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Is the galaxy full of orphans?
Astronomers said Wednesday that space is littered with hundreds of billions of planets that have been ejected from the planetary systems that gave them birth and either are going their own lonely ways or are only distantly bound to stars at least as 10 times as far away as the Sun is from the Earth.
The new work was done using a method known as gravitational microlensing, which is more sensitive to far-out planets. It relies on the ability of the gravitational field of a massive object — in this case a planet and its star — to bend light and act as a magnifying lens, as predicted by Einstein’s general theory of relativity.
Astronomers from two groups — Microlensing Observations in Astrophysics, based in New Zealand and Japan, and the Optical Gravitational Lensing Experiment, based in Poland and Chile — monitor the light from a vast field of background stars, looking for brief blips of increased brightness caused by a planet and its host star passing in the foreground.
The group recorded 10 such events consistent with being caused by planet-size objects but did not detect the corresponding blips from these planets’ host stars, suggesting either that they did not belong to any star, having been ejected by gravitational pinball games earlier in their lives, or that they were very distant.
"This is an amazing result, and if it's right, the implications for planet formation are profound," says astronomer Debra Fischer at Yale University in New Haven, Connecticut.
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 imaging9 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.