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Yale astronomer Debra Fischer compares planet-hunting in the 1990s to stamp collecting – finding whatever happens to come your way. But now NASA’s Kepler mission means scientists can focus their search, locating life-sustaining planets by the dozens.

The quest for intelligent life beyond our world has pestered humanity for centuries. But the Kepler mission offers a way to actually answer the question. NASA launched the Kepler Space Observatory into Earth orbit in 2009. It has since found 2,321 exoplanet candidates, meaning planets that orbit other stars in our galaxy.

The discoveries leave all of us wondering if one of those hosts intelligent life, which begs the question: What comes after Kepler?

Kepler has given scientists mountains of data to sort through in three years of planet hunting and is expected to run through 2016. The spacecraft finds exoplanets by observing stars and looking for planets passing across them, creating a kind of shadow. Scientists then study light curves in Kepler’s data, looking for more signs of a planet orbiting another star.

Fischer, an astronomy professor at Yale University who specializes in exoplanets, said planet hunting used to require long and arduous stints of observation.

“Before Kepler we would go to the observatory, we would observe a star. We would find planets. This is a very long process. So after 15 years, we had something like 600 planets,” said Fischer, one of many experts and citizen scientists around the world working with the Kepler data. “And then Kepler was launched and then in three years, [scientists] announce almost 3,000 candidates. From a standpoint of statistics, it’s completely changed."

Scientists and amateurs also are building databases to expand our understanding of the galaxy. Fischer stressed that exoplanets can help us comprehend how our solar system and Earth itself formed, and yes, even how to look for more subtle signs of intelligent life.

“I think that’s gonna be one exciting aspect in helping us understand if there’s life elsewhere,” she said, giving as an example the unique levels of oxygen in our atmosphere. “Gee, there’s something that’s there.”

Kepler has proved popular with the public, even yielding a website, PlanetHunters.org, where citizen scientists can sift through the data Kepler sends back to Earth and look for signs of an exoplanet that NASA space explorers and other experts may have missed.

Edna DeVore is the deputy chief executive officer and director of education and public outreach at the SETI (Search for Extraterrestrial Intelligence) Institute based in Mountain View, Calif.

“I think it’s popular with the public because it’s got a really simple goal,” DeVore said. “It’s looking for a planet like the Earth with water on it around a star like the sun in our galaxy. That’s an engaging goal.”

“There’s an enormous public interest in finding exoplanets,” she said.  “The big question is ‘Does E.T. have a home?’”

DeVore is a co-investigator of Kepler data at SETI and also works to expand public knowledge of the mission by encouraging citizen science.

“Beyond that,” DeVore said, “citizen science projects engage people in actively contributing to science. They’re actually using the brain power of hundreds of thousands of citizens to assist scientists in making discoveries… The human eye is a terrific device for pattern recognition, for seeing things that are harder for a computer to find.”

Apparent anomalies such as planets that orbit two stars have been confirmed as more common than rare because of the Kepler mission. They’ve been nicknamed “Tatooine” systems after Luke Skywalker’s home world in Star Wars. But they are  technically called circumbinary stars.

“Everything we knew about planet formation has been turned upside down,” Fischer said. The rationalization for looking for Earth-like planets, Fischer said, is that life started here. So, why wouldn’t it exist somewhere else?

“If you look at all the things that are required to get life started, it’s harder to imagine on a gas giant planet,” she said. “It’s not impossible. But we have a huge disadvantage because we’d be looking for something that we have no idea what it looks like.”

Jason Wright, assistant professor of astronomy and astrophysics at Pennsylvania State University, recently released a paper on a mostly ignored nearby star cluster that has proven useful in finding sun-like stars. Nearby is a relative term of course and means 1,000 light years away in this case - the distance travelled over 1,000 years at the speed of light, or 186,000 miles per second. 

“Studies of clusters give us the models that we use to tell what the stars Kepler’s looking at are really like,” Wright said.

He cautioned that not all of the stars Kepler is looking at are sun-like. Furthermore, not all exoplanets that the mission discovers are Earth-like, and even Earth-like planets may not have life.

“We don’t know if there’s any intelligent life in the galaxy,” he said. “Certainly if there’s a lot of Earth-like planets are there that tells us something about intelligent life.”

Wright is skeptical about the existence of intelligent life in our galaxy, though he said he thinks it might exist elsewhere in the universe.

“It would be much older than humanity because the galaxy is 10 billion years old and humanity" is tens of thousands of years old, he said.

It is entirely possible that an alien race of our comparable age exists in this galaxy. Hominids are millions of years old and even our own species, Homo sapiens, are believed to be 160,000 years old. However, Wright believes that, if there were intelligent beings in our galaxy, they wouldn’t be so hard to find.

“Chances are intelligent life in the galaxy [would be] billions of years old. They would have populated the galaxy by now. Their presence would be obvious.”

Other galaxies, he said, is another question.