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An artist's impression shows a super-Earth discovered by HARPS. Researcher estimate that about 40 per cent of the habitable zones of red dwarf stars in the Milky Way contain a super-Earth. (M. Kornmesser/ESO)

Tens of billions of rocky planets in the Milky Way are the right temperature to support liquid water, a new study estimates.

In fact, within 30 light years of our own solar system, there are likely about 100 planets orbiting in the water-friendly "habitable zones" of their stars, according to a news release Wednesday from the European Southern Observatory.

The study to be published in the journal Astronomy & Astrophysics involved using an instrument called the HARPS planet-finder to search around red dwarfs in our galaxy, which make up about four out of every five of the Milky Way's stars. That means our galaxy contains about 160 billion of the long-lived stars, which are fainter and cooler than the sun.

HARPS looked specifically for super-Earths, rocky planets that are more massive than the Earth, but less than 10 times more massive. No such planets exist in our own solar system, but they seem to be common elsewhere.

Field guide to planets

  • Rocky planets are relatively small and have a solid surface. In our solar system, such planets include Earth itself, along with Mercury, Venus and Mars.
  • Gas giants are typically much bigger – at least 10 times bigger – than rocky planets. They have an extremely thick atmosphere of hydrogen and helium and no solid surface to speak of. In our solar system, the gas giants include Jupiter and Saturn.
  • Ice giants are a type of gas giant with a thinner atmosphere of hydrogen and helium surrounding a molten ice interior that may contain water, ammonia and methane. Examples include Neptune and Uranus.
  • Hot jupiters are gas giants that orbit extremely close to their star, giving them surface temperatures close to 1,000 C. They don't exist in our solar system, but have been detected around other stars.
  • Waterworlds are a newly discovered type of planet with atmospheres extremely high in water vapour. High temperatures and pressures turn the water into states of matter that don't exist on Earth, such as hot ice or superfluid water.

"Our new observations with HARPS mean that about 40 per cent of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet," Xavier Bonfils, the French researcher who led the study, said in a statement. Bonfils is based at the Observatoire des Sciences de l'Univers in Grenoble.

While a planet that can support liquid water could also potentially support life as we know it, living organisms may not be that likely on a super-Earth in the habitable zone of a red dwarf, cautioned Stéphane Udry, a member of the HARPS team who is based at the Geneva Observatory.

Because red dwarfs are smaller and cooler than the sun, their habitable zones are much closer to the star than the sun's, resulting in some potential hazards.

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

HARPS looks for planets by precisely measuring the colour of a star. The gravity of a planet tugs on a star as it orbits, pulling it toward and away from a distant observer on Earth. That movement, in turn, slightly changes the colour of the star as seen from Earth due to the Doppler effect.

In this study, the researchers made their estimates from a study of 102 red dwarfs seen from the southern hemisphere over a six-year period. It looked at the number of super-Earths that had been found around them, how many were in the habitable zones of their stars, and estimated the fraction of planets that had been discovered around the stars. They used that to create a similar estimate for all stars in the galaxy.

A separate study using NASA's Kepler telescope estimated there are more planets than stars in our galaxy or at least 100 billion.