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Kepler mission on the verge of more planet discoveries

Posted: July 28, 2010

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The Kepler space telescope has uncovered a treasure trove of candidate planets the size of Earth circling other stars, potentially reshaping scientists' view of the universe.

Artist's concept of Kepler. Credit: NASA
Dimitar Sasselov, an astronomer at Harvard University, presented the latest round of Kepler data at the TEDGlobal conference in the United Kingdom earlier this month.

One slide of Sasselov's presentation showed a breakdown of Kepler candidate planets, showing about 140 of the potential worlds are "like Earth."

"You can see here the small planets dominate the picture," Sasselov said.

Launched in March 2009, Kepler's sensitive camera stares at a field of stars in the constellations Cygnus and Lyra. The telescope looks for tiny dips in each star's brightness, a sign something is passing between the star and the Kepler spacecraft.

Sasselov noted the targets in his presentation were just candidates, but he said the data points to an exciting possibility.

"The statistical result is loud and clear," Sasselov said. "And the statistical result is that planet like our own Earth are out there. Our own Milky Way galaxy is rich in these kind of planets."

But don't pop the cork on the champagne yet.

Credit: Dimitar Sasselov/TEDGlobal
NASA officials are rushing to define the meaning Sasselov's presentation.

"We're very happy that Kepler is making very precise measurements that allow us to identify planets as small as the Earth, although as yet, we have not talked about Earth-like orbits around sun-like stars," said Jon Morse, the director of NASA's astrophysics program. "I think some of the confusion out there is a matter of language and what people are interpreting."

William Borucki, Kepler's top scientist at the Ames Research Center, agreed that the confusion is a matter of Sasselov's choice of words.

"When an astrophysicist like Dimitar Sasselov talks about planets, he's interested in the structure of the planet," Borucki said. "He has no interest in whether there's life on it or not. That's not his business."

Media reports last week picked up Sasselov's announcement, concluding that Earths are scattered throughout the universe, potentially teeming with life.

"A lot of the misunderstanding occurred because he used the inappropriate term for talking to the public. He should not have said Earth-like. He should have said Earth-sized," Borucki said in an interview Tuesday.

There could be countless Earths scattered throughout the galaxy, but it's far too early for researchers to make any such announcement.

For one thing, Kepler is not equipped to search for life.

"The Kepler mission can measure the size of planets by the depth of the dip and the size of the star," Borucki said. "The one thing it cannot do is tell you whether the planet is Earth-like. People, when they think of Earth-like, they think of things like water, they think of reasonable temperatures and possible life. We can't do that. That's a future mission, I'm sure."

Kepler is staring at stars in the constellations Cygnus and Lyra. Credit: Carter Roberts
The telescope also is not yet able to find planets in the so-called habitable zone, a sweet spot just the right distance from a parent star.

Sasselov clarified his presentation in a blog posting Tuesday night on NASA's website.

"Indeed, Kepler has not discovered Earth-like planets in habitable zones. We have not found Earth-size planets; at this time we have found only planet candidates - 706 of them as of June 15, 2010, based on only 43 days of data with 306 released and discussed in a paper by the Kepler team," Sasselov wrote in the blog.

Kepler is currently only searching for planets with orbital periods of less than three months, so any new rocky worlds would reside far too close to the parent star to be habitable. Such planets could harbor oceans of lava.

The European Space Agency's COROT mission, another planet-hunting telescope, discovered the smallest extrasolar planet. When announced in February 2009, scientists said the COROT-7b planet is just 1.7 times the size of Earth, but it orbits hellishly close to its parent star.

Sasselov's 140 "Earth-like" planet candidates were estimated to be less than twice the size of Earth.

Borucki said Kepler scientists will expand their search for planets in the habitable zone once they finish writing an advanced software program to analyze stars over longer periods of time.

Kepler rotates on its axis every three months, meaning light from a specific star falls on a different set of CCDs inside the telescope's 95-megapixel camera.

"The different CCDs have different sensitivities, so it looks to us as if from three months to three months there's a big change in all the brightnesses of the stars," Borucki said.

A computer progarm has to stitch together observations from each three-month period to find planetary transits occurring at longer intervals.

"The computer program can't do that yet," Borucki said. So we can't find anything with significantly longer periods. We simply can't find things in the habitable zone until we finish the computer program. That's what we're working on."

Planets in more distant orbits would pass in front of the parent star at greater intervals. For example, an extraterrestial telescope in the faraway galaxy could only see Earth transit the sun once per year.

"We want to be able to say not only can we find small planets, but we can find small planets in long-period orbits, orbits that would be in the habitable zone," Borucki said.

Artist's concept of a planetary transit in front of a star. Credit: European Southern Observatory
The Kepler team's scientific prudence means it could be several years before NASA is prepared to make an Earth-shattering announcement.

"It takes an Earth a year to go around the sun, so if you're looking for an Earth-sized planet in an Earth-like orbit around a sun-like star, they want to see several transits before they can verify that it's a planet and not something else," Morse said.

Once Kepler observes a dip in light from one of its 156,000 target stars, astronomers catalog the data and use ground-based telescopes to follow up on the planet candidate.

According to Borucki, about half of Kepler's potential discoveries turn out to be false positives. Other systems often include two or three stars, and it is difficult to determine whether a single transit is from a planet or another star.

Kepler has so far found five extrasolar planets, all of which are massive "hot Jupiters" about the size of the gas giant planets in our own solar system.

Scientists hope to announce more planets this winter, Borucki said.

Before the mission launched, Kepler officials advertised finding at least 50 Earth-sized planets inside habitable zones, assuming such worlds are common.

Scientists released Kepler data on more than 150,000 stars in June, including about 300 stars with planetary candidates. Kepler officials retained data on approximately 400 stars to do their own follow-up observations with ground telescopes this summer.

"We found a lot of candidates," Borucki said. "Many of them are smaller than Neptune-sized, and that's wonderful."

The data only covers 43 days of observations because it takes about four months to process observations into usable formats, according to Borucki.