Rogue planets could be common
The find was made by looking for an effect known as gravitational
microlensing. An international team of astronomers claim to have found
free-floating ‘planets’ which do not seem to orbit a star. Writing in
Nature, 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.
An international team of astronomers claim to have found
free-floating ‘planets’ which do not seem to orbit a star. |
The objects revealed themselves by bending the light of more distant
stars, an effect called “gravitational microlensing”. Objects of large
enough mass can bend light, as Albert Einstein predicted. If a large
object passes in front of a more distant background star, it may act as
a lens, bending and distorting the light of that star so that it may
appear to brighten significantly.
The researchers examined data collected from microlensing surveys of
what is called the Galactic Bulge, the central area of our own Milky
Way. Using the data, they found evidence of 10 Jupiter-sized objects
with no parent star detected within 10 Astronomical Units (AU). One AU
is equivalent to the distance between our Earth and Sun.
Further analysis led them to the conclusion that most of these
objects did not have parent stars. Based on the number of such bodies in
the area surveyed, the astronomers then extrapolated that such objects
could be extremely common.
They calculated that they could be almost twice as common as
“main-sequence stars” - such as our own Sun - which are still burning
through their hydrogen fuel stock.
Co-author Takahiro Sumi, an associate professor at Osaka University
in Japan, said these free-floating planets were “very common, as common
as a regular star. The existence of free-floating planets like this is
expected from planetary formation theory. What is surprising is how
common they seem to be.”
According to astronomical convention, planets orbit a star or stellar
remnant, so if these objects do not have a host star, then they are not
technically planets, even if they may have formed in the same way as
what we call planets.
Indeed, the researchers hypothesise these objects were formed in a
planetary disc, like the planets in our own Solar System, before
gravitational forces ejected them from these systems.
Professor Joachim Wambsganss of the University of Heidelberg in
Germany, who reviewed the study for Nature, said this was the “most
plausible theory”.
However, he added there was a minority view that planets could form
the same way that stars do, but fail to reach the critical point of
thermonuclear ignition.
He too agreed the most “shocking” element of the data was the
projected frequency of such objects. Dr Martin Dominik of the University
of St Andrews in Scotland agreed, and said he would be “a bit cautious”
about the results.
“There is this theory that planets formed around a star and due to
the gravitational effects between planets, one of them gets ejected from
the system, so people have predicted that there are planets out there
that are no longer bound to stars,” he said. “But they don’t predict
this number of them.”
- BBC
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