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May
8, 2008: You know the planets of our solar system,
each a unique world with its own distinctive appearance, size,
and chemistry. Mars, with its bitter-cold, rusty red sands;
Venus, a fiery world shrouded in thick clouds of sulfuric
acid; sideways Uranus and its strange vertical rings. The
variety is breathtaking.
Now
imagine the variety that must exist in hundreds of
solar systems. There may be worlds out there that make Venus
seem hospitable and Uranus positively upright. Only 20 years
ago, astronomers were unsure whether any such worlds existed
beyond our own solar system. Now, they've found more than
280 of them, each with its own planetary "personality,"
each a fascinating example of what a world can be.
Above:
Artist T. Riecken's concept of planets orbiting a distant
sun-like star.
Yet
the heyday of planetary discovery is only just beginning.
This fall, astronomers will start a massive search for new
planets by observing about 11,000 nearby stars over 6 years.
This number dwarfs the roughly 3,000 stars that astronomers
have searched to date for the presence of planets. Scientists
estimate that the NASA-funded project, called MARVELS (Multi-object
Apache Point Observatory Radial Velocity Exoplanet Large-area
Survey), will find at least 150 new planets—perhaps many more.
"We're
looking in particular for giant planets like Jupiter,"
says Jian Ge, principal investigator for MARVELS and an astronomer
at the University of Florida in Gainesville. Ge likens big planets
to "beacons of a lighthouse" signaling the presence
of entire solar systems. "Once we find a big planet around
a star, we know that smaller planets could be there, too."
MARVELS
will do much more than just catalogue a few hundred more planets.
By surveying the Jupiter-like planets around such a large
number of stars, MARVELS aims to give astronomers the data
they need to test competing theories for how planetary systems
form and evolve.
To
look at so many stars, MARVELS will use a telescope that can
separately image 60 stars at a time, and this number will
eventually be increased to 120 stars. The telescope, which
will be housed at the Apache Point Observatory in the Sacramento
Mountains of New Mexico, has a 2.5 meter primary mirror and
a wide field of view that covers 7 square degrees of the sky—an
area that would appear 35 times larger than the Moon.
An
array of 60 fiber-optic threads will carry light from the
telescope's focal plane to highly sensitive interferometers.
These instruments can detect tiny changes in the frequency
of a star's light. How does this help find planets? Ge explains:
When a star is tugged to and fro by the gravity of an orbiting
planet, the star's light is shifted to and fro in frequency--an
effect called the Doppler
shift. The powerful gravity of Jupiter-sized planets exerts
a big tug on the parent star, making them relatively easy
to find using the Doppler shift method.
Right:
Each of the red fiber optic cables in the MARVELS instrument
can monitor its own star allowing astronomers to survey many
stars at once. [more]
If
Ge and his colleagues see a star's frequency slowly increasing
and decreasing in a repeating cycle over days, weeks, or months,
it's a good bet that a planet is there.
Scientists
are keen to learn what kinds of stars have orbiting gas giants.
One theory for how these planets form predicts that stars
rich in heavy elements such as silicon, oxygen, and nickel
should be more likely to have Jupiter-like planets. Imagine
a planet-forming disk surrounding such a star: The disk, like
the star itself, would be rich in heavy elements. Those heavier
elements would form rocky chunks in the disk, and these dense
chunks would collide and merge to create a "planet seed"
with strong enough gravity to gather gas around itself and
grow into a behemoth.
So
if MARVELS finds more gas giants around stars containing heavier
elements, the survey would support this theory. But some gas
giants might not need these heavy elements to form. Another
theory suggests that Jupiter-like planets can arise simply
because a disturbance in the planet-forming disk starts the
gravitational collapse of a region of gas and dust—no seed
required.
Right:
The Apache Point Observatory in New Mexico where the MARVELS
survey will take place. [more]
By
examining a large number of stars with a variety of heavy
element fractions, MARVELS may be able to distinguish between
these two ideas.
Data
from MARVELS will also shed light on other questions about
planet formation, such as how often the orbits of gas giants
migrate closer to their stars, and how planets sometimes end
up with highly eccentric orbits instead of the nearly circular
orbits predicted by theory. By surveying an unprecedented
number of stars, MARVELS could deliver the data scientists
need to find patterns about the conditions most favorable
for planet creation, knowledge that can guide future, detailed
observations of individual stars.
Follow-up
observations might eventually use space telescopes powerful
enough to make out the rough appearance of those many worlds.
The planets we know may only hint at the marvels waiting …
out there.
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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