Telescope sets the stage for a decades-long search for life on other planets.
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.