One third of giant planet systems may also harbour Earth-like planets

Because of the way we currently look for planets around other stars, most that have been detected so far have been gas giants like Jupiter or Saturn.

But one-third of these giant planet systems may also harbour worlds like our own, according to an analysis by scientists in the US.

Details of the research appear in the latest issue of the journal Science.

The study focused on a type of planetary system containing so-called "Hot Jupiters". These massive bodies orbit extremely close to their parent stars - in some cases closer than Mercury circles the Sun.

The technique commonly used to find these distant planets detects their gravitational pull on a star. So there is a bias towards finding massive worlds orbiting close to their parent "sun".

Hot Jupiters are thought to have migrated inward toward their stars as their planetary systems were forming.

Debris field

At this stage the evolving planetary system is populated with small planetoids, icy and rocky debris and gas in a "protoplanetary disc".

A gas giant ploughing through this orbiting material creates considerable disturbance. But how exactly this affects the evolving planetary system is the matter of some debate.

Previously, researchers have proposed that inward migration of gas giants caused the disc material either to be vacuumed up or ejected from the cosmic neighbourhood.

"There is certainly scattering, but one of the things people did not take into account was the dense gas in these protoplanetary discs," said co-author Avi Mandell, from Pennsylvania State University, US.

"This gas provides the force that migrates the giant planets inwards but it also damps down the scattering and ejection of particles allowing them to stay caught in the orbit of the star.

"They stay in rather chaotic and often wide orbits; but usually if they remain in the system for millions of years, they settle down and return to a circular orbit where they can merge with other bodies and turn into full planets."

Habitable zone

The researchers, led by Sean Raymond, of the University of Colorado at Boulder, carried out computer simulations of gas giant migration in forming planetary systems. They started off with more than 1,000 virtual Moon-sized objects made of rock and ice and simulated about 200 million years of planetary evolution.

The team concluded that about one of every three known planetary systems could harbour Earth-like planets in habitable zones further out than the Hot Jupiters.

The habitable zone is the region of a planetary system which is neither too hot nor too cold - such that water on the surface of a planet will neither evaporate nor freeze, but instead can stay liquid.

"Our simulations show that you can form a giant planet where Jupiter is in our system and move it through to the inner part of the system. Planets like Earth can still form outside them - as long as they don't get too close to the giant planet," Avi Mandell told the BBC News website.

"If you get too close, the gas giant will scatter material outwards. But if you are far enough away, material can continue to form outside it."

Earth-like planets in habitable zones form in the presence of large amounts of water, the researchers suggest. This raises the possibility that planets like ours orbiting in habitable zones could be covered in oceans - providing all the key constituents required for life.

"We now think there is a new class of ocean-covered - and possibly habitable - planets in solar systems unlike our own," said Dr Raymond.

The simulations also showed that rocky planets known as Hot Earths often form inside the orbits of Hot Jupiters. A team at the University of California, Berkeley, has already found one such example, orbiting just two million miles from its parent star.