JUST a few years ago, we knew of no planets outside the nine in our own Solar
System. Astronomers have since found scores of other planetary systems, but a
new survey suggests that we shouldn鈥檛 limit our search to the environs of other
stars. Spanish researchers say space is peppered with free-floating planets that
are not attached to any star. So many, in fact, that astronomers are having to
rethink their theories about how stars and planets form.
Last year, a team led by Rafael Rebolo at the Astrophysics Institute of the
Canary Islands first spotted planets floating light years from the nearest star
(快猫短视频, 14 October 2000, p 20).
Now the astronomers have
surveyed the number of objects of different sizes in the Sigma Orionis
cluster.
In a sample of 64 low-mass objects鈥攔anging in size from brown dwarf
stars to Jupiter-sized planets鈥攖hey found that the smaller the mass, the
more of them there were. If this relation continues down to planetary objects
too small to see, there could be hundreds of isolated planets within 30 light
years of the Sun, making them as numerous as Sun-like stars.
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Such large numbers pose a problem for the current theory of how stars and
planets form. This says that dense clouds of dust and gas collapse under their
own gravity to form stars, while planets form later in the disc of debris that
surrounds a newly formed star. Simulations suggest that objects less than a
tenth the mass of the Sun just don鈥檛 have enough gravity to directly collapse a
gas cloud. 鈥淚t鈥檚 almost impossible to make something like a planet this way,鈥�
says Rebolo.
So theorists are scrambling for new ideas. Some think isolated planets may
have formed around a star like a normal planet. 鈥淚n time they can enter unstable
orbits and be ejected,鈥� says Maria Rosa Zapatero Osorio of the University of
California at Santa Barbara. But there鈥檚 controversy over how likely this is to
happen.
Alan Boss of the Carnegie Institution in Washington DC has another idea. He
says that if massive stars form first in a gas cloud, they could radiate
strongly enough to ionise material in the remains of the cloud. Magnetic effects
could then make the ionised material spread out, helping it to fragment into
small clouds before the pieces have a chance to collapse. 鈥淵ou can make very
small objects, down to one or two Jupiter masses,鈥� says Boss.
But even this can鈥檛 explain why some clouds turn into a mixture of huge stars
and isolated planets, while others make a load of medium-sized stars. 鈥淚t
depends on the detailed dynamics of the system, and that鈥檚 hard to predict,鈥�
says Boss.