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GIANT planets in faraway solar systems are marching in towards their parent
stars, and banishing small planets like Earth into outer space in the process.
That鈥檚 the conclusion of astronomers in Canada who think they know why these
giant planets end up so close to their host stars.

Around 60 extrasolar planets have been discovered so far. Most are gas giants
several times the size of Jupiter and orbiting very close to their parent
stars.

This has baffled astronomers because giant planets in our own Solar System
formed in the cold outer regions and have not shifted inwards from their
birthplace. Jupiter is about five times as far away from the Sun as the Earth
is, but in other solar systems giant planets are sometimes closer to their star
than Mercury.

One possibility was that the extrasolar giants also formed in the outer
regions of their systems, but lost energy and gravitated towards their stars
when loose gas rubbed against them. However, this would leave them in small
circular orbits. Instead, these giants seem to have very elongated or
鈥渆ccentric鈥� orbits.

Now Norman Murray of Toronto University and his team say small planets like
Earth or Mars could explain the giants鈥� close orbits. They considered a system
in which an outer giant planet completes one orbit in the time it takes a closer
small planet to orbit twice. They found that the effect of the two planets
periodically lining up is enough to force other small planets in the system into
eccentric orbits.

Eventually the orbits of the small planets become so wild that they either
swing out into space forever or fall in towards the star. At the same time, the
giant planet soaks up extra angular momentum and shifts into a tighter, more
eccentric orbit. 鈥淭his mechanism is robust and can operate in real physical
systems,鈥� says Eric Ford, who studies giant planet behaviour at Princeton
University in New Jersey.

A team led by Garik Israelian at the Astrophysics Institute of the Canary
Islands has found something similar happening in a system with at least two
extrasolar planets. The star鈥檚 atmosphere contains lithium-6, which is found in
planets but only survives in a star for a few million years (Nature,
vol 411, p 163). This suggests a giant planet sent its smaller companions
careering into the star relatively recently. 鈥淵ou don鈥檛 want to be around when
that giant planet gets moving,鈥� says Murray.

But why haven鈥檛 the giant planets in our Solar System turfed their weedy
companions out into space? Murray suspects it鈥檚 because there aren鈥檛 many small
planets inside Jupiter鈥檚 orbit, which means the process would take a very long
time鈥攍onger than the time the Sun will take to blow up into a red giant
near the end of its life. 鈥淪o I wouldn鈥檛 worry about it,鈥� he says.