Hundreds of billions of large planets like Jupiter and Saturn wander the Milky Way unattached to any star. Astrophysicists estimate that there are more lost, lonely planets without a sun wandering the galaxy than there are stars to call home.
image credit NASA JPL Cassini
One of the predictions of Einstein's theory of relativity is that a large mass such as a planet or star will bend space and therefore bend light traveling around it. This bending of light acts as a lens that causes a brightening of light before and after the darkening caused by transit of a planet in front of a star.
Link to NSF Video on microlensing. The embed code is unstable.
One application of gravitational microlensing is the search for planets orbiting distant stars. The radial velocity technique has proved to be successful for detecting gas-giant planets like Jupiter orbiting nearby stars, especially ‘hot Jupiters’ orbiting close to their parent stars.
Extra-solar planets orbiting the lens star of a gravitational microlensing event can betray their presence through perturbations to the light curve (Gould & Loeb, ApJ Vol. 396, p. 104; Bennett & Rhie, ApJ Vol. 472, p. 660). This technique of searching for planets is complementary to the radial velocity technique, as it is sensitive to less massive planets than Jupiter orbiting at larger distances than those of the hot Jupiters. Also, it is sensitive to planets in distant regions of the Galaxy.
A large number of planets may be ejected from developing solar systems by gravitational instabilties.
Previous observations have spotted free-floating planet-like objects within star-forming clusters, with masses low as roughly three times that of Jupiter. Scientists debate over whether these gaseous bodies form like stars or planets. Small, dim stars called brown dwarfs form like stars, growing from collapsing balls of gas and dust, but they lack the mass to ignite their nuclear fuel and shine with starlight. It's possible that brown dwarfs could be as small as planets.
It's also possible that planets get the "boot" from their early solar systems, a rambunctious time when growing planets can be ejected from their orbits due to close gravitational encounters with other planets or stars. Without a star to circle, these planets would move through the galaxy as our sun and others stars do, in stable orbits around the galaxy's center. The fact that the new survey found 10 free-floating Jupiters supports the "ejection" scenario, though it's possible that both mechanisms are at play.
"If free-floating planets formed like stars then we would have expected to see only 1 or 2 of them in our survey instead of 10," said Bennett. "The results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth."
A large Saturn like planet with a large moon like Titan in an orbit tight enough to generate large amounts of tidal friction for to heat an ocean might be capable of transporting primitive life across the galaxy.
To find the wanderers, scientists turned their telescopes towards the Galactic Bulge surrounding the centre of the Milky Way. Using a technique called gravitational microlensing, they detected 10 Jupiter-mass planets wandering far from light-giving stars. Then they estimated the total number of such rogue planets, based on detection efficiency, microlensing-event probability and the relative rate of lensing caused by stars or planets. They concluded that there could be as many as 400 billion of these wandering planets, far outnumbering main-sequence stars such as our Sun. Their work is published today in Nature1.
Study author Takahiro Sumi, an astrophysicist at Osaka University in Japan, says the deduced number of homeless exoplanets surprised him. "The existence of free-floating planets has been predicted by planetary formation theory, but nobody knew how many there are," he says.
And because current theories of planet formation hold that lower-mass planets are more readily flung from developing planetary systems than are higher-mass planets, there could be a huge number of lighter planets on the loose. "They might be littering the Galaxy," says Fischer.
image credit: NASA JPL Cassini; "This false-color composite image shows the glow of auroras streaking out about 1,000 kilometers (600 miles) from the cloud tops of Saturn's south polar region."
Most of the heat Saturn radiates to space is internally generated. Solar heating makes a minor contribution to its temperature. Complex physical processes internal to Saturn generate strong magnetic and electric fields. This image of Saturn's auroras is evidence of strong internal activity. A Saturn like planet ejected into space could heat its moons like Saturn does by tidal friction and perhaps by electric and magnetic fields.
Image credit: NASA "This artist's concept shows a glowing patch of ultraviolet light near Saturn's north pole that occurs at the "footprint" of the magnetic connection between Saturn and its moon Enceladus. The footprint and magnetic field lines are not visible to the naked eye, but were detected by the ultraviolet imaging spectrograph and the fields and particles instruments on NASA's Cassini spacecraft. The footprint, newly discovered by Cassini, marks the presence of an electrical circuit that connects Saturn with Enceladus and accelerates electrons and ions along the magnetic field lines. In this image, the footprint is in the white box marked on Saturn, with the magnetic field lines in white and purple."