Free-floating planets prove too light for dark matter

17 April 2000

The most sensitive search ever undertaken for small substellar
objects has revealed 13 free-floating planets and more than 100
young brown dwarf stars in an active star-forming region of the
Orion Nebula. The discovery was made using the new infrared
camera at the UK Infrared Telescope (UKIRT) in

Although the appeal of small lumps of gas and
rocks may not be immediately obvious, some astronomers hope
that such objects might help to solve one of the great mysteries of
cosmology. Invisible dark matter makes up 90% of our universe
and possible candidates include just such small, faint

Previously, only two free-floating planets had been
discovered. The fact that there are 13 in one small region of the
sky makes it likely that there be many more in other star-forming
areas across the universe. The survey also revealed more than
100 young brown dwarf stars. These dull mini-stars are too tiny
to shine brightly because their cores never heat up enough to
trigger the fusion of hydrogen. A small amount of energy is
generated by the fusion of deuterium.

Despite the large
number of objects discovered in the new survey, their total mass
is nevertheless low compared with the total mass of the stars.
Indeed, if this is a typical cluster, it is unlikely that planets and
brown dwarves contribute significantly to dark matter.

*For more dark matter developments, see feature ‘Not enough stellar objects to fill the galactic halo?’

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