Artist's impression of the trio of super-Earths discovered using the ESO's 3.6-m telescope at La Silla, Chile, after five years of monitoring. The three planets, having 4.2, 6.7, and 9.4 times the mass of the Earth, orbit the star HD 40307 with periods of 4.3, 9.6, and 20.4 days, respectively. (ESO)

European astronomers said Monday they have discovered five "super-Earths" — including three orbiting a single star — which they say suggests Earth-like planets could be very common outside our solar system.

The discoveries, revealed at a conference in France, were made using a spectrograph at the La Silla Observatory in Chile, which is run by the European Organization for Astronomical Research in the Southern Hemisphere, or ESO.

Planet hunter Michel Mayor, best known for discovering the first confirmed planet outside our solar system, said in a statement the finds suggest the search for Earth-like planets has just begun.

"Does every single star harbour planets and, if yes, how many? We may not yet know the answer but we are making huge progress towards it," said Mayor, the principal investigator at the ESO's Geneva Observatory.

The term Earth-like planet is a rather broad definition and includes planets with a mass below 30 Earth masses; in other words, planets smaller than gas giants like Jupiter or Saturn. But because they aren't as easy to spot as gas giants, even the largest of these planets evaded detection until recent years.

These planets are often discovered, but not seen directly, by monitoring the slight changes their gravitational pull have on the velocity of their stars. Occasionally a planet will actually pass in front of the Earth's view of the star and provide an even clearer signal of its presence. But in either case, the more frequently the planet orbits the star, the easier it is to spot.

The ESO said the recent discoveries bring to 45 the total number of Earth-like planets with an orbital period shorter than 50 days it has discovered, and suggests one in three stars similar to our sun may harbour such planets.

The star with the three Earth-like planets, called HD 40307, is slightly less massive than our sun and is located 42 light-years, or about 400 trillion kilometres, away from Earth.

The planets have masses 4.2, 6.7 and 9.4 times that of Earth, and orbit the star with periods of 4.3, 9.6, and 20.4 days, respectively.

The astronomers discovered the other two Earth-like planets in two separate star systems. One planet with 7.5 times the mass of the Earth, was discovered orbiting the star HD 181433, a star that is already known to host a Jupiter-like planet. The second system contains a planet with 22 times the mass of Earth.

More than 270 planets have been discovered outside our solar system, but as instruments become more precise, research is increasingly focusing on planets similar to Earth, particularly those at a distance similar to our own planet's distance from the sun.

Astronomers see this as the "sweet spot," since the temperature and conditions might allow for the presence of liquid water, a key ingredient for supporting life.