Planet hunting has morphed from the marvellous to the mundane

Prof Geoff Marcy of San Francisco State University

It brings the total number of planets discovered circling other stars, so-called exoplanets, to 50.

The second known example of a star orbited by more than one planet was also announced.

It could be the first of many as scientists say they are now seeing the first tantalising hints that multiple planets may circle many other stars.

The rush of new planetary announcements comes on top of Friday's news of an exoplanet around Epsilon Eridani. At a mere 10.5 light years from Earth, this world could be visible to the Hubble Space Telescope.

Saturn-sized planets

During the last five years, 41 exoplanets had been detected orbiting nearby stars, but until now only one system, Upsilon Andromidae, had been shown to have more than one planet going round a sun.

Geoff Marcy: "We are finding planets faster than we can investigate them"

At the International Astronomical Union (IAU) General Assembly being held in Manchester, UK, scientists reported the discovery of a second extrasolar planetary system.

It contains two Saturn-sized planets orbiting a star called HD 83443. This star is slightly less massive than our Sun (0.8 solar masses).

It lies in the constellation Vela and is 141 light years away from our Solar System. The two gas giant planets both orbit very close to the star. One of them orbits the star in 2.98 days, the other in 29.8 days.

Astronomers are at a loss to explain why one circles the star almost exactly 10 times faster than the other. The inner planet has the shortest orbital period and has the smallest distance from its parent star than any other exoplanet yet discovered.

Unexplained wobbles

The number of stars found to have more than one planet in orbit about them is set to grow rapidly.

This is the first time anyone has noticed that such a high percentage of stars with one known planet show evidence of a second companion

Dr Debra Fischer, University of California at Berkeley

A team of astronomers based at the University of California, Berkeley, is beginning to detect hints that many extrasolar planets may have smaller companions.

Dr Debra Fischer, of the University of California at Berkeley, has looked at data for 12 stars around which single planets are known to circle.

She says she is beginning to see patterns that these planets may have siblings. Of the 12 stars she has looked at, she has found that five exhibit unexplained wobbles that could result from the tug of a companion - whether another planet, an unseen star or something in-between.

"This is the first time anyone has noticed that such a high percentage of stars with one known planet show evidence of a second companion," Fischer said.

Signature in the dust

"We're now at a stage where we are finding planets faster than we can investigate them and write up the results," said Professor Geoff Marcy of San Francisco State University. "It's wonderful. Planet hunting has morphed from the marvellous to the mundane."

Nasa astronomers also reported to the IAU conference that they have studied the patterns imprinted on the dust disks around well known, nearby stars to determine whether they have orbiting planets.

The planets are still hidden but they write their signature in the dust, say the researchers. The astronomers estimate that the Beta Pictoris star has a planet 10 times the mass of Earth orbiting at a distance of about 10.4 billion km (6.5 billion miles). And Vega, one of the brightest objects in the sky, appears to have a planet twice the mass of Jupiter in an orbit about 8 billion km (5 billion miles) from the star.

These distances from the parent star are larger than for any of the planets in our Solar System.

Earth-like clones

The exoplanets so far discovered have forced astronomers to think long and hard about their theories of how planets form around a central star.

Professor Marcy told the BBC: "The planets we are finding about other stars all orbit in elongated, elliptical orbits. It's quite frightening that virtually all the planets we've found orbit close to their stars where they heat up and then move out to where they cool down.

"Water would boil and then freeze - and of course, such characteristics are not conducive to life."

But Professor Marcy remains confident that Earth-like planets will be found. "There must be rocky planets out there that have just the right temperature so that the water is not frozen into ice nor vaporised into steam but is in liquid form," he said.

"It may be that our Solar System is one in a hundred or one in a thousand, but let's not forget that our Milky Way galaxy contains one hundred billion stars so there are certainly Earth-like clones out there."