MYRIAD nomadic planets may be roaming our Galaxy free from the clutches of
parent stars. Two teams of astronomers think they have detected 25 of these
free-floating planets, and say there could be hundreds of millions of them
wandering the Milky Way.
The nomadic planets have turned up in regions of space where stars are known
to form. A star is created when a cloud of gas and dust collapses under its own
gravity, generating heat that eventually sparks nuclear reactions. But if the
collapsing cloud is less than roughly 80 times the mass of Jupiter, its nuclear
reactions quickly peter out, leaving a 鈥渂rown dwarf鈥� that then fades and cools.
Planets, however, form when clouds smaller than 14 Jupiter masses
collapse鈥攖oo small to start a nuclear reaction.
Philip Lucas of the University of Hertfordshire and Patrick Roche of Oxford
University looked for signs of brown dwarfs and planets in a region around 3
light years across in the Trapezium cluster, part of the Orion nebula. They used
the United Kingdom Infrared Telescope on Mauna Kea, Hawaii, which can reveal
warm objects. 鈥淭hey are only bright and easy to see when they鈥檙e very young and
still have some heat from their formation,鈥� says Lucas.
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The astronomers found hundreds of infrared point sources. They worked out
their masses by comparing their brightness at different wavelengths with
theoretical predictions of how different masses of collapsing balls of gas
should look.
This revealed that around 150 objects were brown dwarfs. But, intriguingly,
they found 13 objects that seemed to be only 5 to 13 times as heavy as
Jupiter鈥攍ight enough to be planets. However, unlike the planets of the
Solar System, they did not form while orbiting a star. 鈥淭hese are no ordinary
planets鈥攖hey鈥檙e on their own,鈥� says Lucas. His results will appear in
Monthly Notices of the Royal Astronomical Society.
Using a similar method, a team led by Rafael Rebolo at the Institute of
Astrophysics of the Canaries in Tenerife has discovered a dozen nomadic planets
in another region of Orion. These have masses in a similar range to those found
in the Trapezium cluster.
Japanese researchers found hints of two nomadic planets in 1998. But the
sheer number in the new surveys is the first evidence that they are ten a penny
in the Milky Way. 鈥淭he number of free-floating planets in this mass range could
amount to several hundred millions,鈥� says Rebolo.
Lucas adds that life is unlikely to have emerged on any of these gas giants,
which probably have low levels of carbon and oxygen. Both teams plan to confirm
their discoveries by studying the spectra of the objects to see if their surface
temperatures and gravity match theoretical predictions.