Using a powerful telescope on a mountaintop in Chile, they detected three planets around a nearby star.
They say one has a rocky surface like Earth and another lies in a zone that could support life.
The planets – around a star called HD 69830 – are the size of Neptune in our own solar system. What is more, the new “family” – shown here in an artist’s impression – is believed to have an asteroid belt too, just like our own.
The star, which is slightly smaller than our own sun, lies 41 light years away in the constellation of Puppis.
It is just visible with the naked eye on a dark night from southern countries – but not from the UK, Ireland or the northern USA where it never rises above the horizon.
The astronomers, led by Christophe Lovis, of Geneva, spent two years studying HD 69830 with the European Southern Observatory’s 3.6-metre telescope at La Silla in Chile.
That is like looking at the sky with an “eye” that is nearly 12ft wide.
An ultra-precise spectroscope called HARPS was used to spot little wobbles in the light caused by the movement of the three planets, the science journal Nature reports.
Accurate measurements of these wobbles showed that the worlds orbit their own “sun” in 8.67, 31.6 and 197 days.
Graduate student Lovis, from Geneva Observatory, Switzerland, said yesterday: “For the first time, we have discovered a planetary system composed of several Neptune-mass planets.”
Colleague Michel Mayor, also from Geneva, said: “Only ESO’s HARPS instrument made it possible to uncover these planets. Without any doubt, it is presently the world’s most precise planet-hunting machine.”
He added: “The planetary system around HD 69830 clearly represents a Rosetta stone in our understanding of how planets form.”
The new planets are between ten and 18 times the mass or size of the Earth. The innermost is mainly rocky, the middle one a mixture of gas and rocks and the outermost is thought ot have a rocky. icy core.
Observations also indicate that the outer planet lies within the inner edge of a “habital zone” where liquid water could exist on its surface.
Astronomers say that the “cherry on the cake” is recent observations of the star from Nasa’s orbiting Spitzer Space Telescope that suggests it probably has an asteroid belt just like our own sun.
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