Free-floating planets found unbound to stars

Ten planets the size of Jupiter have been found which do not appear to orbit any star.

ASTRONOMERS SAY THEY have found a previously inconceivable phenomenon: planets that do not appear to be anchored to a host star but instead wander the heavens unbound.

In a two-year scan of the cosmos, 10 planets with roughly the mass of Jupiter, the largest planet of our Solar System, have been found at such enormous distances from the nearest star that some of them could be said to float freely through the Milky Way.

"Although free-floating planets have been predicted, they finally have been detected, holding major implications for planetary formation and evolution models," says Dr Mario Perez, exoplanet program scientist at NASA's headquarters in Washington DC.

The investigation, published in the science journal Nature, breaks new ground in the science of exoplanets, or planets that exist beyond our Solar System. More than 500 such planets have been identified since 1995. But these are the first that have been found to be orbiting at such a huge range from the nearest star or seem to be "unbound" from it.

Distant and unshackled

The new planets were found in a search that looked for objects ranging between 10 and 500 astronomical units (AU) from a star. The AU is a standard measurement comprising the span between Earth and the Sun, nearly 150 million kilometres.

By comparison, Jupiter is just over five AU from the Sun, while Neptune, the outermost recognised planet, is 30. The theory of planetary foundation says that planets are agglomerations of dust and gas and are enslaved by their stars, doomed to orbit around it until the star runs out of fuel.

The new paper suggests these very distant planets unshackled from their gravitational moorings at an early phase. "They may have formed in proto-planetary disks, and subsequently scattered into unbound or very distant orbits," it says.

The study was written by two teams who used gravitational microlensing to analyse tens of millions of Milky Way stars over a two year period. Under this technique, a foreground star passes in front of a distant, background star. Light from the background star is magnified, carrying a telltale "light curve" that can be filtered from the foreground star.

"The implications of this discovery are profound," says German astronomer Dr Joachim Wambsganss in a commentary also published by Nature. "We have a first glimpse of a new population of planetary-mass objects in our galaxy. Now we need to explore their properties, distribution, dynamic states and history."

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