The red dwarf star Gliese 581 is only 20 light years away from Earth, and a number of planets orbiting, including one in the middle of the star's habitable zone that is only three to four times the mass of Earth, with a diameter 1.2 to 1.4 times that of Earth.
Credit: Lynette Cook/NASA
SYDNEY: Planet Earth is not so special after all; there's one orbiting roughly every one in four Sun-like stars, according to a five-year astronomy study.
The study, published in the journal Science, used Hawaii's twin 10-metre Keck telescopes to scan 166 sun-like stars within 80 million light years, or about 757 trillion kilometres.
The team spotted 22 planets around 33 of these stars by looking for tiny 'wobbles' caused by the gravitational tug of the planets - called the Doppler or radial velocity method.
23 Earths for every 100 Suns
"Of about 100 typical Sun-like stars, one or two have planets the size of Jupiter, roughly six have a planet the size of Neptune, and about 12 have super-Earths between three and 10 Earth masses," said astronomer and lead author Andrew Howard, from the University of California at Berkeley.
"If we extrapolate down to Earth-size planets - between one-half and two times the mass of Earth - we predict that you'd find about 23 for every 100 stars."
Limitations in the technique mean astronomers can't yet see planets up to three times Jupiter's mass orbiting within one quarter of the distance of the Earth to the Sun (1 AU or almost 150 million kilometres), or smaller Earth-like planets much close in.
Gliese 581 g most similar to Earth
Planets three times Earth's mass are as small as current technologies can spot when seeking exoplanets, and is the size of Gliese 581 g, probably the most Earth-like candidate so far, 20.3 light-years away in the constellation Libra.
But if Howard and colleagues are right, many more Earth-like planets await discovery, with models predicting that more Earth-like planets exist than Jupiter-sized planets. So far, Jupiter analogues account for almost 400 of the roughly 500 exoplanets found so far.
Currently Earth-like planets are much harder to find.
"Glorious news"
"One of astronomy's goals is to find eta-Earth (ηEarth), the fraction of Sun-like stars that have an Earth," Howard says. "This is a first estimate, and the real number could be one in eight instead of one in four. But it's not one in 100, which is glorious news."
"What this means is that, as NASA develops new techniques over the next decade to find truly Earth-size planets, it won't have to look too far," he says.
Astronomer John Greenhill from the University of Tasmania says his own research agrees that Earth-mass planets are "very common".
Probably have life there
Greenhill's expertise is in the 'microlensing technique', which looks at the bending of light of a source star by an intervening planet-star system and is particularly suitable for finding small-mass planets.
He estimates between 32% and 100% of stars have planets two to 10 times the size of Earth in orbits ranging from 1¬-10 AU.
"That's one heck of a lot of Earth-mass planets," he says. "I don't care how small the probability of life is; some of them are bound to have water on them and will probably have life there."
