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'Super-Earths' may be common -research
Last Updated(Beijing Time):2008-06-17 08:09
The Earth has not one but three peers, if European researchers are to be believed.

A batch of three "super-Earths" is orbiting a nearby star, and there are two other solar systems with small planets as well, they said Monday.

Their findings, presented at a conference in France, suggest that Earth-like planets may be very common.

"Does every single star harbor planets and, if yes, how many?" asked Michel Mayor of Switzerland's Geneva Observatory. "We may not yet know the answer but we are making huge progress toward it."

The three planets orbit a star slightly less massive than our Sun, 42 light-years away toward the southern Doradus and Pictor constellations. A light-year is the distance light can travel in one year at a speed of 300,000 km a second, or about 9.5 trillion km.

The planets are bigger than Earth - one is 4.2 times the mass, the second is 6.7 times and the third is 9.4 times.

They orbit their star at extremely rapid speeds - one whizzing around in just four days, compared with Earth's 365 days, the second taking 10 days and the slowest taking 20 days.

Mayor and his colleagues used the high accuracy radial velocity planet searcher (HARPS), a telescope at La Silla observatory in Chile, to find the planets.

More than 270 so-called exo-planets have been found. Most are giants, resembling Jupiter or Saturn. Smaller planets closer to the size of Earth are far more difficult to spot.

None can be imaged directly at such distances but can be spotted indirectly using radio waves or, in the case of HARPS, spectrographic measurements. As a planet orbits, it makes the star wobble very slightly and this can be measured.

"With the advent of much more precise instruments such as the HARPS spectrograph ... we can now discover smaller planets, with masses between 2 and 10 times the Earth's," said Stephane Udry, who also worked on the study.

The team said it found a planet 7.5 times the mass of Earth orbiting the star HD 181433 in 9.5 days. This star also has a Jupiter-like planet that orbits every three years.

Another solar system has a planet 22 times the mass of Earth, orbiting every four days, and a Saturn-like planet with a 3-year period.

"Clearly these planets are only the tip of the iceberg," said Mayor.

"The analysis of all the stars studied with HARPS shows that about one-third of all solar-like stars have either super-Earth or Neptune-like planets with orbital periods shorter than 50 days."

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