This is
excellent news. Space has been looking
pretty dry and we did need to get a real demonstrator. I personally think that we are going to
discover ample water but so far, it appears to be well hidden, likely by ash
and dust. At least now we have hard confirmation.
The accepted
theory for planet formation argues for self-assemblage from numerous smaller
objects. Thus finding water like this
pretty well confirms that such a model is feasible.
All Good
Pulverized
Asteroid around Distant Star Was Full of Water
The first discovery of a rocky, watery object beyond
our solar system shows how planets might get their oceans
A decimated planetary system around a distant star
holds the relics of a giant asteroid that may have once been flooded with water. The
finding offers intriguing clues as to how planets become habitable, and may
also provide an unsettling peek at what our own solar system might be in for.
This system lies 170 light-years away and is
centered on a star, GD 61, that is nearing the end of its life. It is a white
dwarf—the dense hulk left over after a star has used up its fuel for nuclear
fusion and cast off its outer gaseous layers into space. These stars start out
roughly the size of the sun, and end up condensed into a sphere about the size
of Earth.
Astronomers did a bit of planetary forensics on GD
61, which is surrounded by rubble—the remains of a large asteroid orbiting the
star that seems to have been kicked into a close orbit, where the white dwarf’s
strong gravity ripped it to shreds. Some of the asteroid’s remains are now
scattered over the surface of the star, where they show up as chemical
signatures in the light of the white
dwarf.
The researchers used the Hubble Space Telescope’s
Cosmic Origins Spectrograph to observe GD 61 and split its light into
constituent colors, revealing the chemicals it contains. They found
magnesium, iron, silicon and other heavy elements, which wouldn’t exist
naturally on the surface of a white dwarf, suggesting that they fell onto the
star from an orbiting object. The researchers also found a huge excess of
oxygen—an amount, they say, that indicates the asteroid polluting the star’s
surface was originally composed of 26 percent water. That’s pretty wet—Earth,
by contrast, is only 0.02 percent water. “This work marks the first detection
of water-rich rocks in exoasteroids, and is an important step in developing a
comprehensive picture of exoplanetary systems,” says Kevin France of the
University of Colorado at Boulder, who wasn’t involved in the research.
The find could be significant, because theorists
think Earth, having formed too close to the sun for water to survive, got its
oceans from just such large, wet asteroids that impacted it long ago. “We’ve
got the same kind of object which probably
delivered Earth’s oceans, and we found this around another star,” says
research leader Jay Farihi at the University of Cambridge in England. The
discovery, he says, is a step in the quest to find habitable worlds, and maybe
even life, beyond Earth. “This goes beyond planets in the habitable zone. We
have some actual chemistry that tells you the ingredients for habitable planets
were there.”
Some experts aren’t convinced that the oxygen found
on the surface of the white dwarf is a clear sign that water existed on an
orbiting asteroid, however. “The link of the pollution of a white dwarf to the
inventory of water in an earlier planetary system is a very interesting
scientific question still under investigation,” says exoplanet researcher Lisa
Kaltenegger of Harvard University and the Max Planck Institute for Astronomy in
Germany, who was not involved in the research. Claire Moutou, another exoplanet
specialist at the Laboratory of Astrophysicsof Marseille in France, agreed. “I
find the analysis/conclusions of the paper reasonable, as far as the amount of
oxygen available to lie in H2O molecules is concerned. The interpretation of
the origin of this water content is more speculative.”
The scientists behind the project, which is detailed
in the October 11 issue of Science,
say they took pains to verify that the chemicals they see really do prove the
destroyed asteroid had water. They observed the star GD 61 in many wavelengths
through many telescopes, including NASA’s Spitzer Space Telescope and two
instruments on the W. M. Keck Observatory in Hawaii, along with Hubble. “The
authors seem to have done a careful job of cataloguing the elements and
searching for reasons to explain away the oxygen excess,” says debris disk
expert John Debes of the Space Telescope Science Institute in Baltimore. “The
detection of hydrogen in addition to the oxygen is a really convincing
signature of water.” The finding sheds light on how
planets form and evolve, adds Brice-Olivier Demory, an exoplanet
researcher at the Massachusetts Institute of Technology who also was not
involved in the research. “This is a startling result strengthening the fact
that water can be found in a very diverse range of environments.”
And water is
just one of the mysteries about this system. Scientists don’t know for sure if
GD 61 had, or has, planets, but they say a giant planet most likely pushed the
asteroid in toward its doom near the white dwarf. And they can’t tell how big
the asteroid was that deposited this detritus on the star—based on the amount
of pollution in the white dwarf’s atmosphere, the researchers estimate the
asteroid was at least 90 kilometers wide, but could have been much larger. That
would put it in a class of objects known as minor planets, similar to Ceres and
Vesta in our own solar system, which are also thought to contain large amounts
of water stored under their rocky crusts in the form of buried ice.
The planetary graveyard around GD 61 may be a vision
of what’s to come for the sun and its planets in the far future. The sun, like
98 percent of the stars in the galaxy, will also become a white dwarf
eventually, and its ferocious gravity will probably strip Earth and other inner
planets of their heavy elements. It’s not a pretty picture, but, as Farihi
says, “we have five billion years to work on that.”
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