Dana Berry/Kepler Mission/NASA;
The Kepler-11 system, which was announced last week, sounds like something out of a science fiction paperback. All six of these planets are huddled quite close to their star. Five of the six even have orbits smaller than Mercury’s, the closest planet to our sun—making it the most densely packed solar system ever discovered. Unlike the eight major planets in our solar system, which circle the sun along slightly tilted paths, the Kepler-11 planets’ orbits are surprisingly flat and circular: “flatter than a CD case,” according to Lissauer.
These six small planets are packed so snugly together that they actually drag each other back and forth, causing their orbits (which range from 10 to 47 days for the five inner planets) to vary by as much as 20 minutes, Seager says. Earth’s year, by comparison, “doesn’t even vary by a nanosecond.” That strange dance was a boon to scientists, who—by measuring each planet’s gravitational tug on its neighbours—could calculate their masses. Once they had each planet’s radius (which the Kepler telescope finds by checking how much light it blocks as it passes a star), they could then figure out density, which hints at whether the planet is made up of gas, rock, ice, or some exotic combination.
That’s how we know the planets that orbit Kepler-11 are formed mostly of gases, with rock and maybe some iron, too. These planets might even have some water in their makeup, but it’s hard to imagine life thriving there (or life as we know it, anyway). If there is rock at the centre of one of these planets, it’s “below a massive atmosphere,” says Daniel Fabrycky, a Hubble fellow at the University of California, Santa Cruz. “If you lived on the surface, there would be crushing pressure, like living at the bottom of the ocean. And you’d not see the sun.”
The fact that scientists could confirm there were planets orbiting the Kepler-11 star, using Kepler data alone, is a huge leap forward. Confirming planets with ground-based observations can be a long and difficult task: in September, U.S. astronomers announced they’d found the first potentially habitable planet outside our solar system, Gliese-581g. Astronomers believe there could be six planets orbiting that same red dwarf star, and they’ve been observing it for 11 years. But even then, it seems, something was off. Excitement about Gliese-581g was quickly dampened after other scientists looked over the data and said they doubted the planet even existed. A distant star’s winks and wobbles might suggest it’s hosting a planet, but confirming it is “very time consuming,” says Batalha, a professor of physics and astronomy at San Jose State University, “and telescope time is hard to come by.”
Confirming all 1,235 of the possible planets Kepler has found will be a monumental job, but experts estimate that over 80 per cent of these candidates will turn out to be real planets. For William Borucki, Kepler team leader, one of the most exciting tasks will be checking out the five potential planets that are close in size to Earth, and orbit in the habitable zone of stars that are smaller and cooler than our sun. “In the coming year, we expect to go through them, and determine which we can confirm,” he says.
Some of the 54 candidates in the habitable zone might even have moons with liquid water, he suggests. As the Kepler mission progresses, “we’ll start discovering planets with longer orbital periods,” he says: planets that travel around their suns in 100 days, then 200 days. And, eventually, maybe some that take about 365 days to orbit their own sun—just like Earth. “Very importantly,” Sasselov adds, “Kepler is not finished yet.” To find an Earth-size planet orbiting a star like our sun in a one-year orbit would take three years, since three different sightings are needed to confirm it isn’t a fluke. Their signals are incredibly faint. But Kepler, these scientists believe, can find them.
It might seem myopic to hunt for other forms of life by seeking out planets that look exactly like our own. After all, we still don’t understand how life sprung up here on Earth, and we’re just beginning to learn all the surprising forms it can take on our own home planet. “We don’t have a good definition for life,” Sasselov says. “How do we search for something we cannot properly define?”
And how can we hope to understand what life might look like on planets so far away? “We can’t even begin to imagine what the possibilities are out there in the universe. But we have earthling eyes,” Batalha says. “We look around here on Earth and ask ourselves the question, ‘Where does life exist?’ It exists in every nook and cranny, but they all require liquid water.” So we’ll continue hunting for planets that could support water, like Earth, and maybe even life.
As the Kepler mission is showing us, it’s impossible to predict what we could find. This space telescope is just watching 156,000 stars “out of a couple hundred billion in the Milky Way galaxy,” Jayawardhana says. How many galaxies are out there? “Many billions and billions.”
