Astronomers have discovered a group of planets that have no fixed orbits and instead float freely through the Milky Way.

The scientists surmise the 10 new planets, each roughly the size of Jupiter, were originally part of various solar systems, but got thrown out of orbit to roam like orphans, heading in the direction of the constellation Sagittarius.

Such planets had been predicted by theorists but this is the first time any have been detected. The discovery has big implications for understanding the dynamics of solar systems.

Dr Ian Bond, an astronomer at Massey University in New Zealand, is one of several authors of a new paper in the journal Nature which describes how astronomers in New Zealand, Japan and the US found the planets in some of the emptiest parts of space.

But even though the planets are not anchored to anything, Dr Bond says they are not just zinging around the Milky Way like pinballs.

"They're just subjected to the gravity of the galaxy as a whole, in the same way that stars are, so that these will be orbiting the centre of the galaxy in the same way that our sun does," he said.

"It's a sense of discovery, a sense of just seeing what's really out there. That part of it is really exciting - that sense of exploration that we're doing."

The newly-discovered planets are as large as Jupiter and like that planet, they are also thought to consist of hydrogen and helium.

Their discovery was made possible by a new technique known as gravitational microlensing.

Under this method, when a celestial body passes in front of a distant background star, telescopes detect changes in the light.

Astronomers then measure the changes to deduce the size and mass of the closer object.

"In order to do this, you need the capability of monitoring several tens of millions of stars all at the same time and several times during an observation run during the night," Dr Bond said.

New knowledge

Astronomers say finding the orbitless planets gives them important new knowledge about how planetary systems form and evolve.

Professor Fred Watson, in charge of the Anglo Australian Observatory, says the formation of planets is usually associated with the formation of stars.

"We believe that our solar system, for example, came into being at the same time as the sun was born, about 4.6 billion years ago," he said.

"If you're finding things that look like planets and have about the right mass for planets, but they're not associated with a star, then the big question is, how did they get there?"

Dr Bond believes the planets formed in a solar system, but were then pushed out by the gravity of other planets.

"They don't have to be physically touching, but the gravity of each of those planets, well, the gravity of one planet will affect the orbit of another and so there's complex interactions taking place and it has the effect of moving them around," he said.

"People have done numerical studies which can show that planets can end up actually being moved into unstable orbits such that they actually get ejected."

Dr Bond says it is possible the same thing happened in our solar system.

"We may have a number of planets that got kicked out of our solar system during their early years, so we have some lost siblings wandering interstellar space out there," he said.

The team that found the new planets says they are just a small sample and there will be many more orphan planets out there.

Their next project is to find smaller ones.