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Posted by baalke on April 11, 2007, 7:42 pm
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http://www.jpl.nasa.gov/news/news.cfm?release=2007-039
NASA Shows Future Space Telescopes Could Detect Earth Twin
Jet Propulsion Laboratory
April 11, 2007
For the first time ever, NASA researchers have successfully
demonstrated
in the laboratory that a space telescope rigged with special masks and
mirrors could snap a photo of an Earth-like planet orbiting a nearby
star. This accomplishment marks a dramatic step forward for missions
like the proposed Terrestrial Planet Finder, designed to hunt for an
Earth twin that might harbor life.
Trying to image an exoplanet - a planet orbiting a star other than the
sun - is a daunting task, because its relatively dim glow is easily
overpowered by the intense glare of its much bigger, brighter parent
star. The challenge has been compared to looking for a firefly next to
a
searchlight.
Now, two researchers at NASA's Jet Propulsion Laboratory in Pasadena,
Calif., have shown that a fairly simple coronagraph - an instrument
used
to "mask" a star's glare - paired with an adjustable mirror, could
enable a space telescope to image a distant planet 10 billion times
fainter than its central star.
"Our experiment demonstrates the suppression of glare extremely close
to
a star, clearing a field dark enough to allow us to see an Earth twin.
This is at least a thousand times better than anything demonstrated
previously," said John Trauger, lead author of a paper appearing in
the
April 12 issue of Nature. This paper describes the system, called the
High Contrast Imaging Testbed, and how the technique could be used
with
a telescope in space to see exoplanets. The lab experiment used a
laser
as a simulated star, with fainter copies of the star serving as
"planets."
To date, scientists have used various techniques to detect more than
200
exoplanets. Most of these exoplanets are from five to 4,000 times more
massive than Earth, and are either too hot, too cold or too much of a
giant gas ball to be considered likely habitats for life. So far, no
one
has managed to capture an image of an exoplanetary system that
resembles
our own solar system. Scientists are eager to take a closer look at
nearby systems, to hunt for and then characterize any Earth-like
planets
- those with the right size, orbit and other traits considered
friendly
for life.
In the lab demonstration, the High Contrast and Imaging Testbed
overcame
two significant hurdles that all telescopes face when trying to image
exoplanets - diffracted and scattered light.
When starlight hits the edge of a telescope's primary mirror, it
becomes
slightly disturbed, producing a pattern of rings or spikes surrounding
the major source of light in the focused image. This diffracted light
can completely obscure any planets in the field of view.
To address this problem, Trauger and his colleagues at JPL fashioned a
pair of masks for their system. The first, which resembles a blurry
barcode, directly blocks most of the starlight, while the second
clears
away the diffracted rings and spikes. The combination creates enough
darkness to allow the light of any planets to shine through.
"Mathematically, and sort of magically, this coronagraph blocks both
the
central star and its rings," said Wesley Traub of JPL, co-author of
the
new paper and Terrestrial Planet Finder project scientist.
Scattered light presents the additional hurdle. Minor ripples on a
telescope's mirror produce "speckles" - faint copies of a star,
shifted
to the side, which can also hide planets. In the High Contrast Imaging
Testbed, a deformable mirror the size of a large coin limits scattered
light. With a surface that can be altered ever so slightly by
computer-controlled actuators, this mirror compensates for the effects
of minor imperfections in the telescope and instrument.
"This result is important because it points the way to building a
space
telescope with the ability to detect and characterize Earth-like
planets
around nearby stars," Traub said.
For their next steps, Trauger and Traub plan to improve the
suppression
of speckles by a factor of 10, and extend the method to accommodate
many
wavelengths of light simultaneously.
More information on NASA's planet-finding missions, including
Terrestrial Planet Finder, is at http://planetquest.jpl.nasa.gov .
JPL manages the Terrestrial Planet Finder mission for NASA's Science
Mission Directorate, Washington. JPL is a division of the California
Institute of Technology in Pasadena.
------------------------------------------------------------------------
Media contact: Jane Platt 818-354-0880
Jet Propulsion Laboratory, Pasadena, Calif.
2007-039
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