Science:
A long-running debate on whether free-floating planets really
exist and how common they are may now be resolved, writes Jon
Cartwright for
Science. A
new study
by Takahiro Sumi of Osaka University in Japan and colleagues
lists 10 objects in our galaxy that are very likely "homeless"
planets and posits that such planets are more common than
main-sequence stars, outnumbering them by nearly two to one.
The team used a technique called gravitational microlensing to
measure changes in the brightness of distant stars, whose light
is bent and magnified by the gravity of planets passing in
front of them. Smaller planets create shorter magnification
times. The team found 474 incidents of microlensing, just 10 of
which were brief enough to be planets of around Jupiter's size,
while the other 464 microlensing events were due to bigger
objects such as brown dwarfs. One question arising from the
study is how the free-floaters formed. According to one
explanation, they used gravity to draw in nearby material such
as the asteroids and protoplanets in our solar system. Sumi and
his colleagues think it's more likely that they began their
lives in planetary systems and were subsequently thrown out of
orbit.
