Tens of billions of rocky planets in the Milky Way are in potentially habitable zones with liquid water to support life, according to a multi-year study by astronomers at the European Southern Observatory (ESO), published in the journal Astronomy & Astrophysics on Wednesday.

The European work adds to a wealth of research on life-sustaining planets, particularly NASA's Kepler mission.

ESO researchers say about 40 percent of red dwarf stars – the most common type in the Milky Way – have at least one so-called orbiting "super-Earth." These super-Earths, or planets with masses between one and 10 times of Earth, are at just the right distance from the stars to allow water to flow on their surface.

So far, scientists have confirmed the existence of just 763 exoplanets, or planets outside our own eight-planet solar system.

HARPS technique

Because red dwarf stars are much cooler than the Sun, any planets with liquid water would need to be orbiting much closer to the star than the Earth is to the Sun. That, however, could be troublesome to support life.

"Red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation," Stéphane Udry, a member of the ESO team, said in a statement. "That may make life there less likely."

The team used the HARPS spectrograph on its own 3.6-meter telescope in Chile. The HARPS technique uses an indirect method of detection that infers the existence of orbiting plants from the way their gravity makes a parent star appear to move across the sky.

By comparison, since 2009, NASA's Kepler uses an onboard telescope that continuously and simultaneously monitors the brightness of more than 100,000 stars over the course of three and a half years. Its mission is to find Earth-like planets in other parts of the galaxy.

Last month, the Kepler team published a catalogue that lists a staggering 2,321 candidate planets. The list has grown since May 2009 when the space-based telescope first began watching stars for the shadow of planets passing over their faces. So far, however, only 69 of them have been considered confirmed as planets.

Mixed reactions

Reaction from the research community to the HARPS findings is mixed.

"The real news is the discovery of a few more super-Earths by the HARPS red dwarf program," Frédéric Pont, a professor of astrophysics at the University of Exeter, wrote in an e-mail to DW. "The rest is hype. We know very well from the Kepler mission that there are billions of super-Earths around the red dwarfs in the galaxy."

Mark Wyatt, a researcher in the Institute of Astronomy at the University of Cambridge, begs to differ.

"The HARPS discovery complements the Kepler findings, as the two techniques are sensitive to different types of planets," Wyatt wrote in an e-mail to DW.

"Kepler measures the radii of planets around distant stars in planetary systems that are aligned exactly edge-on, whereas HARPS measures the masses of planets around the closest stars to the Sun (in systems that don't have to be exactly edge-on). By combining the information from both instruments, we will better understand what the planetary systems around these low mass stars are like, but each study is also valuable in its own right."

Author: John Blau
Editor: Cyrus Farivar