Up to 90 per cent of stars may be orbited by nothing more than small, barren worlds, according to a new study by American astronomers.
Nearly 70 extra-solar planets have been discovered in the last six years and the relatively crude search methods have raised hopes that most stars would have planetary systems similar to ours.
But a detailed simulation of the Orion nebula, the nearest star-forming region to Earth, has confirmed that planet formation in such environments is extremely tough.
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Henry Throop, at the University of Colorado in Boulder, led the study and says the Orion nebula has made about 20,000 young stars. 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. All the photons from high-mass stars come out in the ultraviolet, so they act like cosmic blowtorches.鈥�
鈥淭hey just heat up and tear apart a lot of the material nearby,鈥� says Throop. 鈥淪pecifically they destroy the planet-forming discs around many of the low-mass stars.鈥�
Geoff Marcy, at the University of California at Berkeley, has found dozens of extra-solar planets and told 快猫短视频: 鈥淭he diverse and often harsh environments endured by protoplanetary disks may limit the types of planets that can form in the most common star-forming regions.鈥�
Time limit
In the 1990s, astronomers using the Hubble Space Telescope saw these evaporating discs for the first time. Bob O鈥橠ell, at Vanderbilt University, Nashville, made the discovery and called the star-disc systems 鈥榩roplyds鈥�, for protoplanetary discs.
Since then, astronomers have debated whether there is enough time to form planets before the discs vanish. In 1999, follow up work by O鈥橠ell and colleagues suggested not.
The evaporation rate they deduced from spectroscopic data suggested a disc survival time of around 100,000 years 鈥� far too short to form planets, which are thought to need between one and 10 million years. Since the majority of stars are considered to form in regions similar to Orion, this implied that up to 90% of stars were planetless.
Pulling together
Now Throop鈥檚 team have found a loophole. He explains: 鈥淲hen the gas in the disc get heated up, it blows away, taking small dust particles with it, just like a tornado picks up dust from the ground.鈥�
But his modelling reveals that up to 10 astronomical units away from the star, gravity clumps the dust grains together fast enough to resist the gas pressure. 鈥淵ou can make planets like the Earth, no problem,鈥� he says.
However, comets and Jupiter-like planets, which rely on the condensation of the gaseous material, cannot form. This means atmospheres and oceans are probably also prevented from forming on the Earth-like planets, rendering them gigantic lifeless chunks of rocks.
Throop points out that the known extrasolar planets broadly support his model鈥檚 conclusions, because searches for such objects find Jupiters around roughly five percent of the stars they observe.
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