By BBC News Online Science Editor Dr David Whitehouse

New suggestions that the planets in our Solar System have not always been in their current orbits have been put forward by two teams of astronomers.

This work, along with recent speculation that Jupiter may have formed much further from the Sun than its current position, and the discovery of other planetary systems orbiting other stars, is forcing a reappraisal of our understanding of how the planets were formed.

Writing in the journal Nature, researchers from Queen's University in Kingston, Canada, propose that all of the giant planets in our Solar System formed in a narrow region of the gas and dust cloud that surrounded the early Sun. They suggest that they ended up in their present orbits as a result of violent and chaotic scattering.

This would mean that when Jupiter, our Solar System's largest planet, formed, it triggered the birth of other giant planets nearby. In a way, Jupiter was the "midwife" of the Solar System.

Rocky cores

The four major planets in our Solar System are classified into two "gas giants" (Jupiter and Saturn), that have a small rocky core surrounded by a large hydrogen and helium atmosphere and also two "ice giants" (Uranus and Neptune), that have icy mantles around their cores and only a thin atmosphere.

Scientists have always been slightly puzzled by the positions of Uranus and Neptune because in their present locations it would have taken longer than the age of the Solar System for them to form.

The scientists from Queen's University suggest that the four giant planets started out as rocky cores in the Jupiter-Saturn region, and that the cores of Uranus and Neptune were tossed out by Jupiter's and Saturn's gravity.

In the simulations, the ejected planets went into highly chaotic orbits for a few hundred thousand years after which they settled down and gradually migrated to their present, nearly circular orbits.

Stable orbits

Another group of scientists, also writing in Nature, from the University of Toronto, have simulated how planets such as Jupiter may have formed in the first place.

They found that gas and dust circling the early Sun that starts to accumulate to form a proto-Jupiter creates a spiral density pattern in the surrounding disk material. The proto-planet accretes mass rapidly through the spiral arms but when the planetary mass reaches four-to-five-times Jupiter's mass, the disk rapidly fragments into smaller proto-planets.

Over hundreds of thousands, or millions of years the proto-Uranus and proto-Neptune would be flung outwards by the now smaller proto-Jupiter's gravity.

Not too long ago, scientists regarded the orbits that the planets circle our Sun as being the ones they were born in. Now they are realising that this is not the case. Uranus and Neptune may have migrated outwards and Jupiter may have come in from the outer cold.

One of the questions scientists would like to answer is whether the Earth has always been where it is now?