— A new study examines the prevalence of planets around red dwarf stars, the most common type of star in the galaxy.
— 40 percent of these red dwarfs could harbor planets that maintain water in a liquid state on their surfaces.
— But there is no way of knowing from this estimation how many of these worlds will be rocky, and therefore genuinely habitable.
Take the most common type of star in the Milky Way — so-called red dwarf stars that are cooler, smaller and longer-lived than stars like the sun.
Then, survey a sampling for orbiting planets and extrapolate the results. What do you get?
A stunning claim that 40 percent of our galaxy's 160 billion red dwarf stars have plus-sized Earths orbiting the right distance for liquid water to exist on their surfaces, a condition believed to be necessary for life.
If this finding is correct, that would mean the Milky Way is home to tens of billions of planets in habitable zones, concludes a team of scientists using an Earth-based telescope to look for planets beyond the solar system.
The effort is complementary to studies by NASA's Kepler space telescope, which hunts for extrasolar planets around sun-like stars.
About 80 percent of the stars in the Milky Way are red dwarfs, which, on average, are about one-third smaller and 4,000 degrees Fahrenheit cooler than the sun.
Kepler lead scientist William Borucki, with NASA's Ames Research Center in Mountain View, California, said he's not surprised by the finding of the European team, which uses a light-splitting spectrograph called HARPS on a telescope at the La Silla Observatory in Chile to look for planets beyond the solar system.
But claiming that red dwarfs' planets are rocky worlds goes too far, Borucki told Discovery News.
"I am astounded that they're saying they are rocky planets. I don't see any reason to assume they're rocky planets," Borucki said.
Limits of the technology used by the HARPS team, which looks for slight wobbles in starlight caused by an orbiting planet's gravity, make assessments of a planet's density difficult, if not impossible, to determine. The Kepler team, which finds planets as they pass across their parent star's face relative to the telescope's line of sight, likewise is limited by its technique.
"Each technique has some strengths and some real weaknesses. None of them is perfect. None of them give you all the answers you'd really like to have," Borucki said.
"If you put the two (techniques) together, then you can get size (of a planet) and the volume, and you get the density. If you know the density, you've got some idea of whether its rocky or not rocky," he added.
"They don't even have the mass, much less the size," Borucki said. "To say it's rocky I think is a real stretch."
The European team did determine that gas giant planets like Saturn and Jupiter are relatively rare around red dwarf stars, and that super-Earths — planets a few times the diameter of Earth — are common.
Since the bulk of the stars in the Milky Way are red dwarfs, "one cannot avoid studying them to understand planet formation or to evaluate the habitability potential of our galaxy," HARPS researcher Xavier Bonfils, with France's Observatory of Sciences of the Universe of Grenoble, wrote in an email to Discovery News.
Bonfils and colleagues hope to refine their observations with a new spectrograph that analyzes infrared light, which is where red dwarf stars shine most of their light. The astronomers also hope to catch planets in transit.
"With a habitable planet transiting a bright red dwarf, it will be possible to analyze the atmospheric composition of the planet," Bonfils said.
Also on tap: a new generation of telescopes, including NASA's James Webb Space Telescope, which could find chemical fingerprints of oxygen, carbon dioxide, water and other molecules in the atmospheres of extrasolar planets. Those measurements would be key in the search for life beyond Earth.
Bonfils' research will be published in the journal Astronomy & Astrophysics.