by Kate Lunau on Tuesday, February 15, 2011 11:21am
Just one generation ago, the thought of finding a planet that might support life was the stuff of science fiction. Last week, NASA scientists announced they’d discovered a whopping 1,235 potential planets orbiting faraway stars, using the Kepler space telescope. If confirmed, this would almost triple the number of known planets outside of our solar system (called “exoplanets”), which currently stands at just over 500. “What we’re anxious to learn is whether there’s other life in our galaxy,” says Kepler co-investigator Natalie Batalha. She and other members of the team are trying to learn whether planets like our own are abundant or rare. “The answer will drive all future missions,” she says.
Among Kepler’s haul were 54 possible planets in the habitable zone, where temperatures could allow for liquid water at the surface, which is necessary to support all life as we know it. Five are close in size to Earth, and orbit in the habitable zone of stars that are smaller and cooler than our sun. The rest range in size from so-called “super-Earths” (up to twice the size of our planet) to ones bigger than our solar system’s kingpin, Jupiter. Most of Kepler’s findings still need to be confirmed as actual planets, but it’s almost certain the vast majority of them will be.
The mission’s goal is to find other planets like Earth, but along the way, we’re finding all sorts of things we didn’t expect: like a system of six confirmed planets orbiting a sun-like star called Kepler-11, packed so tightly together that, according to Jack Lissauer of NASA Ames Research Center, who led the work on Kepler-11, “we didn’t know such systems could even exist.” It’s becoming clear that the universe is much more diverse, and more prolific, than we ever imagined.
Launched in March 2009, the Kepler space telescope orbits our sun and stares unblinkingly at some 156,000 stars—which range from a few hundred to a few thousand light years away—searching for the telltale winking of light that might signal a planet passing in front, like a moth flying by a porch light. It’s taking a sample from one neck of the Milky Way galaxy, from which planet hunters hope to discover whether Earth twins are statistically common or not. “We ultimately want to look for life,” says Kepler co-investigator Dimitar Sasselov, who leads Harvard University’s Origins of Life Initiative. “This is how we get to that point.”
“It’s a golden age for astronomers,” says Jaymie Matthews, a professor of astronomy and astrophysics at the University of British Columbia. “I’m personally convinced that 400 years from now, people will look back at this era—even this past decade—in the same way we look back at the times of Copernicus, Kepler and Galileo. We’re the first generation in the history of our species capable of searching for another Earth. And we’ve only had that capability for a few years.”
Imagine looking out at the Empire State building at night, with all its window shades open and the windows lit up from the inside. Now, imagine the dip in brightness that would occur if one person stood at one of those windows, and pulled down one window shade by just seven centimetres. “Kepler was designed to measure light variations to that level in a bright star,” Matthews says, “and even smaller.”
Matthews is mission scientist on MOST, Canada’s first space telescope, which was designed to take such precise measurements, too. Both MOST and Kepler use a photometer (light meter) to measure the brightness of stars. Originally intended as a one-year mission, MOST launched in 2003 and is still collecting data today. Unlike Kepler, which is a planet hunter, MOST was built specifically to study stars. With a budget just a fraction of Kepler’s, “we’re the Zellers of space telescopes,” Matthews jokes. “But we showed you can achieve this precision and that Kepler, if launched, would work.”
MOST can study about 40 stars at once, but Kepler can observe many, many more. “Kepler is the exoplanet equivalent of a long-form census,” he says. “It’s doing the demographics of 150,000 citizens of our galactic city.” Kepler continuously monitors an entire field of stars around the constellations Cygnus and Lyra. Every 30 minutes, it photographs them. (Kepler is equipped with the largest digital camera NASA has ever flown in space.) If a planet happens to be passing in front of one of these stars, Kepler might catch a wink in brightness, which can last from about an hour to half a day.
The Kepler space telescope represents a huge leap forward for planet hunters. Of the 500 exoplanets we’ve known about until now, most were discovered using what’s called the Doppler technique, which was pioneered by Canadian astronomers Gordon Walker and Bruce Campbell back in the 1970s. This method involves measuring tiny changes in a star’s spectrum of light (called Doppler shifts) to show that gravity from an orbiting planet is tugging away on it, causing the star to sway, according to the University of Toronto’s Ray Jayawardhana, author of Strange New Worlds: The Search for Alien Planets and Life Beyond our Solar System.
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Using the Doppler technique, planet hunters have found plenty of massive planets orbiting extremely close to their host stars, because they’re easier to see. But they haven’t found many smaller, cooler, rocky planets like Earth. The first exoplanet ever discovered around a sun-like star, 51 Pegasi b, was announced in 1995; it’s about the same mass and size as Jupiter. This became the model for a bizarre new type of planet, “hot Jupiters,” which look like our Jupiter but orbit extremely close to their suns. It’s hard to imagine life existing in a place like that—but, until Kepler, our ability to find planets more like our own has been quite limited.
Kepler scientists shy away from talking about the search for “Earth-like planets,” since this conjures up visions of oceans, a breathable atmosphere, maybe even some kind of native life. Our current technology isn’t good enough to detect those things from so far away. Instead, their mission is very specific: to search for Earth-size planets in an Earth-like orbit around a sun-like star, so these planets sit the right distance from their host star for liquid water on the surface.
With each month that goes by, that goal gets closer. In January 2010, Kepler announced its first five new exoplanets, all of them “hot Jupiters.” This January, it revealed its first rocky planet, named Kepler-10b. About 1.4 times the size of Earth, it’s the smallest planet ever found outside our solar system, but it’s nowhere we could possibly live. One side of the planet constantly faces its star, which it whips around in less than one Earth day. (Kepler-10b is more than 20 times closer to its star than Mercury is to our sun.) Its starlit side, scientists say, must be a lava sea.
On the hunt for other Earths, Kepler’s finding places so strange we never could have imagined them. “We started out finding planets Jupiter’s mass and size, and now we’re down to super-Earths,” Matthews says. “There’s no analog for that in our solar system—we never even thought about these things before.” Of the 1,235 potential planets Kepler has found so far, 662 are the size of Neptune, a gas giant with no solid surface. “That’s crazy, because no one really understands how Neptune formed,” says Kepler team member Sara Seager, professor of planetary science and physics at the Massachusetts Institute of Technology. But maybe Neptune-like planets are far more common than we realized.
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.”
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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.”