The Kepler Mission: The Search for Earth-like Planets

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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