Triumph of the Planet Hunters

Recent discoveries suggest that our galaxy contains billions of Earth-size worlds. Now astronomers are narrowing the search—for planets that most closely resemble ours in the particular ways that support life

BY MICHAEL D. LEMONICK

BESIDE A MODEL of the Kepler spacecraft at NASA’s Ames Research Center, William Borucki, the mission’s chief scientist, lectures on the space observatory, which orbits the sun in search of distant Earth-like planets.

LOOK UP ON A CLEAR, MOONLESS night, far from city lights, and you can see thousands of stars twinkling in the dark vault of the sky. Astronomers, of course, will assure you that the stars aren’t really twinkling. Rather, it’s an illusion caused by the shimmering of Earth’s atmosphere, something like the rippling air you see coming off the hood of a car on a hot summer day.

In fact, many stars are twinkling, if only in a way that a clear-eyed telescope orbiting high above the atmosphere can make out. More precisely, the stars are winking at us, dimming just a bit on a schedule as precise as clockwork. Others are wobbling in place, moving toward us and away, then back again with the same sort of rhythm.

The explanation for all this winking and wobbling is that most stars aren’t drifting through the Milky Way in solitude. They’re accompanied by planets, just as the sun is; those winks and wobbles are proof of that. When a planet passes precisely in front of its star from Earth’s point of view, the star dims subtly. Even when things aren’t lined up quite so perfectly, the planet’s gravity tugs the star to and fro as it orbits, causing measurable changes in starlight.

KEPLER TAKES AIM at the Milky Way. Each rectangle indicates a specific target region covered by one of the 42 paired photographic CCD elements.

By noting the winks and wobbles with exacting precision, astronomers have found thousands of alien worlds, called exoplanets, in an almost bewildering variety of sizes and orbits, since the first was discovered just two decades ago. As yet, none truly resemble our Earth—a world of similar size, orbiting at just the right distance from its star, so its temperature is neither too hot nor too cold to harbor life.

Astronomers are on the verge of finding one, though, and given how short a time they have been in the planet-hunting business, when they do, it will be an extraordinary achievement. “They discovered the first planet orbiting a sunlike star in 1995, when I was in college,” says Eric Ford, an astronomer at Penn State, “and now I get to help search for the first Earth-like planets. That’s pretty cool.”

It could also be transformative. Since ancient Greece at least, philosophers have argued the question of whether we are alone in the cosmos. While finding a second Earth won’t end that debate right away, it will take astrobiologists working at the intersection of astronomy and biology a big step closer by giving them somewhere to look.

FOR YEARS, THE ODDS-ON FAVORITE to make such a discovery was the Kepler mission, a telescope whose sole function since its 2009 launch was to stare ceaselessly at a field of some 156,000 stars located in the northern sky between the constellations Cygnus and Lyra, waiting for any to wink as a planet passes by. The project was developed by Bill Borucki at the NASA Ames Research Center in California (and was rejected four times before NASA finally gave it the go-ahead). Borucki reasoned that if you focus on just one star hoping a planet will pass it, you’ll almost certainly fail, as planet and star have to line up just right for the event to be visible. Look at many tens of thousands, however, and you’re a lot more likely to catch a planet in the act.

Kepler has succeeded, spectacularly. At last count it had discovered more than 4,000 “planet candidates,” and follow-up observations have confirmed nearly a quarter of them as bona fide planets. Still, true twins of Earth remain elusive. While Kepler identified more than 100 Earth-size planets, virtually all of them are too hot for life as we know it. Then again, extrapolating from what Kepler has seen in that small patch of sky, there could be as many as 40 billion Earths in the Milky Way alone—with the nearest no more than 12 light-years away. Of the current search field, Harvard’s Courtney Dressing, who made the calculation, says, “Statistically speaking, this is like a stroll across the park.”

Kepler also revealed the existence of a class of worlds that doesn’t exist in our own solar system: super-Earths, which lie in size midway between Earth and Neptune. It’s not yet clear whether these tend to more closely resemble Earth—mostly rock, with a relatively thin atmosphere—or Neptune, whose rocky core is surrounded by a deep shroud of gas. In all likelihood, says Andrew Howard of the University of Hawaii, the answer is both: “There are a lot of ways to make a planet twice the size of Earth,” Howard says. “For me, it’s kind of amazing that we keep expecting planetary systems like our own, and they keep turning out to be different.”

