WASHINGTON, Aug 5: Our solar system may be unique after all, despite the discovery of at least 120 other systems with planets, astronomers said on Wednesday. All the other solar systems that have been found have big, gassy planets circling too close to their stars to allow them to be anything like Earth or its fellow planets, the British and US-based researchers said.

If that is the case, Earth-like planets will be very rare, the astronomers write in the latest issue of the Monthly Notices of the Royal Astronomical Society. "Maybe these other extrasolar systems ... contain only the giant planets," said Mario Livio of the Space Telescope Science Institute in Baltimore.

Livio and colleagues took a close look at what is known about the other planetary systems that have been discovered. "In (our) solar system the orbits are very circular. Most of the giant planets observed in extrasolar systems have very elliptical orbits," Livio said in a telephone interview.

This could mean that astronomers have been wrong in assuming that all planets formed in basically the same way. Livio said most experts thought that planets formed out of dust. "This dust coagulates and forms small rocks and the rocks combine and form small bodies and then those bodies form things like Earths," he said.

"The Earths collect and accrete gas and then they form giant planets like Jupiter. That is one model." But so far no one has found a planet outside our solar system that looks like it formed that way.

"Then there is a second model that has been suggested specifically for the formation of giant planets like Jupiter. You start with a gas disk and this disk becomes unstable and it breaks up into large clumps and those clumps are the things that form giant planets," Livio said.

"In that model it is not obvious at all how planets like Earth may have formed." It could be our solar system formed in the first way and most of the others formed in the second way, Livio said.

But he said it is hard to tell as planets outside this solar system can only be detected through indirect observation and these methods are not able to detect smaller planets like Earth.

Either way, it is time to start thinking about the possibility that our system is unique or at least unusual, Livio said. What has been seen up to now does not bode well for the main purpose of seeking other planets - finding life outside our solar system. "If the orbit is very elliptical then the planet may come very close to its sun at some point and that doesn't appear to be very healthy for life," Livio said. -Reuters