NASA Planet Hunter to Search Out Other Earths

This story was corrected on 4 March 2009.

PHOTO: Kim Shiflett/NASA

27 February 2009—For centuries, humans have looked up at the sky and wondered if they were alone in the universe. Now we’re one step closer to finding an answer. On 6 March, NASA’s first planet-hunting spacecraft, Kepler , will embark on a three-and-a-half-year journey in search of Earth-like planets outside our solar system. Kepler is the first space telescope capable of discovering such planets orbiting in a distant solar system’s habitable zone—the area in which liquid water, and possibly life, can exist.

”We’re at the threshold of answering questions that date back to the ancient Greeks,” says James Fanson, Kepler project manager at NASA’s Jet Propulsion Laboratory, in Pasadena, Calif. Although astronomers have found more than 330 planets in other solar systems, none of these planets have the size and location believed necessary to support life.

Named for the 17th-century astronomer who discovered the laws of planetary motion, Kepler will orbit the sun instead of the Earth. The craft will focus on about 100 000 stars between the Cygnus and Lyra constellations. (To find this area, in the Northern Hemisphere, look for a triangle of the three brightest stars in the summer sky, which make up the Cygnus constellation. If you make a square with your hands and hold them at arm’s length in the direction of the constellation, you’ll see the area Kepler will explore.) Kepler will find planets using the ”transit method,” looking for periodic slight dips in the brightness of stars that can signal a planet orbiting a star.

” Kepler was a technically difficult challenge,” Fanson says. It took nearly 25 years to go from a far-off dream to reality. The mission is the brainchild of Bill Borucki, Kepler ’s principal science investigator at NASA Ames Research Center, at Moffett Field, Calif. ”Bill proposed the mission several times, but the reviewers were skeptical,” Fanson says. ”When an Earth-size planet passes a star, brightness only dims by 84 parts per million. Many thought the technology wasn’t available to measure that, and it took years for scientists to prove it was possible.”

”We didn’t have the detector technology 25 years ago to make those kinds of measurements,” says Riley Duren, chief engineer for Kepler . But in the past decade, charge-coupled devices (CCDs) with the parameters needed for finding other Earths have been demonstrated in the laboratory. Kepler uses the latest detectors but didn’t require inventing new technologies. ”We didn’t have to reinvent the wheel,” says Duren. ”We just took the best CCDs and made custom versions, and then we built custom electronics and software to get the most out of our system.” The result? A telescope that’s the equivalent of a 95-megapixel camera—the largest ever flown in space—made up of 42 CCDs, each with a bit more than 2 megapixels.

Ball Aerospace & Technologies Corp., in Boulder, Colo., the contractor responsible for building Kepler , began work on the US $500 million mission in 2002. The engineers had to face many challenges: keeping the camera pointed accurately enough so that the stars remained motionless in the images, creating a large field of view to increase the chances of finding stars with orbiting planets, and implementing low-noise electronics to make it possible to read data from the CCDs, according to John Troeltzsch, Ball Aerospace program manager for civil space systems.

”The specification for our pointing accuracy is 9 milliarc seconds of drift over 15 minutes,” Troeltzsch says. That drift is only about 1/56th of the angle resolved by a good ground-based telescope. There aren’t many disturbances in space, except for the solar wind pushing up against Kepler ’s photovoltaic panels. To make sure the telescope doesn’t move, four CCDs in the camera monitor the position of 24 stars continuously. If those stars move in the field of view, a control system repositions the telescope.

Building a 1-meter-diameter telescope with a field of view 33 000 times as great as that of the Hubble Space Telescope was no easy undertaking, but that large field of view was absolutely necessary. Planetary orbits are randomly oriented in the sky. In order to see a transit, your line of sight must precisely match up with the plane of the orbit. The more stars you see, the more likely you are to find one with the right orbital plane. The array of CCDs in Kepler ’s camera form a focal plane of 900 square centimeters.

Behind that focal plane is a 0.5-meter box containing more than 20 000 electronic components. ”The difficulty was designing all those parts to be low noise and maintain performance in a radiation environment,” Troeltzsch says. ”It was eight years of putting designs together and seeing if they’d work.” Part of the solution was to operate the camera at –85 °C, which helped reduce the noise.

Once a month, Kepler will turn toward Earth to align a fixed high-gain antenna and transmit its data to NASA’s Deep Space Network (DSN). The DSN is an international network of antennas that supports the agency’s interplanetary spacecraft missions and radio and radar astronomy observations. Scientists at the Ames Research Center will then analyze the results to verify any possible planets. Kepler will also rotate once every 90 days to keep its solar panels directed at the sun.

Planet hunting has been around for more than a decade, but it’s been done mostly from the ground. Kepler isn’t even the first spacecraft to search for planets. In 2006, the French space agency, CNES, launched the Convection Rotation and Planetary Transits (COROT) mission, but it was never expected to find Earth-like planets. Unlike Kepler , COROT orbits Earth, which partly obscures the view of the sky from the craft. This means that COROT can observe a patch of sky for only weeks at a time, not years. ”In order to have a firm detection of a planet, we need to see at least three orbits so we can verify that we see a body moving in a fixed orbital period,” Fanson says. ”To find planets like Earth, which take a year to go around a star, you need to observe the same stars for three to four years.” Also, compared with Kepler, COROT is much smaller, less sensitive, and has a smaller field of view.

Fanson, Duren, and Troeltzsch say that even if Kepler finds no Earth-like planets, that itself will be a monumental discovery—the fact that planets like ours are rare. But the researchers don’t think that will be the case.

”I’m an optimist, and I hope the galaxy is filled with many Earth-like planets,” Fanson says, adding that the Kepler mission is the highlight of his long career at the agency.

”I’ve been working for NASA for 25 years and have been involved in many exciting missions, including being on the team that fixed the Hubble Space Telescope,” Fanson says. ”But this is the most exciting one, because we have the chance to answer a question that has been in the minds of people for as long as we have records.”