Kepler 11’s six planets orbiting their sun

Kepler 22b, the first planet found to orbit the “habitable zone,” where water can exist

Gas giant H189733b passing its star

Planet Kepler 16b circling its two stars

In fact, the galaxy is well furnished with all sorts of oddball systems. Early hints of this emerged with the discovery of the first extrasolar planet back in the mid-1990s. Found by Swiss astronomers Michel Mayor and Didier Queloz, who noticed the parent star wobbling in place, the world known as 51 Pegasi b was “just weird,” says Harvard planet hunter David Charbonneau. It was about half as massive as Jupiter but much closer to its star than Mercury is to the sun. Its year was just four days long, which theorists had considered impossible for a giant planet.

Yet as new planets trickled in over the next few years, it became clear that “hot Jupiters” weren’t uncommon at all. And when the Kepler came online, the strangeness just got stranger. In 2011, for example, the probe found a planetary system around a star, Kepler 11, that contains no fewer than six planets, all significantly bigger than Earth, and five of which were crammed inside an orbit equivalent to Mercury’s. “It’s remarkable,” astronomer Ford, a co-author of the discovery paper, told TIME . “We never expected to see something like this.”

Likewise, astronomers couldn’t have imagined Kepler 16b, a Saturn-size planet found late in 2011 that circles not one star but a pair whirling around each other in a tightly bound orbit. Theorists had doubted such worlds could exist in such a gravitationally unsettled environment, but once again the universe didn’t seem to care. “When we first saw it,” said co-discoverer Joshua Carter of the Harvard-Smithsonian Center for Astrophysics, “I thought, Wow, this is just amazing. It’s hard not to get excited. This is too much fun.”

Unfortunately, Kepler ran into a problem. In 2013, a reaction wheel, which steadies the telescope so its gaze stays solidly locked on target stars, failed. Now with only two functioning reaction wheels, Kepler no longer has the precision to discern those revealing winks. Still, terabytes of unprocessed data from the first four years of the mission continue to produce discoveries.

Maybe the most exciting: Kepler 186f, announced nearly a year after the reaction-wheel failure. It is Earth-size, almost certainly rocky, and orbits in the habitable zone of its star. At 500 light-years away, however, it’s too distant for follow-up observation—for example, to determine if it holds significant amounts of life-sustaining water. In any case, Kepler 186f isn’t likely to be the final surprise the mission uncovers. “The candidates it’s finding,” says David Latham of the Harvard-Smithsonian Center, “will keep us busy for years.”

Harvard’s David Charbonneau heads up the MEarth project, which led to the discovery of GJ 1214b.

EXOPLANET GJ 1214B, DEPICTED orbiting its small, reddish M-dwarf star, which is about 40 light-years from Earth. It is the first exoplanet whose atmosphere was analyzed.

The embarrassment of riches from Kepler is being matched by those arising from other telescopes, both in space and on the ground. The SuperWASP (for Wide Angle Search for Planets) survey, with scopes in South Africa and the Canary Islands, has found 26 planets of its own. Among them is WASP-12b, a world strangely rich in carbon and so big that it’s probably mostly gas, though there could be smaller, solid planets nearby. Raising the potential dazzle factor, co-discoverer Nikku Madhusudhan, now at Yale, told TIME, “On a carbon-rich world, you could have big landforms made of pure diamond.”

Then there’s the MEarth project, led by Charbonneau, which focuses not on sunlike stars but on the much smaller, dimmer, redder and more numerous M-dwarfs that make up 70% or more of the Milky Way. “I was taught in school that we orbit an average star,” says Charbonneau, “but it’s a lie. If the sun is a 100-watt bulb, most stars are like little Christmas lights.” That’s an advantage for planet hunters, though. A planet that passes in front of an M-dwarf blots out a bigger percentage of the tiny star’s light. Plus, the star can be made to wobble more easily.

MEarth’s biggest payoff to date is GJ 1214b, a super-Earth revealed in 2009, just a month before Kepler found its own first planet. Like the carbon planet found by SuperWASP, this one has an unusual makeup: less than a third the size of Earth, it seems to be half rock, half water. “It’s a top-of-the-top discovery in the quest for Earth-size planets,” said University of California, Berkeley’s Geoff Marcy, the world’s leading (human) planet hunter, when GJ 1214b was announced.

But even superlative discoveries are destined to be overtaken in the fast-moving world of exoplanetology. In March 2012, a European team operating a telescope in the high desert of Chile said it had found no fewer than nine super-Earths in a scan of 108 nearby M-dwarf stars, including two in stars’ habitable zones. At that rate, there should be more than 3 billion such planets in the Milky Way—a number that would have been mind-blowing a few years ago. Yet, says Harvard planet hunter John Johnson, that calculation “was greeted almost with yawns, because it’s gotten to the point where we’re bored with super-Earths.”

