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May 18, 2011

400 Billion wandering planets in the Milky Way

Free-floating planets may be more common in our Galaxy than stars.

BBC News - Japanese astronomers claim to have found free-floating "planets" which do not seem to orbit a star. They say they have found 10 Jupiter-sized objects which they could not connect to any solar system. They also believe such objects could be as common as stars are throughout 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

Nature - Unbound or distant planetary mass population detected by gravitational microlensing

Nature - So many lonely planets with no star to guide them

Scattered about the Milky Way are floating, Jupiter-mass objects, which are likely to be planets wandering around the Galaxy's core instead of orbiting host stars. But these planets aren't rare occurrences in the interstellar sea: the drifters might be nearly twice as numerous as the most common stars.



Planetary scientist David Stevenson at the California Institute of Technology in Pasadena has considered how the temperatures on ejected planets might compare with those on star-bound bodies2. If Jupiter were kicked out of the Solar System, its surface temperature would drop by only about 15 kelvin, he says – although it would still be unsuitable for supporting life. However, "when you eject a planet that is quite massive, it could have carried along an orbiting body", Stevenson adds. "And that might be a more attractive possibility for life."

Unbound Earth-mass planets might still be capable of carrying liquid water, Stevenson says, even in the frozen reaches of interstellar space – as long as they have a heat-trapping hydrogen atmosphere. "That can bring the surface temperature up to 300 kelvin [about 27 °C]," he says. "And then you can have oceans."

Since 1995, more than 500 exoplanets have been detected using different techniques of which 12 were detected with gravitational microlensing. Most of these are gravitationally bound to their host stars. There is some evidence of free-floating planetary-mass objects in young star-forming regions but these objects are limited to massive objects of 3 to 15 Jupiter masses with large uncertainties in photometric mass estimates and their abundance. Here, we report the discovery of a population of unbound or distant Jupiter-mass objects, which are almost twice as common as main-sequence stars, based on two years of gravitational microlensing survey observations towards the Galactic Bulge. These planetary-mass objects have no host stars that can be detected within about ten astronomical units by gravitational microlensing. However, a comparison with constraints from direct imaging suggests that most of these planetary-mass objects are not bound to any host star. An abrupt change in the mass function at about one Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.

35 pages of supplemental information

Nature - Astronomy: Bound and unbound planets abound

Two teams searching for extrasolar planets have jointly discovered a new population of objects: ten Jupiter-mass planets far from their host stars, or perhaps even floating freely through the Milky Way.

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