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PLANETS like our own could be common, say astronomers in the US and
Canada.

No one has spotted Earth-like planets outside the Solar System. However, the
researchers have found a surprisingly large amount of the rocky material such
planets would be made of around stars like our Sun. “We think this is a very
exciting discovery,” says team member Brian Chaboyer of Dartmouth College in New
Hampshire.

The usual techniques for tracking down “extrasolar” planets rely on detecting
tiny changes in a star’s motion caused by the gravitational tug of an orbiting
planet. However, since stars are big and difficult to budge, this technique only
works if the planet is a giant like Jupiter.

The team, led by Norman Murray of the University of Toronto, reasoned that if
rocky asteroids—the building blocks of Earth-like planets—were
orbiting a Sun-like star, some would inevitably rain down on the star itself.
Many of the asteroids that orbit between Mars and Jupiter have suffered such a
fate.

Chaboyer and his colleagues examined the spectra of 642 nearby Sun-like stars
for signs of iron, a common constituent of rocky planets. They believed any iron
sucked up by a star would get stuck in the star’s outer “mixing” layer. “Since
this layer generally accounts for only a few per cent of the star’s mass, even a
small amount of iron ‘pollution’ should show up in the spectrum,” says
Chaboyer.

But the team had to find a way of distinguishing between iron from asteroids
and any iron the star was born with. In a paper submitted to Astrophysical
Journal, they say that because Sun-like stars of low mass have thicker
mixing layers than those of high mass, any collected iron will be more diluted
in low mass stars and the spectral signature due to iron will be weaker. If the
star’s mixing layer were thin, then the signature of iron would be stronger.
“This is exactly the pattern we found,” says Chaboyer. “It was a striking
result.” If all the iron in a star was home-grown, the thickness of the mixing
layer would not affect its iron signature.

Other astronomers urge some caution. “However, if the correlation holds up
under scrutiny, then it is clearly a reasonable hypothesis that the increased
iron comes from rocky material from which the primordial hydrogen and helium has
been stripped,” says Simon Jeffery of Armagh Observatory, Northern Ireland.

As a further test of the idea, the astronomers looked at older Sun-like stars
which had exhausted the hydrogen fuel in their cores. These are known to have
very deep mixing layers. “Sure enough, their iron was very diluted relative to
the other stars just as would be predicted if the stars were accreting
material,” says Chaboyer.

The surprising thing was that 80 to 90 per cent of the stars the team
examined appear to be accreting rocky material. “It seems that the majority of
stars like the Sun have terrestrial material around them,” says Chaboyer. “This
can only boost the chance of finding Earth-like planets.”