DARK, frozen and lifeless鈥攖hat鈥檚 the prospect for a planet forming
around a brown dwarf star. Yet these inhospitable planets could be very common,
the AAS heard.
Brown dwarfs are tens of times heavier than Jupiter but not massive enough to
ignite nuclear fusion, the energy source of stars. Astronomers can鈥檛 decide
whether these mysterious objects are failed stars or super-planets that have
escaped the planetary system of their birth. Now observations reported at
Pasadena suggest that they have more in common with stars than planets.
An international team of astronomers used the 3.5-metre New Technology
Telescope (NTT) of the European Southern Observatory in La Silla, Chile, to
study infrared light from the Orion Nebula, an active star-forming region some
1200 light years from Earth. They found more than 100 faint brown dwarfs
floating around the nebula, and 60 per cent of them turned out to emit unusual
amounts of infrared radiation, indicating that they are surrounded by discs of
warm dust.
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The Hubble Space Telescope has already photographed some of these dusty discs
at visible wavelengths. Astronomers couldn鈥檛 see what was at the centre of the
discs in the Hubble pictures, so they assumed that they were normal young stars
whose light was absorbed by dust in the discs. But dust doesn鈥檛 absorb infrared
radiation, so the central objects are visible to astronomers using the NTT. Many
of them have turned out to be low-mass brown dwarfs, according to August Muench
of the University of Florida.
Dust discs are known to form around young stars, so this provides 鈥渢he most
compelling evidence so far that brown dwarfs form as stars do鈥�, notes Geoffrey
Marcy, a planet hunter based at the University of California in Berkeley. But he
warns that this may be just one of many ways to make brown dwarfs. 鈥淧erhaps some
of them do really form around stars, just like planets,鈥� he says.
If the dust discs around brown dwarfs do regularly form planets, they will be
very different from the planets we know, says Charles Lada of the
Harvard-Smithsonian Center for Astrophysics, who was a member of the observing
team. For one thing, brown dwarfs continually cool during their lifetime, so the
planet鈥檚 climate would also grow colder with time. 鈥淚f the Earth was orbiting a
brown dwarf, it would now be just 20 degrees above absolute zero.鈥