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 simliar 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 Jupiters 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 Neptunes," 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."