THE HOTTEST KNOWN PLANET in the Milky Way is WASP-12b, discovered by SuperWASP (for Wide Angle Search for Planets). The carbon-rich exoplanet is so close to its yellow dwarf star that it superheats to 2,800°F and bulges due to tidal forces.

That is almost surely an exaggeration. While astronomers are ultimately looking for a mirror Earth similar in size and composition to our own (carbon planets and giant blobs of water need not apply), it’s not clear that larger planets couldn’t be life-sustaining as well. Says Dimitar Sasselov, director of the Harvard Origins of Life Initiative, “I don’t see a dividing line [for planets friendly to life] anywhere between one Earth mass and five Earth masses and even 10 Earth masses.”

Johnson isn’t even slightly bored with planets in this size range: He’s building a “micro-observatory” atop Mount Palomar, in California, in which four modest-size telescopes will work in concert to find nearby habitable super-Earths. “We’re just going to hammer away at the nearest, brightest stars,” he says, “and basically shake the tree and see what falls out.” A follow-on space mission to Kepler, known as the Transiting Exoplanet Survey Satellite, is set to do something similar, with far greater precision.

BUT IF SUPER-EARTHS MAY harbor life, the only place we know life exists for sure is on Earth itself, so a true twin of our sustaining home base remains the smartest kind of planet to seek. Scientists are working now to isolate a promising handful of relatively nearby candidates, after which they know they will have to observe them more intensely: Winks and wobbles can prove a planet exists, but life has different telltale signs, such as ozone molecules in the atmosphere.

Problem is, those indicators can be an elusive quarry for our existing telescopes. When exoplanets first started to show up in the 1990s, NASA was on its way to a solution, announcing a mission called the Terrestrial Planet Finder (TPF), four huge space telescopes flying in tight formation out in the neighborhood of Jupiter. To astronomers’ frustration, though, that project was soon back-burnered, replaced with a far less ambitious design—a single telescope armed with technology that blots out the light of a star so planets orbiting around it can show through. Unfortunately, even this slimmed-down version has proved too expensive for a cash-strapped NASA to fund at the moment.

Astronomer John Johnson

So for now, hope rests with the James Webb Space Telescope. Slated to launch by 2018, it may be able to shoulder some of the important planet-imaging duties until a version of TPF is authorized. Given the astonishing progress planet hunters have made over the past few years in finding worlds closer and closer to the size and temperature of our own, there will almost certainly be plenty of candidates for it to look at.

Will any of them betray the presence of life? That, of course, remains an open question. But there is great satisfaction in knowing we are on track to answer it, one way or another, before much more time goes by. “Over the past 15 years,” says Johnson, “this idea has moved from the realm of science fiction to credible discussion. There are a lot of people left over from the old days who will still scoff at the notion. But we now know that the galaxy is teeming with Earth-size planets. I mean, we know this.”

PLANET 55 CANCRI E orbits its sun so closely that it heats to more than 3,000°F. Life-sustaining planets would be thousands of degrees cooler and difficult to detect, but astronomer John Johnson believes they could exist.

Goldilocks Worlds: where things are just right for life

THE ARTIST’S EYE

No, this Earth-like world does not exist, but similar ones are surely out there.

TOO CLOSE

The heat from a star can boil off water from planets that venture too close to it and warm their surfaces to deadly temperatures. Dry, airless Mercury and hothouse Venus illustrate the perils of proximity.

TOO FAR

Space is a cold place, and you don’t have to edge far from your home star before water freezes solid. Atmosphere retains heat, and Mars might have been a thriving world if it had held on to more of its air.

JUST RIGHT

Earth exists in the habitable zone, where liquid water can be present in abundance. Life as we know it can’t exist without water. Life as we haven’t imagined it is, admittedly, more of a riddle.

Three Ways to Spot Planets

1. WOBBLE

As a planet orbits, it gravitationally tugs its parent star this way and that. By measuring this motion, scientists can verify a planet’s existence and infer its mass.

2. TRANSIT

Light from even the brightest star is slightly dimmed as an orbiting planet passes in front of it. The degree of dimming indicates the size of the planet.

3. GRAVITATIONAL MICROLENSING

Gravity bends light. Hence, a planet may distort the image of its star. This reveals the existence of a planet but little more.

Planets galore, everywhere astronomers look

New Kids on the Block

NASA’S Kepler Hall of Fame: Of the more than 1,000 verified plants found by the kepler Space Telescope, eight are less than twice Earth-size and in their star’s habitable zone. All eight orbit stars that are cooler and smaller than our sun.

