Science & Tech

Kepler starts search for other Earths

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Harvard-Smithsonian scientists part of NASA telescope mission

As NASA’s Kepler space telescope this week begins scanning the Milky Way for planets that might harbor life, scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) are keeping their fingers crossed and waiting for the data to start flowing.

The information stream is their reward for years of work dedicated to getting the space telescope off the ground and on the job. Kepler, which Astronomy Professor and Co-Investigator Dimitar Sasselov described as a giant space camera, was launched March 6 atop a Delta II rocket from Cape Canaveral in Florida.

Kepler’s primary mission is the detection of habitable planets orbiting other stars. The spacecraft’s exquisitely sensitive camera is able to detect the slight dimming of distant starlight that indicates a planet is passing between the star and the camera. The amount of starlight thought to be obscured by a small, rocky planet like Earth is so tiny — 10 parts in a million — that detecting it has proved impossible for ground-based telescopes.

With Kepler’s ultrasensitive equipment freed from the Earth’s obscuring atmosphere, astronomers on the project are confident that they will finally answer the question of whether planets like Earth, with conditions conducive to life as we know it, are common or rare in the universe.

John Geary, another co-investigator based at the Harvard-Smithsonian Center for Astrophysics, has been working furiously since the launch to get the telescope commissioned for its science mission, analyzing image data and serving on technical review committees to get the instrument running smoothly. During a video conference call Monday (May 11), NASA gave the go-ahead to begin the telescope’s scientific mission. Observing was set to begin Tuesday, Geary said.

Since the telescope’s launch, Kepler has traveled more than 3 million miles into space, trailing the Earth on its orbit around the sun. Geary, an expert in the CCD (charge-coupled device) technology that will capture the images taken by Kepler, has been involved in the project for the past decade. Since his involvement has mainly been with the mission’s hardware, now that everything checks out, he expects his role in the telescope’s ongoing mission to be reduced.

“Finally, it’s there,” Geary said. “It’s as good as it’s going to get.”

Seven faculty members and researchers at the CfA are part of Kepler’s 46-person scientific team, including Sasselov, Geary, and co-investigator David Latham, senior lecturer on astronomy. Andrea Dupree and Soren Meibom are members of the science working group, while Lecturer on Astronomy Matt Holman and David Charbonneau, the Cabot Associate Professor of Astronomy, are participating scientists. Sasselov and Latham sit on the five-member Kepler Science Council that oversees the project. The project’s principal investigator, William Borucki, and deputy principal investigator, David Koch, are based at NASA’s Ames Research Center in California.

For the next several years, Kepler will observe stars in the Cygnus-Lyra region of our Milky Way galaxy. Though some of the stars are 3,000 light-years away, the Milky Way is so vast that scientists consider them to be Earth’s neighbors.

The discovery of planets around other stars, also called “extra solar planets” or “exoplanets,” has grown rapidly since 1995, when the first planet was discovered around a star similar to our own sun. To date some 350 planets have been discovered orbiting other stars.

Most of those planets, however, have been large gas giants like Jupiter or Saturn, easier to detect because of their size and mass.

There are two major ways that astronomers detect exoplanets. The most successful has been an indirect technique called the “radial velocity” method. As a planet orbits a star, it exerts a gravitational pull that causes the star to wobble slightly in its orbit, moving it slightly closer to Earth and then farther away. This wobble can be detected as a shift in the wavelength of the light reaching the Earth, analogous to the Doppler shift one hears from a passing train.

The second method is the “transiting” method used by Kepler where the planet dims the star’s light as it crosses in front of it.

Researchers led by Latham have spent years winnowing out the 14 million stars in Kepler’s field of view to about 150,000 candidates most likely to have habitable planets orbiting them. The catalog of those stars took five years of work and eliminates those that are too young and hot, or too old and cool, or which have other characteristics that either make them unlikely to have planets harboring life or that make it unlikely Kepler will detect them.

Of those 150,000 stars, just a small percentage will be oriented so that their planets cross in front of Kepler’s field of view. Even with that taken into account, with such a large sample to start from, there should be more than 1,000 star systems with the proper orientation, Geary said.

Latham, who has been involved in Kepler for more than a decade, said a significant part of the challenge will come after the planets have been detected. Exoplanet research so far has indicated that nine out of 10 detections of likely planets turn out to be false. Weeding out these false positives will be an important part of the mission, he said.

Like Geary, Latham said there were a few moments of anxiety when the rocket carrying the telescope launched, even though Delta II rockets have proven quite reliable.

“There’s always that moment of doubt when you see them light the fuse,” Latham said.

Part of the follow-up will involve the use of a second space telescope, Spitzer. After five and a half years of observing the universe in infrared wavelengths, Spitzer’s primary mission is ending, freeing up valuable telescope time for other purposes. Charbonneau proposed a new project to both the Kepler and the Spitzer teams that would use Spitzer to confirm likely planet detections from Kepler. Spitzer’s detections would come through the radial velocity method, providing confirmation not just by a different telescope, but also through a different technique.

“Kepler is absolutely revolutionary,” Charbonneau said. “It will take the first census of habitable worlds in the galaxy and find out whether habitable planets like Earth are relatively common or whether Earth is a cosmic rarity.”

Astronomers are looking for planets within a star’s “habitable zone” where water on the planet would be liquid. They can tell by the speed with which a planet transits a star’s surface how fast it is moving and how close to the star it is. For a star the size of our sun and a planet the size of the Earth, a favorable orbit would be similar to Earth’s, with a transit across the sun once a year. To confirm the planet’s presence, Kepler will watch the same section of the sky continuously for the duration of its mission, allowing detection of five or six transits, for example, of a planet like Earth.

Given the accelerating pace of exoplanet discovery in recent years, astronomers on the project are confident they will find a significant number of potentially habitable planets. Even if they don’t, however, Kepler’s mission design and large sample size will help answer the question of whether planets like Earth are common or not.

Sasselov said he expects some planets — though likely not habitable — to be found almost right away. Short-period planets that orbit very fast and close to their stars will likely be discovered within the first few months. Analysis of the data, however, will delay the first announcement of Kepler’s results until the end of the year or early 2010, he said.

In addition to finding new planets, Sasselov said that three previously discovered planets — including one announced by the CfA in January — are in Kepler’s field of view and can be studied in greater detail.

“It took years of planning through many interesting turns on the road,” Sasselov said. “I’m very confident Kepler will have a good yield.”