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THE search for planets like our own that may support life could be long and
frustrating. A new study of the Orion Nebula, the nearest star-forming region to
the Earth, suggests that barren lumps of rock may be all that鈥檚 orbiting up to
90 per cent of stars.

In the past six years, relatively crude methods have turned up nearly 70
planets beyond our Solar System. So astronomers were hopeful that the galaxy
might be teeming with environments welcoming to life. But Henry Throop of the
University of Colorado in Boulder says that his model of the Orion Nebula
suggests that planet formation is far from easy.

About 20,000 young stars have formed in the Orion Nebula. 鈥淢ost of those are
low-mass stars like the Sun, but a handful are high-mass stars with a brightness
of about 1000 times the brightness of the Sun,鈥� says Throop. Such stars blast
out ultraviolet photons. 鈥淭hey act like cosmic blowtorches,鈥� he says. 鈥淭hey just
heat up and tear apart a lot of the material nearby. Specifically they destroy
the planet-forming discs around many of the low-mass stars.鈥�

Geoff Marcy of the University of California at Berkeley, who has found dozens
of extrasolar planets, agrees that nascent planets face a struggle. 鈥淭he diverse
and often harsh environments endured by protoplanetary discs may limit the types
of planets that can form in the most common star-forming regions.鈥�

In the 1990s, a team of astronomers led by Bob O鈥橠ell of Vanderbilt
University in Nashville observed these evaporating discs for the first time
using the Hubble Space Telescope. Since then, astronomers have debated whether
the discs could last long enough to form planets. In 1999, follow-up work by
O鈥橠ell and colleagues suggested not.

The evaporation rate they deduced from the discs鈥� spectra suggests they
survive around 100,000 years, says O鈥橠ell. This is far too short to form
planets, which could take between 1 and 10 million years. Since the majority of
stars probably form in regions similar to the Orion Nebula, this implies that as
many as 90 per cent of stars have no companion planets.

But Throop doesn鈥檛 think the situation is quite so bleak. His modelling
reveals that close in on the disc鈥攔oughly the distance from the Sun to
Saturn鈥攇ravity clumps dust grains together fast enough to resist the
ultraviolet blast. 鈥淵ou can make planets like the Earth, no problem,鈥� he
says.

Gas, however, would be blown away. So comets and Jupiter-like planets, which
are made of gas, cannot form. But don鈥檛 get too optimistic about finding life on
other planets quite yet. Any Earth-sized planets would form without atmospheres
and oceans, rendering them lifeless chunks of rock.

Throop points out that the known extrasolar planets broadly support his
model鈥檚 conclusions. Astronomers have only found planets around roughly 5 per
cent of the stars they observe.