4,175 Candidate plants found by the Kepler Space Telescope as of January 2015. Here's the breakdown:

Exoplanet candidates are findings that have yet to be confirmed as actual exoplanet discoveries. Candidate planets are 80% to 90% likely to be verfied.

Finding a Second Earth

Lisa Kaltenegger uses data about our planet’s geology and meteorology to identify possible siblings

BY JEFFREY KLUGER

LISA KALTENEGGER at the Max Planck Institute for Astronomy in Germany, one of several sites she uses in her quest for just-right exoplanets

In a galaxy with 300 billion stars, there are surely untold billions of planets out there. Is anyone home on any of them?

Few astronomers are approaching this question as creatively as Lisa Kaltenegger, a 36-year-old exoplanet investigator who is a lecturer at Harvard University, a professor at Cornell University, and leader of a research group at the Max Planck Institute for Astronomy in Heidelberg, Germany. The focus of her work is not actually discovering exoplanets, planets orbiting distant stars; that is mostly the job of the Kepler space telescope. Rather, she and her team are modeling them—hoovering up massive amounts of data from Kepler, the Hubble Space Telescope and various ground telescopes and processing it through computer models to determine which worlds could harbor life. These days, so-called Big Data is inescapable, from algorithms that predict what you’ll buy to government surveillance that watches what you do. So it only makes sense that it could also be the key to finding extraterrestrial life.

Kaltenegger’s model is a complex one, factoring in a planet’s size, mass, composition and orbit—specifically, whether it is in the habitable zone around its star, where temperatures would remain hospitable and water would remain liquid. Just as important are the size, nature and temperature of the star itself, since those like our sun have a very different profile from, for example, a red giant’s or a white dwarf’s. Kaltenegger’s calculations even include a dash of the fantastical. “What if you have more than one host star? What if you see Tatooine?” she asks, referring to the childhood home of Star Wars hero Luke Skywalker.

All that is impressive but not groundbreaking. Where Kaltenegger shakes things up is in her use of data from the only planet in the universe that, by definition, cannot wear the exo prefix: Earth. Her models are based on data about Earth’s meteorology, geology and volcanology, plus one other important feature: its history.

Our planet, seen by extraterrestrials, would look very different depending on the moment when it is observed. Take a look at us 3.9 billion years ago, and we would have had a barren, globe-girdling ocean and an atmosphere made mostly of hydrogen sulfide, carbon dioxide and nitrogen. Not exactly the rain forest. Check back 2.4 billion years ago, and Earth’s atmosphere was mostly nitrogen, carbon dioxide and methane; blue-green algae were blooming in the seas. Not long after that, photosynthesis began flooding the atmosphere with oxygen, leading to an explosion of our modern forms of life.

Every bit of this could have been observed by faraway civilizations studying Earth with a technique known as spectral analysis. Since light coming from a planet breaks down into different wavelengths depending on the planet’s chemical composition, all you need to know is which elements are represented by which spectra, and you can pretty much figure out what’s going on in the atmosphere. In similar fashion, we are able to make observations about other worlds. “We’ve determined how this spectral fingerprint looks for a young and an older Earth,” Kaltenegger says. “We use that as an alien ID chart for other planets.”

On this score, Kaltenegger is well ahead of the curve. Telescopes can’t yet resolve exoplanets visually. In 2017, though, NASA will launch the Transiting Exoplanet Survey Satellite specifically to look for exoplanet atmospherics. Kaltenegger, as a pioneer in the field, will be one of the mission scientists. Once instruments like that and others come online, it should be even easier to find potentially life-bearing planets than astronomers thought possible.

A study released early this year by Kaltenegger, who will be one of the scientists on the mission, strongly supports this conclusion. She and Cornell research associate Ramses Ramirez considered not just planetary systems with mature stars but also those with stars in what is known as the pre-main sequence phase, when they are larger and brighter and have not yet contracted enough to ignite nuclear fusion.

During this period, which can last 2.5 billion years, life has plenty of time to emerge. Once the star contracts, the planet becomes more visible to telescopes, no longer lost in the glare of solar fires. And though the radiated warmth of the star inevitably decreases with its luminosity, any organisms that have already taken hold on the planet could adapt by moving belowground or underwater.

“Our study adds new targets to the search for habitable worlds,” Kaltenegger told Astrobiology magazine, a NASA-sponsored journal, shortly before her research was published. “On Earth, life emerged within the first billion years.” The odds, she believes, increasingly favor the possibility that it has emerged in other places—maybe many other places—as well.

“With billions of rocky worlds,” she says, “life would have to be extremely picky not to be able to evolve out there, wouldn’t you say?”