Alan MacRobert Sky and Telescope
 An
artist's concept of a rogue giant planet plunging into the star
HD 82943. Copyright 2001 Lynette R. Cook.
Using two
of astronomy's most powerful new instruments, European researchers
have found telltale evidence that an innocent-looking Sun-like
star in Hydra swallowed a planet sometime in the past. The evidence
is the rare isotope lithium-6, which the astronomers detected
in the star's atmosphere. Normally this form of lithium is quickly
consumed by nuclear reactions during a star's youth. The only
plausible way it could show up in this star's surface, write Garik
Israelian (Astrophysics Institute of the Canary Islands) and three
colleagues in tomorrow's issue of Nature, is if planetary-type
material fell in after the star was fully formed and its internal
layers had settled into their final configuration. One or more
giant planets totaling about two Jupiter masses would do the job.
So would a terrestrial planet having three Earth masses or most
likely, three Earths' worth of asteroids and comets.
This is the
first time lithium-6 has been definitely found in a star with
a composition similar to the Sun's. The astronomers used the high-resolution
UVES spectrograph on the 8.2-meter Kueyen telescope at the European
Southern Observatory's Very Large Telescope to tease the spectral
signature of lithium-6 from that of the more common lithium-7.
Lithium-7 is generally destroyed in a star's interior as well,
but less easily; the evidence it presents is open to interpretation.
By contrast, lithium-6 is a smoking gun.
The star is
HD 82943, a 6th-magnitude G0 star older than the Sun and located
90 light-years away. It was already known to have at least one
giant planet orbiting it, and just last month a second (still
disputed) body was announced.
The possibility
that HD 82943 swallowed a third planet, or pieces of one, comes
as no great surprise. Theorists modeling the formation of planetary
systems find that planets tend to spiral inward while they are
still embedded in the massive disk of gas and dust that gave them
birth. However, that process may occur too soon for any lithium-6
to survive the star's unsettled early life. A different inward
route is suggested by the fact that the system's planets have
eccentric (elliptical) orbits. This may be a sign that they went
through chaotic interactions with other planets in ages past.
According to Alessandro Morbidelli (Observatoire de la Cote d'Azur),
planets with such eccentric orbits could toss lingering asteroids
into the star. In fact, some 25 percent of the primordial asteroid
belt was thrown into the Sun. This latter scenario could happen
late enough for lithium-6 to survive in the star's atmosphere.
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