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Just 20 years ago, astronomers didn’t know if there were any planets at all outside our own solar system—whether other places like Earth, which is brimming with life, are common, exceedingly rare or even non-existent. Two years ago, NASA scientists announced that, using the powerful Kepler space telescope, they’d found well over 1,000 new planets, more than doubling the number they’d previously known about. It was a stunning revelation, but few people realized, even then, that this was just the beginning.
Astronomers now believe our galaxy alone is filled with literally billions of planets—maybe even more planets than stars. There are at least 100 billion planets in the Milky Way, and some think that estimate is conservative. Some are more bizarre than anything dreamed up in science fiction: diamond worlds and double-sunned worlds, and worlds where another planet hangs in the sky like our moon. Others are eerily similar to Earth. A few of them, like a newly found planet orbiting Alpha Centauri, just 4.3 light years away, are tantalizingly close. That planet is nearer to its host star than Mercury is to our sun, and would be blisteringly hot—far too hot for life as we know it. But where there’s one planet, there are often several, and astronomers are scouring the skies around Alpha Centauri for more worlds in our own cosmic backyard.
Given the breathtaking pace of discovery, it seems only a matter of time before we find another Earth. Maybe, one day, we could even pay it a visit. With current technology, it would take us about 75,000 years to reach our nearest star, but a small team of scientists and engineers is working on next-generation starships, using exotic propulsion systems to put these far-off worlds within reach. Achieving interstellar travel would be a feat unparalleled in history, and a number of people believe it’s impossible. But planet hunting was once seen as fringe, too, and now it’s changed our understanding of the universe.
The view of the night sky will never be the same. “Look down at this ball of rock beneath your feet,” says François Fressin of the Harvard-Smithsonian Center for Astrophysics. “Now look up at the starry night. Planets are everywhere,” orbiting all those points of light.
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On Jan. 7, at the meeting of the American Astronomical Society (AAS) in Long Beach, Calif., Kepler scientists announced their latest findings: 461 new “planet candidates” (which have yet to be confirmed as planets, but almost certainly will be), including four that are less than twice the size of Earth and orbit in their sun’s habitable zone, where liquid water could exist at the surface. The Kepler space telescope, which orbits our sun, is taking a galactic census, to help us answer the question of whether there’s life elsewhere in the Milky Way. Since its mission began, in May 2009, it’s pinpointed 2,740 potential planets orbiting 2,036 stars. Kepler finds them by monitoring 156,000 stars, watching for tiny dips in brightness that could indicate a planet has passed by (called a “transit”). This tells astronomers about the size of a planet, and about its distance from the host star, depending on the length of time between each wink.
To know a planet is really there, astronomers need to double-check from the ground. By now, 105 of the Kepler candidates have been confirmed as true planets. But there simply aren’t enough telescopes on Earth to follow up on each of Kepler’s finds within a reasonable amount of time, so Fressin came up with another solution. In a wide-ranging data simulation, which corrects for any errors or incompleteness in Kepler’s list, Fressin concluded that just one-tenth of the huge number of planet candidates are false positives, and that virtually every sun-like star hosts a world. “One in six stars has an Earth-sized planet, at least,” he says. “If you look at a star, it will have planets. I don’t say they will be welcoming, but they will be there.”
There are about 100 billion stars in the Milky Way, so if Fressin is right, that gives us at least 17 billion Earth-sized planets—never mind all the giants out there, Jupiter-sized and bigger, or tiny shrimps like Mercury, and everything in between. An increasing number of astronomers, like Fressin, are saying this galaxy is bursting with worlds. A paper in Nature in January concluded that stars with planets are the rule, not the exception—and that, because many stars will have several, planets are more common than stars.
Another new study reaches a similar conclusion. Led by Jonathan Swift of the California Institute of Technology, it looked at a star called Kepler-32. Unlike our sun, it’s a red dwarf, a small and relatively cool type of star that emits light in infrared. “About seven of every 10 stars in the galaxy is a red dwarf,” says co-author John Johnson of Caltech. “Whatever’s going on there,” he says, “is going on throughout the galaxy.” Five planets circle snugly around Kepler-32, all within a distance one-third the size of Mercury’s orbit. Astronomers think it could be representative of what’s happening across the Milky Way, suggesting there are billions of planets orbiting red dwarves.
On the hunt for another Earth, Kepler is looking for something specific: planets of an Earth-like size, in an Earth-like orbit, around stars similar to our sun. Our planet is the only one we know of that can sustain life, so it makes sense to search for its twin. But Johnson suggests we flex our imaginations a little. “My prediction is that the first habitable Earth-sized planet [that Kepler finds] will be around a red dwarf,” he says. Not only are these stars far more common than ones like our sun; because they’re cooler, planets can orbit much closer to them and still remain temperate, making more transits per year—and more chances to catch them in a star’s wink. “I would be so bold as to say the majority of Earth-sized planets in this galaxy have a red dwarf in their sky,” he says: a massive red sun overhead, instead of our little yellow one.
