The Kepler Mission: The Search for Earth-like Planets

BOULDER, Colorado - The hunt forEarth-like worlds orbiting distant suns will get a big boost next year with theliftoff of NASA's Kepler mission. That spacecraft's job isto monitor 100,000 stars in a stellar staring contest intended todetect periodic decreases in a star's brightness--a falloff of light due toplanets transiting their parent stars.

Kepler'spursuit of rocky Earth-sized planets is a step forward in taking on some toughbut major questions, such as: Are terrestrial planets common or rare? What aretheir sizes and distances?

What'smore, how often are such worlds detected in the habitable zone--the regionaround a star where liquid water should be available on a planet, perhapsmaking it a homely place for life?

Kepleris a trailblazer for other innovative searches for terrestrial planets. Futureplans of planet hunting researchers were detailed here January 26-28 at a mediaworkshop sponsored by the University of Colorado's Center for Astrobiology.

Earth trailing orbit

NASAselected 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 doggedlyadvocating the idea since 1992--under an admittedly lackluster designation ofFrequency of Earth-size Inner Planets, or FRESIP for short.

Themission was renamed after Johannes Kepler (1571-1630), the first to correctlyexplain planetary motion, thereby, becoming founder of celestial mechanics.

Launchdate for the $500 million Kepler mission is November 2008, to be lofted into aheliocentric, Earth trailing orbit by a Delta 2 booster.

Thespacecraft's operational life is four years, toting along enoughlife-sustaining expendables for six years, Borucki said. That two year missionextension, he said, would greatly enhance Kepler's ability to spot planetssmaller than Earth and reliably identify Earth-size planets in orbits correspondingto that of Mars with two year periods.

Magic moment

Inessence, Kepler is part digital camera on steroids and part light meter.Special purpose charge coupled devices (CCDs), like those utilized in homedigital camera gear, have the needed photometric attributes to take on planethunting duties.

Kepler'sphotometer is an array of 42 CCDs, acting as a single purpose scientificinstrument. The spacecraft is outfitted with a 37-inch (0.95-meter) apertureSchmidt photometer with a 55-inch (1.4-meter) primary mirror. It features afocal plane array with more than 95 million pixels that will appraise thebrightness of stars every 15 minutes.

Usingprecise photometry, the spacecraft can detect small decreases in stellarbrightness as a planet transits its star. Three transits with a consistentperiod, brightness change, and duration would provide scientists the magicmoment of proclaiming detection of an extrasolar planet.

Thephotometer will constantly gauge the brightness of 100,000 stars, searching forplanets as they pass in front of their parent star. In seeing any brightnesschange, Kepler data can be used to determine the planet's size and orbitalperiod.

Keplerwill stare at one large area of the sky in the constellation Cygnus.

BallAerospace here in Boulder is the prime contractor for NASA's Kepler Mission,building the photometer and spacecraft, as well as managing system integrationand spacecraft testing.

Valuable insight

Boruckireported that Kepler should detect numbers of terrestrial plants, many of themexpected to be within the habitable zone--if they are common. A null resultwould 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 theuniverse? This is the first part of that answer...a step."

Ofcourse, even if Kepler discovers that these planets are rare, it would providevaluable 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 haveStar Trek because they'll be nowhere to go."

TheKepler mission is a kind of spotter scope, an invaluable leg up for futureplanet searches, such as those projected for NASA's Space InterferometryMission (SIM) and the Terrestrial Planet Finder (TPF)--although both projectsappear to be in a never-never land of funding support at present.

Made in the shade

There'sa new advance in the possibility for direct observation of exoplanets, reportedWebster Cash, Director of the Center for Astrophysics and Space Astronomy hereat the University of Colorado.

Cashhas been working on an idea tagged as the New Worlds Observer, receiving earlyfinancial support and encouragement from the NASA Institute for AdvancedConcepts (NIAC).

NewWorlds Observer "is a better way to solve the problem that the TerrestrialPlanet Finder team has been struggling with," he suggested.

Theproposal calls for two spacecraft--a large, self-propelled, flower-shaped starshadeand a conventional-quality telescope positioned far apart in space.

"Thekey thing is that the starshade stops light from the star from ever gettinginto the telescope," Cash said. "This is really what you want. This is theideal goal for direct study of exoplanets."

Thehuge starshade--at least 100 feet (30 meters) from tip-to-tip--is a space-basedocculter that blocks light from a target star. "If luck is with you, you'll seea little cluster of tiny faint actual direct signal from the planetswith no interference from the big bright star," Cash noted.

Viewedas a low-cost replacement for the Terrestrial Planet Finder, Cash said the NewWorlds Observer starshade would have a price tag of some $500 million,optimized for use with about a 13-foot (4 meter) diameter telescope costingupwards of $1 billion.

"We'vegot 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."

Cashhas also sketched out a New Worlds Imager a much more complicated concept for thefuture. "It involves at least five spacecraft and has the goal of taking trueimages of the surfaces of exoplanets," he told

Avoiding wild goose chases

Thereis no doubt that technology, inventive approaches, theoretical work, along withground and space-based observations are melding to yield surprising findings inthe search for far-off worlds.

Atthe moment, some 200 extrasolar worlds have been charted, primarily huge, gasgiant planets. That number will continue to grow in years to come--as the rosterof detected planets shrink in size.

MargaretTurnbull, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, said as newinstruments return new data, extrasolar planet notions are sure to undergoadjustments, even abandonment...but sometimes reaffirmation.

"Inthe case of planet formation, observations of 'hot Jupiters' told us that ournice little theory of how to make a solar-type system of planets did notaccount for all possible scenarios," Turnbull told

"Inthat instance, the theory actually delayed the discovery of those hotJupiters," Turnbull pointed out, "because no one believed that they couldexist. Thus, very little funding was allocated for those searches andscientists who pursued that line of research were considered fools and openlymocked. Now those researchers are winning awards for their discoveries."

Turnbull,however, strongly cautioned against criticizing theorists "because we arealways walking a fine line between wanting to be creative and yet not wantingto spend our limited funding on wild goose chases."

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