A debris
disk spied recently around a distant dead star is likely the remains of an
asteroid that was vaporized when the star died, scientists say.
The
discovery, detailed in the Dec. 22 issue of the journal Science, could
be a sign of what will happen in our own solar system in a few billion years.
Because the crushed asteroid was probably gravitationally lassoed in by one or
more planets, the finding also provides evidence that planetary systems can
form around massive stars.
While analyzing
the light spectra of several hundred white dwarfs, astronomer Boris Gänsicke of
the University of Warwick discovered evidence of a cool dust cloud around the
white dwarf G29-38. White
dwarfs are the dead stellar remains of relatively small stars like our Sun that have run out of fuel and sloughed
their outer layers off into space.
G29-38
"had very, very unusual calcium emission lines in the red end of the
spectrum, which white dwarfs shouldn't have, or which most stars shouldn't have anyway," Gänsicke
told SPACE.com.
The chemical
signature of light from the white dwarf suggested it was girdled by some kind
of rotating gas disk, he said.
"It's the
first time we can really actually prove that there is a disk of debris material
going around the white dwarf," Gänsicke said.
He and his
colleagues believe the disk was created by a tidally disrupted asteroid, pulled out of its orbit by
a large object. They think the most likely scenario is that one or more planets
disrupted the asteroid's orbit, causing it to tumble closer to the star. The
star's gravity eventually ripped it apart, and the heat of the star evaporated
the debris to form a ring of rotating gas. Such a disk would be short-lived
because the material falls onto white dwarf, according to Gänsicke, so it must
have formed relatively recently.
This means
it is likely that one or more of the planets
originally surrounding the star survived the swelling red
giant phase to disrupt the asteroid, Gänsicke said.
This star
and its possible planetary system provide a model of what our solar system will look like in a
few billion years.
"It looks
similar to how our solar system will look once the Sun ends its life," Gänsicke
said.
When the Sun
becomes a red giant, it will grow to somewhere between the present-day
orbits of Earth and Mars.
"So what
will happen is that the Sun becomes a red giant, probably destroys Mercury and Venus and the Earth, but Mars, the
asteroid belt, Jupiter, Saturn, and all the other planets will
survive, and they will move maybe just a little bit further out," Gänsicke
said.
Eventually,
the Sun would become a white dwarf with asteroids and the remaining planets orbiting around it. It is
possible that Jupiter could disrupt the orbit of asteroid, causing it to fall
toward the Sun, forming the same disk that Gänsicke discovered, he said.
G29-38is now
about 75 percent of the mass of our Sun, but it was originally four or five solar
masses. Astronomers have been uncertain whether or not planets could form
around massive
stars, since they don't live as long.
The
discovery of this disk around the white dwarf is good evidence of the existence
of planets, according to Gänsicke. "While we haven't found a planet directly,
we have quite strong indirect evidence that there must be a planet," he said.
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