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NASA's Hubble Space Telescope has found the building blocks for
Earth-sized planets in an unlikely place, the atmospheres of a pair of
burned-out stars called white dwarfs. The dwarfs are being polluted by
asteroid-like debris falling onto them. This discovery suggests that
rocky planet assembly is common in stars, say researchers.
The white dwarfs reside 150 light-years away in the Hyades star
cluster, residing in the constellation Taurus the Bull. The cluster is
relatively young, only 625 million years old.
Hubble's spectroscopic observations identified silicon in the white
dwarfs' atmospheres, a major ingredient of the rocky material
constituting Earth and other terrestrial planets in our solar system.
The silicon may have come from asteroids that were shredded by the white
dwarfs' gravity when they veered too close to the stars. The rocky
debris likely formed a ring around the dead stars, which then funneled
the material onto the stellar relics.
The material detected whirling around the white dwarfs suggests that
terrestrial planets formed when these stars were born. After the stars
collapsed to white dwarfs, surviving gas-giant planets may have
gravitationally perturbed members of any leftover asteroid belts into
"We have identified chemical evidence for the Lego building blocks of
rocky planets," says Jay Farihi of the University of Cambridge in
England, lead author of a new study that appeared in the May 2 issue of
the Monthly Notices of the Royal Astronomical Society. "When these stars
were born, they built planets, and there's a good chance they currently
retain some of them. The material we are seeing is evidence of this.
The debris is at least as rocky as the most primitive terrestrial bodies
in our solar system."
Astronomers commonly believe that all stars formed in clusters. But
searches for planets outside our solar system have only detected a
handful of them orbiting cluster stars. Farihi suggested that it may be
harder to make the precision measurements needed to find extrasolar
planets in clusters because the stars are young and more active,
producing stellar flares and other outbursts.
The team, therefore, searched planets around retired cluster stars.
"Using Hubble to analyze the atmospheres of white dwarfs is the best
method for finding the signatures of solid planet chemistry and
determining their composition," Farihi explains. "Normally, white dwarfs
are like blank pieces of paper, containing only the light elements
hydrogen and helium. Heavy elements like silicon and carbon sink to the
Besides finding silicon in the Hyades stars' atmospheres, Hubble also
detected low levels of carbon, another sign of the debris' rocky
nature. Astronomers would expect carbon to be depleted or absent in
rocky, Earth-like material. Carbon is a key element that helps
astronomers determine the properties and origin of the planetary debris
raining down onto white dwarfs. It leaves fingerprints only in
ultraviolet light, which cannot be observed from ground-based
telescopes. Finding its chemical signature required Hubble's Cosmic
Origins Spectrograph (COS).
"The one thing the white dwarf pollution technique gives us that we
just won't get with any other planet-detection technique is the
chemistry of solid planets," Farihi says. "Based on the
silicon-to-carbon ratio in our study, for example, we can actually say
that this material is basically Earth-like. If you put this stuff into
the hand of a child, or an adult, and you ask them, `What is this?' Any
human being would be able to respond, ‘It's a rock!' They wouldn't need
to be a scientist. They would know exactly what it is, as it's something
familiar to all of us."
Farihi suggests that asteroids less than 100 miles (160 kilometers)
across were probably gravitationally torn apart by the white dwarfs'
strong tidal forces. The pulverized material may have been pulled into a
ring that eventually fell onto the dead stars. "It's difficult to
imagine another mechanism than gravity that causes material to get close
enough to rain down onto the star," he says.
The team estimated each asteroid's size by measuring the amount of
dust being gobbled up by the dead stars, about 10 million grams per
second, equal to the flow rate of a small river. They then compared that
data with measurements of material falling onto other white dwarfs.
The Hyades study offers insight into what will happen in our solar
system when our Sun burns out 5 billion years from now. When the Sun
exhausts its hydrogen fuel, it will puff up to a red giant and swallow
Mercury and Venus, and perhaps the Earth. As the Sun begins to eject its
outer layers, it loses mass. The balance of gravitational forces
between the Sun and Jupiter changes, disrupting the main asteroid belt.
Some of these asteroids could veer too close to the Sun, which breaks
them up. The debris could be pulled into a ring around the dead Sun,
similar to the inferred rings around the Hyades white dwarfs.
The two "polluted" Hyades white dwarfs are part of the team's search
of planetary debris around more than 100 white dwarfs, led by Boris
Gänsicke of the University of Warwick in England. Team member Detlev
Koester of the University of Kiel in Germany is using sophisticated
computer models of white dwarf atmospheres to determine the abundances
of various elements that can be traced to planets in the COS data.
The team plans to analyze more white dwarfs using the same technique
to identify not only the rocks' composition but also their parent
bodies. "The beauty of this technique is that whatever the universe is
doing, we'll be able to measure it," Farihi said. "We have been using
our solar system as a kind of map, but I don't know what the universe
does. Is there another recipe for Earth-like or habitable planets? The
chemistry can tell us. Hopefully, with Hubble and its powerful
ultraviolet-light camera COS, and with the upcoming ground-based 30- and
40-meter telescopes, we'll be able to tell a story. We hope to create a
picture of hundreds of rocky planet building blocks and tell how often
they look like Earth and how often they look different, or even exotic.
Who knows, maybe we'll find some stuff we haven't thought of yet."
Space Telescope Science Institute, Baltimore, Md.