LONG BEACH,
Calif. – New observations of chewed-up asteroids around old dead stars called
white dwarfs bolster the idea that the Earth and other rocky planets in our
solar system are far from alone in the universe.
Astronomers
used NASA's Spitzer
Space Telescope to peer at six such white dwarf stars and found the
signature of asteroid debris circling the stars. An analysis of the light
coming from the systems show the rings are made of some of the same materials
as rocky bodies in our own solar system.
"It
strengthens suspicions that Earth-like planets are common," said Michael
Jura of UCLA here Monday at the 213th meeting of the American Astronomical
Society.
Planet
building
Asteroids
and planets form from the dust and gas that swirls around young stars. As the
dust sticks together, it forms clumps that eventually become full-fledged
planets, according to the leading theory. Asteroids are the "leftover
building blocks that didn't get incorporated in the planets," Jura said.
As stars
like our own sun near the end of their life, they puff up into red giants that
consume their innermost planets and jostle the orbits of outer planets and asteroids.
Eventually the stars blow off their outer layers and shrink down into white
dwarfs.
Occasionally,
a perturbed asteroid will wander too close to the white dwarf, whose gravity
rips the rocky body to shreds, forming debris. (This is similar to what
happened to Comet Shoemaker-Levy 9 when it was torn
apart by Jupiter's gravity before impacting the planet in 1994.)
Spitzer had
previously observed shredded asteroid pieces around two white dwarfs; the new
observations bring the total count (of white dwarfs with asteroid debris) to
eight.
"Now
we've got a bigger sample of these polluted white dwarfs, so we know these
types of events are not extremely rare," Jura said, noting that they about
1 percent of white dwarfs are estimated to have these signatures.
The
details
All eight
white dwarf systems observed showed signatures of a glassy silicate mineral
similar to olivine, which is common on Earth.
"This
is one clue that the rocky material around these stars has evolved very much
like our own," Jura said.
The spectra
(light broken into its components) of the asteroid debris also showed no carbon
signature – what Jura calls a "carbon desert" – another similarity to
the asteroids and rocky planets in our own solar system, which have relatively
little carbon.
A
single asteroid is thought to have broken apart within the last million
years or so in each of the eight white-dwarf systems. The biggest was once
about 124 miles (200 km) in diameter, a bit larger across than Los
Angeles County.
Continued
observations of this white dwarf debris could help astronomers further figure
out the composition of exoplanets, which so far has proven difficult.
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