BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 5, 2009
Credit: NASA
In a galaxy of 200 billion or more stars, one could argue planets like
Earth - orbiting suitable stars in habitable "Goldilocks" zones where
water exists in liquid form and the temperature is not too hot or too
cold - must be common. After all, if only a tiny fraction of the Milky
Way's stars harbor such planets, one could still expect untold
thousands, if not millions, of potentially habitable worlds. And with
numbers like that, it's not at all unreasonable to suppose life arose
on at least some of them. Extending that argument to the countless
galaxies populating the observable universe, one could easily assume
the cosmos must be teeming with life.
But statistics alone prove nothing about how solar systems might be
constructed. While more than 340 planets have been detected orbiting
other stars, the limitations of Earth-based observations mean, with a
few exceptions, only huge, Jupiter-class gas giants have been
identified to date, many of them orbiting hellishly close to their
parent stars.
Whether such seemingly strange solar system architectures are common
or whether they are simply easier to detect using current methods, is
not yet clear. Whether Earth-like planets are common or rare is simply
unknown. But it is a question with profound implications, one at the
heart of any debate about the possibility of life elsewhere in the
universe.
NASA plans to take a major step toward answering that question, one
way or the other, with launch of a Delta 2 rocket Friday carrying a
sophisticated solar-powered satellite called Kepler that is equipped
with one of the most powerful digital cameras ever built for space.
The $591 million mission is scheduled for liftoff from the Cape
Canaveral Air Force Station at 10:49:57 p.m. EST.
"To this point in time, if you asked me are there other earth's out
there, I'd say absolutely, there have to be, we can't be so special,"
said NASA science chief Ed Weiler. "But if you ask me to prove it, I
can't."
The Kepler mission "very possibly could tell us that earths are very,
very common, that we have lots of neighbors out there. Or, it could
tell us that earth's are really, really, really rare, perhaps we're
the only Earth," Weiler said. "I think that would be a very bad answer
because I for one don't want to live in an empty universe where we're
the best there is! That's a scary thought to many of us. But Kepler
will tell us that answer and it will tell us soon."
Trailing the Earth in its orbit around the sun, the Kepler spacecraft
will aim a 95-megapixel camera on a patch of sky the size of an
out-stretched hand that contains more than 4.5 million detectable
stars. Of that total, the science team has picked some 300,000 that
are of the right age, composition and brightness to host Earth-like
planets. Over the life of the mission, more than 100,000 of those will
be actively monitored by Kepler.
Kepler during launch preps in Florida. Credit: Ben Cooper/Spaceflight Now
The spacecraft's camera will not take pictures like other space
telescopes, rather it will act as a photometer and continually monitor
the brightness of candidate stars in its wide field of view and the
slight dimming that will result if planets happen to pass in front.
By studying subtle changes in brightness from such planetary transits
- comparable to watching a flea creep across a car's headlight at
night - and the timing of repeated cycles, computer analysis can
ferret out potential Earth-like worlds in habitable-zone orbits.
The probability of finding sun-like stars with Earth-like planets in
orbits similar to ours - and aligned so that Kepler can "see" them -
is about one-half of 1 percent. Given the sample size, however, that
still leaves hundreds of potential discoveries.
But it will take three-and-a-half years of around-the-clock
observations to capture the repeated cycles needed to confirm
detection of an Earth-like world.
"Kepler is designed to find hundreds of Earth-size planets, if such
planets are common around stars, dozen of these planets, if they are
in the habitable zone," said William Borucki, Kepler principal
investigator at NASA's Ames Research Center.
"If we find that many, it certainly will mean that life may well be
common throughout our galaxy because there's an opportunity for life
to have a place to evolve. If, on the other hand, we don't find any,
that will be another profound discovery. It will mean that Earths must
be very rare, we might be the only life in our Universe. In fact, it
will mean there will be no Star Trek."
But he quickly added: "We are hoping to find hundreds, of course. ...
Although Kepler will not find E.T., it is hoping to find E.T.'s home."
Jon Morse, director of astrophysics at NASA headquarters in
Washington, described Kepler as "our planetary census taker."
"We're going to get the full sweep of the types of planets in
different types of orbits around different types of stars through a
big cross-section of our galaxy," he said. "It is going to shape the
way that we formulate our plans for future missions on our quest to
find Earth-like planets and study their atmospheres and look for the
bio-markers like the types of molecules in our atmosphere that may
indicate life."
Said Michael Bicay, director of science at the Ames Research Center:
"The ramifications of the results of this mission, whatever those
results are, are going to be significant in our understanding of the
frequency of Earth-size planets in the local galaxy and the habitable
zones."
Where Kepler will look. Credit: NASA
See larger image here
Named in honor of Johannes Kepler, the 17th century Copernican
astronomer who formulated the laws of planetary motion, NASA's newest
science satellite weighs 2,320 pounds and measures 15.3 feet from top
to bottom. It is equipped with four solar panels capable of generating
1,100 watts of power, a radiation-hardened PowerPC flight computer and
a Ka-band communications link to relay science data back to Earth. The
spacecraft was built by Ball Aerospace of Boulder, Colo.
Following launch from Complex 17B at the Cape Canaveral Air Force
Station, Kepler will pass the moon's orbit in just two days as it
heads into a 371-day orbit around the sun, separating slowly from
Earth. It will aim itself at a patch of sky near the left wing of
Cygnus the Swan, midway between the stars Deneb and Vega.
And then, Kepler will simply stare at the same stars for three-and-a-half years.
