for more-distant planets, the chances of catching a transit for HD 40307g as well would seem remote. Astronomers are interested in such detections because a transit would not only provide confirmation of the planet's existence, but also allow astronomers to infer a great deal about the planet, including its density.
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