Stars can swallow their children
26 August 1999
PHILIP BALL
There are a lot of guilty-looking stars in the sky, according to
astronomers Mario Livio and Lionel Siess of the Space Telescope Science
Institute. They claim that as many as one in twelve stars in our galaxy
are red in the face after swallowing their own planets.
It is only four years since we have had confirmation that other
stars had planets in the first place. It was almost inconceivable that
they didn't – there is nothing special about our own star, the Sun. But
it was not until 1995 that astronomers in Switzerland saw the signature
of a planet orbiting a star other than the Sun. They inferred the
presence of the planet by the wobble that it induced in the rotation of
the Sun-like star 51 Pegasi.
The planet seemed to be about the same size as Jupiter – that is,
much bigger than Earth. But, surprisingly, it was closer to 51 Pegasi
than Mercury is to the Sun, whereas all the giant planets in our solar
system are beyond Mars and the asteroid belt. The surface temperature
on such a planet would be over a thousand degrees Centigrade.
Since then, several other stars have been found to possess
Jupiter-like planets, many of them shockingly close to their respective
suns. Now Siess and Livio have asked what fate holds for such steamy
worlds. Their predictions, to appear in the Monthly Notices of the Royal Astronomical Society, are sobering: they suggest that, as the star gets older, it will devour the nearby planets.
Stars become planet-eaters after they have burnt up most of their
hydrogen fuel, at which point their outer atmosphere of gas expands and
cools to a red glow. Such a star (a fraternity of which our Sun will
eventually be a member) is called a red giant.
If a Jupiter-sized planet is orbiting a red giant, its fate takes a
bizarre twist. As the star expands and engulfs the planet, it releases
gravitational energy and heats up. This sudden injection of heat causes
the star to throw out a shell of hot gas, which emits infrared
radiation. As a result, the star glows unusually brightly in the
infrared part of the spectrum for that stage in its evolution. Hence
stars that have committed infanticide, Siess and Livio point out, are
betrayed by the light they emit.
Around 8 per cent of stars glow in this way, which cannot easily be
accounted for otherwise. But there are other ways to recognize
planet-eaters. As the composition of a planet is likely to differ from
that of a star, the new elements acquired from a consumed planet are
evident from the gaps that they strip out of a star's spectrum. These
gaps are due to the absorption of light at well-defined wavelengths by
the elements concerned, and they leave a kind of bar-code signature in
the starlight.
For example, Siess and Livio predict the presence of an excessive
amount of lithium. They say that some evidence for this sort of
contamination has been seen for stars thought to possess planets,
suggesting that they have already devoured others. And there are many
lithium-laden giant stars already known whose enrichment in this
element has been hard to explain.
In addition, as the star sucks up the material from its planet, it
starts to spin more rapidly. Again, this would explain why some giant
stars seem to be spinning faster than we would expect. In other words,
we can recognize planet-eaters because they'll be dirty, glowing
infrared and turning fast.
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