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Scientists have catalogued 450 exoplanets in the past 15 years, but most finds include up to three planets per star, usually gargantuan balls of gas similar to Jupiter or Saturn. ((Illustration by France's space agency, the Centre National d'Etudes Spatiales) )

Scientists say they've identified a sun-like star with as many as seven different planets, including one that might be the smallest ever found outside the solar system.

If confirmed, the planetary system around HD 10180, a star more than 100 light years distant, would be the richest ever discovered. One astronomer says it's part of a growing body of evidence that the universe is full of planets, and that several could be similar to our own.

"The really nice thing about finding systems like this is that it shows that there are many more out there," said Alan Boss, of the Washington-based Carnegie Institution for Science, who wasn't involved with the find. "Mother Nature really had fun making planets."

Although most of the planets identified are large — about 13 to 25 times the mass of our home — those behind the discovery, announced Tuesday at an international conference in France, say they're nearly certain they've identified one only 1.4 times the size of Earth.

Smallest ever spotted

Planets found outside the solar system are called "exoplanets," and this would be the smallest one ever spotted.

Scientists have been successfully hunting exoplanets for about 15 years, and they've now catalogued some 450. But most finds have been limited to one or two or three planets per star, usually gargantuan balls of gas similar to Jupiter or Saturn.

But at up to seven planets, the new discovery nearly matches our own solar system, which counts eight.

Christophe Lovis of Geneva University, one of the scientists behind the find, said the first five were most comparable to Neptune.

"They are made essentially of rocks and ice. They have a solid core. But on top of that is a layer of gas, of hydrogen and helium most likely," he said. "They are probably not habitable."

'Year' would take 1 day

The sixth is possibly a Saturn-like planet, while the seventh, the smallest, would be so close to its star that its "year" would take just over a day.

Lovis and his team haven't been able to observe the planets directly, which is typical. Few planets can be seen against the blazing light given off by their much more massive parent stars.

The European Southern Observatory compares the challenge to "spotting a dim candle in front of a raging forest fire."

So the scientists used the observatory's 3.6-metre telescope at La Silla, Chile, to study the star itself. Over six years, they took 190 measurements, checking it for the telltale wobbling caused by the gravitational forces of nearby planets.

Boss noted that the method was "biased toward finding the big guys" because the greater the planet, the greater its gravity and the more it made its parent star wobble. But he said the discovery showed that finding smaller planets was still possible.

"This field has gone from zero to close to 500 planets in just 15 years," he said. "Fifteen years ago, we did not know about the big guys. Earth-like planets are going to be quite commonplace."

The find was made by researchers from Switzerland, France, Germany and Portugal, and has been submitted to the journal Astronomy and Astrophysics.