At least one
in every 100 white dwarf stars may be orbited by asteroids and rocky planets, new
observations from NASA's Spitzer Space Telescope suggest. The finding could
mean that these now dead stars once hosted solar systems similar to our own.
Also, the
work could help scientists determine whether other rocky,
Earth-like planets are orbiting around other stars.
White dwarf
stars are the compact, hot remnants left behind when stars like our own sun
reach the end of their lives.
As a
middleweight star like our sun ages, it eventually swells into a red giant
phase. Stars of this mass aren't heavy enough to end their lives in the
spectacular explosions known as supernovas; instead, they gradually expel their
atmospheres until they shrivel into the hot, dense dead stars called white dwarfs.
The
atmospheres of white
dwarfs usually consist entirely of hydrogen and helium, but sometimes
heavier elements such as calcium or magnesium are detected contaminating the
stellar material.
Data from
Spitzer suggest that at least 1 to 3 percent of white dwarf stars are
contaminated in this way.
Scientists
think that the out-of-place elements come from a gradual rain of orbiting dust
onto the sun. The dust emits infrared radiation which Spitzer
detects.
The dust is
entirely contained within what is called the Roche limit of the star, or close
enough that any object larger than a few kilometers would be ripped apart by
gravitational tides. (This is the same phenomenon that produced Saturn's rings.)
Because of the location of the dust, scientists think that the dust may
originate from rocky bodies such as asteroids (also known as minor planets)
that were torn apart in this way.
This could
mean that as many as 5 million white dwarfs in our own Milky Way are surrounded
by orbiting asteroids.
For the
asteroids to get within the Roche limit to be pulled apart at the seams, they
must be perturbed from an orbit farther out from their star — the asteroids
could be nudged by as yet unseen
planets.
Because the
white dwarfs descend from main sequence stars like the sun, the team's work,
presented at the European Week of Astronomy and Space Science conference in England, implies that at least 1 to 3 percent of main sequence stars have terrestrial
planets around them.
"In
the quest for Earth-like planets, we have now identified numerous systems which
are excellent candidates to harbor them," said study team member Jay Farihi
of the University of Leicester in England. "Where they persist as white
dwarfs, any terrestrial planets will not be habitable, but may have been sites
where life developed during a previous epoch."
The
composition of the crushed asteroids can be measured by detecting the heavy
elements present in white dwarfs, so scientists hope to learn more about the
solar systems that may once have circled around the star.
"With
high quality optical and ultraviolet observations (for example, the Hubble Space
Telescope), we should be able to measure up to two dozen different elements in
debris-polluted white dwarfs," Farihi said. "We can then address the
question, "Are there rocky extrasolar planets we find similar to the
terrestrial planets of our own solar system?"
The hunt
for other Earth-like worlds is the primary mission of the Kepler spacecraft,
which recently sent
back its first images.
advertisement
