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The existence of vagabond planets, that have lost the gravitational link with their host star after an interaction within the planetary system, now seems certain. What still remains very uncertain is how many exist in the Galaxy and in the Universe at large.
In today's issue of Nature, there is a first estimate of that number, and it's quite impressive: there are about twice as many planets more massive than Jupiter moving freely in space as there are stars in the Galaxy, about the same as the expected number of normal planets.
Given that the chances of a planet being ejected from a planetary system are higher for lower mass planets, it is quite possible that there are more planets in open space than around stars!
These results, that may mark the defining of a new class of planet, come from a study by a group of researchers led by David Bennett (University of Notre Dame, Indiana). The group used observations of the bulge of the Milky Way made between 2006 and 2007, as part of a survey by a Japan/New Zealand collaboration, using the 1.8 metre telescope of the Mt. John University Observatory (NZ).
The work consisted of searching for so-called micro-lensing events, which show up as brief increases in brightness of background stars when a body passes infront of it. This method, which does not require the detection of any reflected light from the planet, can detect objects down to the mass of Jupiter or Saturn.
The gravitational deformation of space caused by the planet's mass causes the light from the background star to be amplified, as if it were passing through a magnifying glass.
The effect is not large and lasts only for a day or so, making detections very difficult. Despite this, Bennett and colleagues have identified 10 events, consistent with the passage of as many Jupiter sized planets, in regions where there are no stars to which they could be gravitationally bound.
Even though these planets find themselves in the total darkness and cold of interstellar space, there is still a possibility that they are not completely frozen and dead. The heating from internal radioactive decay combined with an extremely thick atmosphere might result in surface temperatures high enough for elementary forms of life; though those searching for life would probably do well to look elsewhere first!
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