Mystery of free-floating 'planets'

A possible planet free-floating in Orion (Image: Science)

By BBC News Online science editor Dr David Whitehouse

Astronomers have discovered 18 planet-like objects, drifting through space in a part of the constellation of Orion.

If they are planets, these "free floaters" will pose a challenge to theories about how planets form.

Planets are thought to arise as gas and dust in the disc swirling around a young star condenses and clumps together.

But the newly-discovered objects seem to have a different origin and evolution.

They lack a central star and they form part of a star cluster called Sigma Orionis, which is no more than five million years old.

"The formation of young, free-floating, planetary-mass objects like these are difficult to explain by our current models of how planets form," said Maria Rosa Zapatero Osorio, of the Instituto de Astrofísica de Canarias, in Tenerife, Spain.

Young and cool

The researchers chose the Sigma Orionis star cluster for their planet hunt because it is nearby, young, and largely free of dust and gas clouds that might obstruct the view.

Using visible and infrared light-detecting sensors on telescopes in Spain, the Canary Islands, and Hawaii, they found 18 objects whose relatively dim, reddish light suggested they could be planetary.

Orion is a region of intense star formation

The team used measurements from spectrographs on the Keck telescopes in Hawaii to study the range, or spectrum, of energy emitted by three of their candidates.

This confirmed the new objects were cool with planet-like temperatures.

"The spectrographic results corresponded to our expectations that these were young giant planets," said Zapatero Osorio.

The researchers say that their estimates of the masses of the objects are well within the range for planets.

Brown dwarf stars

Sigma Orionis is probably five million years old, so if the objects are equally old, they are probably 8-15 Jupiter masses in size.

If they are only one million years old then the fainter ones could be as small as five Jupiter masses.

It is still a possibility that the scientists have found unusually small, cool, brown dwarf stars.

But the astronomers say that judging from previous sky surveys, to find 18 brown dwarfs concentrated in such a relatively small area would be unlikely.

"The most intriguing question now is how can we explain the formation and evolution of planetary-mass objects outside the Solar System?" said Zapatero Osorio.

The research is published in the journal Science.