More than 170 planets outside our own Solar System have been spotted in the past decade or so.

But almost all of them have been gaseous Jupiter-sized giants that race around their star at a close range. Their atmosphere would be too scorching and too dense to have liquid water, an essential ingredient for life as we know it.

In a paper published in the British journal Nature, a team led by Christophe Lovis from the Geneva Observatory in Switzerland located a smaller planetary system orbiting the star HD 69830.

The star is 41 light years from Earth in the Puppis constellation and is about four-fifths the mass of our Sun.

The three planets are big, being 10, 12 and 18 times larger in mass than the Earth. That makes them about the size of our Neptune, although a lot smaller than Jupiter, the biggest planet in our Solar System -- and they appear to be solid planets too, made of rock, not gas.

The two innermost planets are probably so close to HD 69830 that they would be blisteringly hot, but the outermost one lies in what planet experts call the "Goldilocks zone" -- a comfortable distance where water could exist as a liquid.

Another discovery is that HD 69830 also hosts, like our Sun, an asteroid belt, the rubble left over from the building of planets from dust and gassy debris that clump together through gravitational attraction.

No one is suggesting that these planets contain life or the conditions for it, and it would be ludicrous anyway to think of them as a potential home from home, given that our puny chemical rockets and their passengers could never reach there.

But it shows that with patient searching and the right tools, astronomers can uncover ever-smaller exoplanets in ever-wider orbits from their star, which may one day lead to finding copies of Earth.

"The planetary system around HD 69830 clearly represents a Rosetta stone in our understanding of how planets form," said co-author Michel Mayor, also of the Geneva Observatory, referring to the stone that opened the way to understanding the hieroglyphics of ancient Egypt.

"No doubt it will help us better understand the huge diversity we have observed since the first extrasolar planet was found 11 years ago."

The three planets were found thanks to a two-year monitoring of the star using a 3.6-metre telescope at the European Southern Observatory (ESO) in La Silla, Chile.

When a planet moves in front of a star, the light from the star "wobbles" in response to the gravitational pull. The "wobble" gives clues to the planet's size, density and orbit.

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