Published online 3 April 2001 | Nature | doi:10.1038/news010404-9

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Lonely planets float free

They're not quite planets and they're not quite stars: meet the planetars.

The Orion Nebula: this stellar nursery has spawned a planetThe Orion Nebula: this stellar nursery has spawned a planet

British astronomers have confirmed that a small cluster of faint objects that they spotted last year in the Orion constellation can be nothing other than free-floating planets.

This is the first time that anything resembling a planet has been found alone in the Universe. The finding has also thrown astronomers into confusion over what to call the objects, the 2001 UK National Astronomy Meeting in Cambridge heard today.

The objects inhabit the Orion Nebula, a 'stellar nursery' where new stars are constantly forming. From Earth the nebula looks like a star -- the middle one of three that make up the dagger of Orion The Hunter.

An analysis of the wavelengths of light that the faint objects give off reveals that they are too cool to be stars. And, at only a few times bigger than the planet Jupiter, these mysterious bodies are also far too small to be stars.

"They are like stars that never made it," says one of their discoverers, Philip Lucas at the University of Hertfordshire who, along with Patrick Roche at the University of Oxford, UK, performed this latest analysis.

By providing a new example of failed stars, the duo's findings may give astronomers a better idea of the way in which successful stars form. But they also identify a completely new object in the Universe that may turn out to be quite common. "Other groups are seeing similar objects elsewhere," says Lucas.

Because the objects formed in the way stars form, yet resemble planets -- which are thought to form by the coalescence of interstellar rock and dust -- and because they do not orbit a star, they are proving hard to name. "There's never been a strict definition of a planet," Lucas explains.

Various labels have been proposed; for example, 'grey dwarves' and 'planetary mass objects'. Lucas and Roche suggest 'planetar'. Whether the astronomical community will embrace this hybrid moniker is another matter. "Some people like it and some people hate it," says Lucas. 

  • References

    1. and Roche's work will be published in a forthcoming issue of Monthly Notices of the Royal Astronomical Society.