Astronomers have discovered what they say is the most Earth-like planet yet detected - a distant, rocky world that is similar in size to our planet and orbiting in a life-friendly zone around a distant star.
The find, announced on Thursday, excited planet hunters who have been scouring the Milky Way galaxy for years for potentially habitable spots outside our solar system. It is in what astronomers call the "Goldilocks zone" where it is not too hot or not too cold for life.
|NASA astronomers Elisa Quintana and Thomas Barclay
The planet was detected by NASA's orbiting Kepler telescope, which examines the heavens for subtle changes in brightness that indicate an orbiting planet is crossing in front of a star. From those changes, scientists can calculate a planet's size and make certain inferences about its makeup.
The newfound object, dubbed Kepler-186f, circles a red dwarf star 500 light-years from Earth in the constellation Cygnus. One light year is nearly 9.5 trillion kilometres.
The planet is about 10 percent larger than Earth and may very well have liquid water - a key ingredient for life - on its surface, scientists said, because it resides at the outer edge of the habitable temperature zone around its star - the sweet spot where lakes, rivers or oceans may exist without freezing solid or boiling away.
NASA launched its Kepler space telescope in 2009 to search about 150,000 target stars for signs of any planets passing by, or transiting, relative to the telescope's point of view. Kepler was sidelined by a positioning system failure last year.
Analysis of archived Kepler data continues. From Kepler's observational perch, a planet about the size and location of Earth orbiting a sun-like star would blot out only about 80 to 100 photons out of every million as it transits.
|Astronomy professor Karl Gebhardt
The pattern is repeated every 365 days and at least three transits would be needed to rule out other possibilities, so the search takes time.
Scientists don't know anything about the atmosphere of Kepler-186f, but it will be a target for future telescopes that can scan for telltale chemicals that may be linked to life.
So far, scientists have found nearly 1,800 planets beyond the solar system.
"The past year has seen a lot of progress in the search for Earth-like planets. Kepler-168f is significant because it is the first exoplanet that is the same temperature and is (almost) the same size as Earth," astronomer David Charbonneau, with the Harvard-Smithsonian Center for Astrophysics, wrote in an email to Reuters news agency.
"For me the impact is to prove that yes, such planets really do exist," Charbonneau said. "Now we can point to a star and say; "There lies an Earth-like planet.'"