News in Science

Homeless planets discovered in deep space

Interstellar wanderers Scientists using theories first developed by Albert Einstein believe they've found a huge population of planets that don't orbit stars.

The discovery by scientists including Dr Takahiro Sumi from Osaka University in Japan, supports previous suggestions that free floating planets between three and 15 times the mass of Jupiter may exist in young star forming regions of interstellar space.

The new research, reported in the journal Nature, also suggests there could be twice as many of these free floating planets as there are stars.

Since the first discovery in 1995, more than 500 planets have been detected beyond our solar system, most by the so-called "wobble method" which looks for tiny movements in a star caused by the gravitational pull of an orbiting planet.

However, 12 extraterrestrial planets have been detected by a different process called gravitational micro-lensing. First theorised by physicist Albert Einstein in 1915, it involves the mass of a foreground object acting as a lens to bend and magnify light from a more distant background source.

Sumi and colleagues examined two years of gravitational micro-lensing survey observations of the galactic bulge of the Milky Way.

By determining the duration of each gravitational micro-lensing event, they were able to estimate the mass of the foreground object causing it.

After ruling out failed stars called brown dwarfs and stellar remnants including white dwarfs, neutron stars and stellar black holes, Sumi and colleagues believe they detected ten Jupiter-mass objects that don't appear to be orbiting a star.

Galaxy full of planets

Based on the frequency of the occurrence of these objects, they believe there are nearly twice as many free floating planets as there are stars.

Sumi and colleagues believe these planets would have formed in proto-planetary disks of molecular gas and dust around new stars similar to the one our solar system formed in.

But they think these free floating planets were then flung out of their systems or pushed in to very distant orbits.

Dr Simon O'Toole from the Australian Astronomical Observatory says it's an intriguing result, but needs more data to back it up.

"We're only speaking about 10 objects, only one of which has enough data to rule out a host star, or at least rule out a star within ten astronomical units of the planet", says O'Toole.

An astronomical unit is the distance between the Earth and the Sun.

"Ten astronomical units isn't that far out, Saturn's only just under that, both Uranus and Neptune are much further out and Pluto (even though it's no longer a planet) is about 30, while the Kuiper belt is 50," he says.

"You can't just use one result to infer that none of these objects are bound to host stars".

Planetary migration

According to O'Toole the paper's support of the theory that planets migrate during their lifetime is most likely correct.

"Good examples are Neptune and Uranus, which may have formed much further in, possibly between the orbits of Jupiter and Saturn, and later moved out to their current positions.

"They may even have swapped places, all due to various gravitational interactions."