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Posted by baalke on October 19, 2006, 12:12 pm
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http://www.spaceflightnow.com/news/n0610/18giantplants/
New insights into composition of giant planets
DIVISION FOR PLANETARY SCIENCES NEWS RELEASE
October 18, 2006
Research by 2006 Urey Prize winner, Tristan Guillot of the Observatoire
de la Cote d'Azur in Nice, provides new insights into the composition
of
giant planets.
In our Solar System, four planets stand out for their sheer mass and
size. Jupiter, Saturn, Uranus and Neptune indeed qualify as "giant
planets" because they are larger than any terrestrial planet and much
more massive than all other objects in the Solar System, except the
Sun,
put together.
According to Dr. Guillot, "the giant planets, because of their
gravitational might, they have played a key role in the formation of
the
Solar System, tossing around many objects in the system, preventing the
formation of a planet in what is now the asteroid belt, and directly
leading to the formation of the Kuiper belt and Oort cloud. They also
retain some of the gas (in particular hydrogen and helium) that was
present when the Sun and its planets formed and are thus key witnesses
in the search for our origins.
In the past few years many giant planets have also be discovered around
other stars. Those that transit, by chance, between us and their star
can be directly characterized: we can measure both their mass and size,
which allows us to infer, with the help of models, their compositions.
Including our own giant planets and the recent discoveries by ground
based surveys and Hubble Space Telescope, we now know the masses and
sizes of twenty giant planets!
By carefully studying the interior structure of these planets, T.
Guillot and his colleagues try to discover what they are made of and
how
they have evolved. This is a delicate task, involving precise
comparison
between detailed measurements of our giant planets and less precise
data
obtained for extra-solar planets. But new data are flowing in, and the
future looks even brighter: Measurements by the Cassini spacecraft are
leading to an extremely precise measurement of Saturn's gravity and
better constraints on its interior composition. The future Juno
mission,
to be launched in 2011 will yield a measurement of Jupiter's gravity
field with a precision that will be only second to that of the Earth.
The Juno spacecraft, skimming over Jupiter's cloud tops will also
measure the abundance of a key ingredient for planet formation: water
in
Jupiter's deep atmosphere. Last but not least, looking out of our Solar
System, an extrasolar planet bonanza is expected from the missions
COROT, to be launched in December this year, and Kepler, two years
later.
What can we say so far? All giant planets seem to be made of a gaseous
envelope of hydrogen and helium -the same elements that our Sun is made
of- surrounding a central dense core made probably of compressed water
and rocks. This core is at the heart of the formation of these planets.
Standard planet formation theories would predict it to be of the order
of ten Earth masses. This works for Uranus and Neptune, but not so well
for Jupiter and Saturn: Jupiter appears to have a rather small core of
a
few times the mass of the Earth, while Saturn seems to have a larger
core, around 10 to 25 times the mass of the Earth [Guillot 2005].
What about extra-solar planets? While more difficult to model,
astronomers have confirmed that they are indeed made of mostly hydrogen
and helium. But according to recent work by T. Guillot and his
colleagues, some of them possess surprisingly large cores, up to one
hundred times the mass of the Earth. Furthermore, there seem to be a
correlation between how rich the star is in elements such as iron,
silicate and oxygen and the mass of the planetary cores. This indicates
that indeed planetary cores grew by the sticking of grain particles,
but
that the process was more effective than previously thought.
Obviously, the road to a clear understanding of how planets form and
how
our Solar System originated is still a long one. But by comparing
detailed observations of the giant planets in our Solar System to the
less precise observations of distant extra- solar planets, astronomers
are hoping to directly test planet formation models and to learn more
about planets. But billions of new worlds are awaiting us in our
Galaxy.
And for sure, as many surprises!
The Harold C. Urey Prize prize is awarded annually (or less frequently)
by the Division for Planetary Sciences (DPS) of the American
Astronomical Society to recognize and encourage outstanding
achievements
in planetary science by a young scientist.
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