The science team will check in twice a week to check the spacecraft's
health and upload commands. Once a month, Kepler will look away from
its target area, point is high-gain antenna toward Earth and downlink
stored data. Every three months, the spacecraft will rotate 90-degrees
around its long axis to keep its fixed solar panels face on to the sun
and its radiator pointed toward deep space.
Kepler's single science instrument is built around a 55-inch mirror
and a 37-inch corrector plate that represents a modified Schmidt
telescope design. Light from the primary mirror comes to a focus on an
assembly of charge coupled devices, or CCDs, similar in operation to
the chips used in commercial video and still cameras.
But unlike commercial cameras, Kepler's Focal Plane Array is made up
of 42 CCDs, each one measuring 2.32 inches by 1.1 inches and
containing 2,200 by 1,024 picture elements, or pixels, for a total of
95 million pixels. The focal plane will be maintained at minus 121
degrees Fahrenheit to improve sensitivity and minimize electrical
"noise" in the system. Special lenses will smear out the starlight
slightly to make slight changes in brightness more easily detectable.
A technician completes the installation of CCD modules into the focal
plane assembly. Credit: Ball Aerospace
"An Earth-like planet passing in front of a sun-like star is going to
cause the brightness of that star to dim by only 1 part per 10,000,"
said Natalie Batalha, a Kepler co-investigator at San Jose State
University. "That's like looking at a headlight from a great distance
and trying to sense the brightness change when a flea crawls across
the surface. But the Kepler instrument is designed to detect such
small changes in brightness."
Kepler will be aimed at the same spot in the sky, measuring 10 degrees
by 10 degrees, for the life of the mission, actively observing more
than 100,000 candidate stars. In contrast, the moon's apparent angular
diameter is 0.5 degrees. The Hubble Space Telescope focuses on areas
the size of a rice grain held at arm's length. A planetary transit
will cause the light from a target star to dim, or wink, on timescales
of one to 12 hours, depending on the type of star and the size of the
planet's orbit. For a planet like Earth passing in front of a star
like the sun, the star's light would dim by just 84 parts per million,
or less than 1/100th of 1 percent.
Kepler is capable of detecting Earth-like planets around stars ranging
from 600 to 3,000 light years away.
The Kepler team is particularly interested in planets that may orbit
within a star's habitable zone, the region around a star where water
on a planet can exist as a liquid. Habitable zones vary in location
depending on a star's size and brilliance. By timing changes in a
star's light as a transit occurs, scientists can figure out the size
of a presumed planet's orbit and thus whether it falls in that star's
habitable zone.
"The habitable zone is where we think water will be," Borucki said.
"If you can find liquid water on the surface, we think we may very
well find life there. So that zone is not too close to the star,
because it's too hot and the water boils. Not too far away where the
water's condensed and ice-covered, a planet covered with glaciers.
It's the goldilocks zone, not too hot, not too cold, just right for
life."
Brief changes in a star's light output imply an extremely hot planet
orbiting close to its parent. Longer transits imply planets at greater
distances.
"We are interested in finding planets that are not too hot, not too
cold, but just right," Borucki said.
An artist's concept of a planet passing in front of its parent star.
Credit: NASA
In its first year of operation, Kepler is expected to discover
Jupiter-class super planets orbiting close to their parent stars with
periods of just a few days. The Kepler team requires multiple transits
to make sure the photometer is not seeing some other phenomenon, like
the passage of an unseen companion star or a major disturbance in the
target star's appearance. Ground-based astronomers then will be asked
to attempt confirmation and only then will results be announced.
"After several months of data processing and confirmation by
ground-based telescopes, scientists hope to announce their first
results approximately in December 2009 at NASA Headquarters ... about
giant planets found in short-period orbits," NASA said in its Kepler
press kit.
"Discovery of Earth-size planets in Earth-like orbits requires nearly
the full lifetime of the 3.5-year mission, although in some cases
three transits are seen in just a little more than two years. Other
results that require the full 3.5 years of data are: Planets as small
as Mars in short period orbits, which utilizes the addition of dozens
or more transits to be detectable; and the detection of giant-inner
planets that do not transit the star, but do periodically modulate the
apparent brightness due to reflected light from the planet."
The first announcement of any Earth-like planets orbiting stars like
the sun is not expected before December 2012.
"What I'm hoping, expecting to see as a community scientist is in the
first six months to a year, the big, massive hot Jupiter's are going
to roll off the Kepler assembly line," said Debra Fischer, astronomy
professor at San Francisco State University and a veteran exoplanet
observer. "This is exciting because these are bizarre planets. We
don't really understand the statistics, how they form, how they moved
into their current position and just the sheer number of these objects
that Kepler will find is going to help us learn a lot about the
systems.
"And then the next class of planets I think will roll out will be
perhaps the hot Neptune's," she said. "Significantly smaller than
Jupiter, these objects are thought to exist around something like 30
percent of stars like our sun and low-mass stars. And if that is
correct, then Kepler should see quite a few of these very large,
something like 17 times the mass of the Earth, orbiting." "And then
the hardest detection and by far the most exciting is going to be the
detection of bona fide Earths - small, rocky planets, Earth-size
planets."
While such planets are often thought of as terrestrial "rocky" worlds,
"I think that the science fiction writers are going to be challenged
to imagine the diversity that we could expect to find, even in this
type of planets," Fischer said. "They may not be rocky worlds, they
may be water worlds without plate tectonics that force the landmass up
above the oceans. These could be worlds that, in fact, have life like
our oceans, OK? But perhaps not sending radio signals to us."
© 2009 Spaceflight Now Inc.
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