BOULDER, Colorado - The hunt for
Earth-like worlds orbiting distant suns will get a big boost next year with the
liftoff of NASA's Kepler mission. That spacecraft's job is
to monitor 100,000 stars in a stellar staring contest intended to
detect periodic decreases in a star's brightness--a falloff of light due to
planets transiting their parent stars.
Kepler's
pursuit of rocky Earth-sized planets is a step forward in taking on some tough
but major questions, such as: Are terrestrial planets common or rare? What are
their sizes and distances?
What's
more, how often are such worlds detected in the habitable zone--the region
around a star where liquid water should be available on a planet, perhaps
making it a homely place for life?
Kepler
is a trailblazer for other innovative searches for terrestrial planets. Future
plans of planet hunting researchers were detailed here January 26-28 at a media
workshop sponsored by the University of Colorado's Center for Astrobiology.
Earth trailing orbit
NASA
selected Kepler in late 2001 as a Discovery-class mission. But William Borucki,
the mission's principal investigator at NASA's Ames Research Center, Moffett Field, California, had been doggedly
advocating the idea since 1992--under an admittedly lackluster designation of
Frequency of Earth-size Inner Planets, or FRESIP for short.
The
mission was renamed after Johannes Kepler (1571-1630), the first to correctly
explain planetary motion, thereby, becoming founder of celestial mechanics.
Launch
date for the $500 million Kepler mission is November 2008, to be lofted into a
heliocentric, Earth trailing orbit by a Delta 2 booster.
The
spacecraft's operational life is four years, toting along enough
life-sustaining expendables for six years, Borucki said. That two year mission
extension, he said, would greatly enhance Kepler's ability to spot planets
smaller than Earth and reliably identify Earth-size planets in orbits corresponding
to that of Mars with two year periods.
Magic moment
In
essence, Kepler is part digital camera on steroids and part light meter.
Special purpose charge coupled devices (CCDs), like those utilized in home
digital camera gear, have the needed photometric attributes to take on planet
hunting duties.
Kepler's
photometer is an array of 42 CCDs, acting as a single purpose scientific
instrument. The spacecraft is outfitted with a 37-inch (0.95-meter) aperture
Schmidt photometer with a 55-inch (1.4-meter) primary mirror. It features a
focal plane array with more than 95 million pixels that will appraise the
brightness of stars every 15 minutes.
Using
precise photometry, the spacecraft can detect small decreases in stellar
brightness as a planet transits its star. Three transits with a consistent
period, brightness change, and duration would provide scientists the magic
moment of proclaiming detection of an extrasolar planet.
The
photometer will constantly gauge the brightness of 100,000 stars, searching for
planets as they pass in front of their parent star. In seeing any brightness
change, Kepler data can be used to determine the planet's size and orbital
period.
Kepler
will stare at one large area of the sky in the constellation Cygnus.
Ball
Aerospace here in Boulder is the prime contractor for NASA's Kepler Mission,
building the photometer and spacecraft, as well as managing system integration
and spacecraft testing.
Valuable insight
Borucki
reported that Kepler should detect numbers of terrestrial plants, many of them
expected to be within the habitable zone--if they are common. A null result
would mean Earths in the habitable zone are rare in our galaxy.
"Ultimately",
Borucki observed, "what we're asking is...what is the place of mankind in the
universe? This is the first part of that answer...a step."
Of
course, even if Kepler discovers that these planets are rare, it would provide
valuable insight about the origin of our Earth.
However,
that notion doesn't sit well with Borucki. "If we don't find them we can't have
Star Trek because they'll be nowhere to go."
The
Kepler mission is a kind of spotter scope, an invaluable leg up for future
planet searches, such as those projected for NASA's Space Interferometry
Mission (SIM) and the Terrestrial Planet Finder (TPF)--although both projects
appear to be in a never-never land of funding support at present.
Made in the shade
There's
a new advance in the possibility for direct observation of exoplanets, reported
Webster Cash, Director of the Center for Astrophysics and Space Astronomy here
at the University of Colorado.
Cash
has been working on an idea tagged as the New Worlds Observer, receiving early
financial support and encouragement from the NASA Institute for Advanced
Concepts (NIAC).
New
Worlds Observer "is a better way to solve the problem that the Terrestrial
Planet Finder team has been struggling with," he suggested.
The
proposal calls for two spacecraft--a large, self-propelled, flower-shaped starshade
and a conventional-quality telescope positioned far apart in space.
"The
key thing is that the starshade stops light from the star from ever getting
into the telescope," Cash said. "This is really what you want. This is the
ideal goal for direct study of exoplanets."
The
huge starshade--at least 100 feet (30 meters) from tip-to-tip--is a space-based
occulter that blocks light from a target star. "If luck is with you, you'll see
a little cluster of tiny faint planets...an actual direct signal from the planets
with no interference from the big bright star," Cash noted.
Viewed
as a low-cost replacement for the Terrestrial Planet Finder, Cash said the New
Worlds Observer starshade would have a price tag of some $500 million,
optimized for use with about a 13-foot (4 meter) diameter telescope costing
upwards of $1 billion.
"We've
got all the technology to do this today," Cash added. "If NASA were to fund it,
we would be able to do this in seven years."
Cash
has also sketched out a New Worlds Imager a much more complicated concept for the
future. "It involves at least five spacecraft and has the goal of taking true
images of the surfaces of exoplanets," he told SPACE.com.
Avoiding wild goose chases
There
is no doubt that technology, inventive approaches, theoretical work, along with
ground and space-based observations are melding to yield surprising findings in
the search for far-off worlds.
At
the moment, some 200 extrasolar worlds have been charted, primarily huge, gas
giant planets. That number will continue to grow in years to come--as the roster
of detected planets shrink in size.
Margaret
Turnbull, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, said as new
instruments return new data, extrasolar planet notions are sure to undergo
adjustments, even abandonment...but sometimes reaffirmation.
"In
the case of planet formation, observations of 'hot Jupiters' told us that our
nice little theory of how to make a solar-type system of planets did not
account for all possible scenarios," Turnbull told SPACE.com.
"In
that instance, the theory actually delayed the discovery of those hot
Jupiters," Turnbull pointed out, "because no one believed that they could
exist. Thus, very little funding was allocated for those searches and
scientists who pursued that line of research were considered fools and openly
mocked. Now those researchers are winning awards for their discoveries."
Turnbull,
however, strongly cautioned against criticizing theorists "because we are
always walking a fine line between wanting to be creative and yet not wanting
to spend our limited funding on wild goose chases."
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