Astronomers believe that Earth-like planets could be all around us, and some may even harbour life that would be much older and more evolved than on Earth.
Using publicly available data from NASA’s Kepler space telescope, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) have found that six percent of red dwarf stars have habitable, Earth-sized planets.
Since red dwarfs are the most common stars in our galaxy, the Milky Way, the closest Earth-like planet could be just 13 light years away, they said.
‘‘We thought we would have to search vast distances to find an Earth-like planet. Now we realise another Earth is probably in our own backyard, waiting to be spotted,’’ said Dr Courtney Dressing, one of the team member.
Red dwarf stars are smaller, cooler, and fainter than our Sun. An average red dwarf is only one-third as large and one-thousandth as bright as the Sun. From Earth, no red dwarf is visible to the naked eye.
Despite their dimness, these stars are good places to look for Earth-like planets. Red dwarfs make up three out of every four stars in our galaxy for a total of at least 75 billion.
The signal of a transiting planet is larger since the star itself is smaller, so an Earth-sized world blocks more of the star’s disk. And since a planet has to orbit a cool star closer in order to be in the habitable zone, it’s more likely to transit from our point of view.
Dressing culled the Kepler catalog of 158,000 target stars to identify all the red dwarfs. She then reanalyzed those stars to calculate more accurate sizes and temperatures. She found that almost all of those stars were smaller and cooler than previously thought.
Since the size of a transiting planet is determined relative to the star size, based on how much of the star’s disk the planet covers, shrinking the star shrinks the planet. And a cooler star will have a tighter habitable zone.
Dressing identified 95 planetary candidates orbiting red dwarf stars. This implied that at least 60 percent of such stars have planets smaller than Neptune. However, most weren’t quite the right size or temperature to be considered truly Earth-like.
Three planetary candidates were both warm and approximately Earth-sized. Statistically, this means that six percent of all red dwarf stars should have an Earth-like planet.
“We now know the rate of occurrence of habitable planets around the most common stars in our galaxy. That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought,” said co-author David Charbonneau (CfA).
Locating nearby, Earth-like worlds may require a dedicated small space telescope, or a large network of ground-based telescopes. Follow-up studies with instruments like the Giant Magellan Telescope and James Webb Space Telescope could tell us whether any warm, transiting planets have an atmosphere and further probe its chemistry.
Since red dwarf stars live much longer than Sun-like stars, this discovery raises the interesting possibility that life on such a planet would be much older and more evolved than life on Earth.
“We might find an Earth that’s 10 billion years old,” speculated Charbonneau.
Dressing presented her findings in a press conference at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.