It's difficult to imagine a more successful mission than NASA's Kepler telescope, which was sent to space with the goal of spotting planets orbiting distant stars. It has been phenomenally efficient at finding them. In the process, the probe has revolutionized what we know about our galaxy's population of planets. It's easy to think of Kepler itself as a key milepost in humanity's understanding of our Universe. But for at least some of the scientists who work on exoplanets, Kepler is only the first step in a plan that will take decades, and will culminate when we begin searching all the stars in our neighborhood for signs of life.

The scientists in question comprised the World Science Festival's panel on exoplanets: Kepler's Natalie Batalha, the Space Telescope Science Institute's Matt Mountain, and MIT's Sara Seager. The group gave a good background on the discovery of exoplanets, and then provided some ideas of where they thought the research was going. All the ideas ended up with the search for life on other planets.

Kepler and the exoplanet catalog

Exoplanet discoveries have become so commonplace that it's easy to forget we didn't even know any existed prior to 1995. The first discovery announced was greeted with a large dose of skepticism, as some people suspected that the variations used to infer the presence of a planet were inherent to the star itself. Over time, however, we'd built up a catalog of about 700 exoplanets, observed using a variety of methods. Most of these were in the hot Jupiter category, as these were the easiest to spot: their size meant they obscured more of their host star with each orbit, and their mass meant that the star shifted more significantly as the planets circled it.

Kepler, as Batalha described it, is a "statistical mission." Its goal is to provide a relatively complete catalog of exoplanets within a specific region of the sky (it's looking near the constellation Cygnus). It detects them by watching for the drop in light from a star that occurs as a planet passes between the star and Earth, just as Venus is doing today. This limits it to catching planets that are orbiting on a path that takes them between their star and Earth, but by correcting for that limit, we can infer how many planets there are in our galaxy.

For Jupiter-sized planets, the process is easy, as Batalha said they block about one percent of the star's light. For something like Earth, though, you'd need to spot a difference of one in 10,000. Batalha's analogy was that Kepler was looking at a 20 story hotel that had lights on in every room, and trying to catch someone dropping the blinds—by one centimeter.

Despite the challenges, Kepler has been enormously successful. It spotted over 2,000 planet candidates and saw somewhere around 90 percent of its candidates confirmed by follow-up observations. The planets themselves are now quite different. Kepler confirmed that the abundance of hot Jupiters was just a sampling artifact, and the majority of the bodies it has spotted are somewhere between Earth and Neptune in size. As time goes on, it is able to spot multiple transits by planets with orbits similar to Earth's. Batalha says it has enough fuel for another 10 years of observation, so if the money can be arranged, we should have a huge catalog of discoveries.

By imaging stellar nurseries, we've found out that every forming star starts off surrounded by a disk of material that can form planets. On average, Mountain said, it looks like every star in our galaxy has an average of at least one planet—and there are hundreds of billions of stars in our galaxy.

Looking for life

But aside from their size and distance from their host star, we can't really say a lot about these planets. "The good thing is we know they're there," Seager said. "The bad news is we don't know much about them." And the details are what will determine whether these planets fit the bill for the panelists' real interest: could they harbor life?

All of them thought we could rule out Jupiter-sized planets. Although Batalha cautioned that "you don't want to be too myopic" about what you consider a habitable planet, she wasn't optimistic. Seager didn't consider them an option at all. The reasoning is that Jupiter-sized planets have very hot interiors due to their formation and retain that even if they're distant from their host star. The atmospheric circulation is such that most of the atmosphere will eventually be swept close enough to the core for temperatures to reach the point where complex chemicals would be destroyed.

Liquid water (as it generally is) was considered an essential ingredient for life. There are other possibilities, but as Matt Mountain said, you have to be a bit like "a drunk looking for your keys," and check the obvious places first.

If we're willing to consider mini-Neptunes as candidates for habitability, then Batalha said that Kepler has found 23 planets that should be the right distance from their host stars to hold liquid water. But Seager said that most of these candidates are probably too hot. She has a graduate student who has done work indicating that the thick atmospheres retained by super-Earths and mini-Neptunes would have such high concentrations of greenhouse gasses that they would be much hotter than their distance from their star would otherwise imply. This reasoning, however, would open the possibilities for liquid water on more distant planets that Kepler might start spotting with their next data release.

If a planet could hold water, what else might we look at that would provide a hint of life? Seager was excited about oxygen, saying, "no processes we know of [other than life] can make our atmosphere 20 percent oxygen." Photosynthesis has done just that on Earth. But to spot oxygen (or some other chemical we decide to look for), we need to have a look at the atmosphere of the planet.

This is possible. As a planet passes in front of its host star, some fraction of the star's light will pass through the planet's atmosphere. Any chemicals in the atmosphere, such as oxygen, will then absorb light of specific wavelengths. By looking for the bits of light that are absorbed, we can get some sense of the composition of the planet's atmosphere. (And we'll be testing that idea during today's transit of Venus.) The problem is that this absorption will be a tiny fraction of the total light from the star. Instead of looking for someone closing their blinds in the hotel, we're probably talking about searching for an insect flying in front of the hotel window.

We have a plan

That's very, very difficult, but it's not impossible. In fact, everybody on the panel (except possibly its moderator, ABC's Dan Harris) was very keen on doing it. And they laid out a persuasive roadmap for how to get it done, one in which the Kepler mission was just the first step.

Since Kepler is a statistical mission, it should give us some sense of how many stars we'd have to look at to spot a moderately sized habitable-zone planet. We can use that information to figure out how many planets we'd have to look at to image a set number of habitable-zone planets (say, for example, 100 of them). That in turn will tell us the optical properties of the telescope we'd need to image them all with an appropriate resolution. With the optical properties, we can start designing the hardware, then figure out what it would cost to put it into space and have a look.

Based on the progression of imaging technology, the panel seemed to think that within a decade we could send up hardware that would be able to spot planets on nearby stars. And a decade after that, we can start thinking about putting up an observatory that would be capable of imaging their atmospheres, providing some hint as to whether they might harbor life. Referring to other orbiting telescopes, Mountain put the project in context: "It's an expensive thing to do, but in return you get the Universe."

"We're on the verge of actually answering this," Mountain said, and he was clearly enthralled by the prospect. At one point, he turned to the audience and said, "You could be the first generation that are around when we answer the question of 'are we alone?' There's no going back from that."

When the moderator suggested that it wouldn't matter because we couldn't really visit the planets, the whole panel suggested he rethink that. Since the search would be limited to nearby planets, Mountain said that it might be possible to build something that could get there in 50 years or so—maybe sooner, if the discovery of life inspires society sufficiently. At that point, Seager turned to the audience and asked whether anybody would be willing to take a one-way, 50-year trip to see the planet. More than a dozen hands shot up, and she said that every audience that she's gotten the chance to hit with that question had a handful of people who would be willing.