Or maybe we’ll find a habitable moon, like the lush Pandora, which orbits a planet around Alpha Centauri in James Cameron’s Avatar. We haven’t yet spotted a moon outside our solar system, but they almost surely exist: this solar system has eight planets (nine if you count Pluto), but Jupiter and Saturn have dozens of moons. Titan, Saturn’s biggest, is covered with a hazy atmosphere and has lakes of liquid methane; Europa, around Jupiter, might harbour a deep ocean of salty water underneath its icy surface. A new study from Ji Wang at Yale University describes Ph2b, a Jupiter-sized planet in the habitable zone of a sun-like star. “No life could evolve on it,” Wang says of the gas giant —not life we know of, anyway. But there might be moons around it, and those are a different story. Chris Lintott of the University of Oxford suggests imagining if Jupiter were hauled into the temperate zone of our solar system: would its watery moons host life?
Scientists still don’t understand how life began on Earth, which makes searching for it elsewhere in the galaxy complicated. Water seems to be a necessary ingredient—hence the search for a planet that’s rocky, where liquid can pool at the surface, and one that’s neither too hot nor too cold, but in the “Goldilocks” zone that’s just right. (Luckily, water is made from hydrogen and oxygen, two common elements in the universe; there seems to be lots of water around, although in many places, like Mars, it’s locked away in ice.) Once we find an Earth-sized planet in the habitable zone of its star, there will be other questions to ask, Johnson notes, such as: does it have an atmosphere? What about a large moon, like ours, which helped stabilize Earth’s tilt? Is it important that a planet share its solar system with a Jupiter-sized giant, as we do, “to hoover up asteroids and toss [water-rich] comets to Earth?” So many factors seem to have contributed to life flourishing here. But with billions and billions of planets out there, it seems increasingly likely that life could have found a foothold elsewhere, too.
In September 2011, Kepler’s team announced they’d seen something out of Star Wars: a planet that orbits two stars, like the fictional Tatooine. Then things got weirder. In August 2012 came the discovery of multiple planets orbiting two suns in a single system, one of which might be in the habitable zone. In October, a planet was announced that has not two suns, but four. We now know that two-sunned planets aren’t rare, but are extremely common in the galaxy. If seeing a double (or quadruple) sunset isn’t enough excitement, there’s the Kepler-36 system, where a star is orbited by two planets that almost graze each other as they go by, so tightly do they sit together. The inner world would provide an incredible view of its neighbour; at their closest approach, every 97 days, the two planets are separated by fewer than five Earth-moon distances. Variety seems to be endless: Yale University astronomers concluded late last year that 55 Cancri e, a super-Earth about the size of our planet, has a fat layer of diamond beneath its surface.
Of all the planets found, none has sparked the imagination so much as a rocky world around Alpha Centauri , announced in October. The nearest star system to us, and one of the brightest in the southern skies, Alpha Centauri is made up of three stars: A and B, which are similar to our sun, and Proxima Centauri, a red dwarf. Roughly the same mass as Earth, the planet discovered there orbits so close to its star, Alpha Centauri B, that it must be inhospitable. But astronomers are betting there are others we haven’t seen yet. “When stars form a planet, they come in a bunch,” says Yale astronomer Debra Fischer, a renowned planet hunter who’s been looking for worlds in Alpha Centauri on a separate project. “Now we’re in a position to search for more.”
Tau Ceti, our closest solitary sun-like star, which is just 11.9 light years away, is an equally tempting target. Another team recently found five planet candidates orbiting Tau Ceti—and one of them looks as though it must be right for life. “The fourth planet, we believe, is deeply embedded in the habitable zone,” says astronomer James Jenkins of the Universidad de Chile in Santiago, one of the system’s discoverers. “I feel like we should drop everything and send a probe there,” Sara Seager, astronomer at the Massachusetts Institute of Technology, said in an email to the New York Times shortly after the Alpha Centauri planet was announced. It’s the understatement of the century to say getting there is no small feat.
Last year, on the 35th anniversary of its launch, NASA’s Voyager 1 was set to break free of our solar system, becoming humankind’s first emissary to interstellar space. That little satellite, which is plutonium-powered, is now more than 18.5 billion km from Earth, but it would still take another 75,000 years to reach Alpha Centauri. The distances that separate us from even the closest stars are mind-boggling. “Short of a scientific miracle of the kind that has never occurred, our future history for millennia will be played out on Earth and in the ‘near space’ environment of the other seven planets,” astrophysicist Adam Frank wrote in a New York Times editorial last year. “Like it our not, we are probably trapped in our solar system for a long, long time.”
Still, interstellar travel has been researched at least since the 1970s, when the British Interplanetary Society launched Project Daedalus to see whether a spaceship could reach our nearest stars in 50 years. The design they came up with was monstrous in size, with an initial mass of 54,000 tonnes, most of that in fuel; but they concluded that, yes, it could be done.
Richard Obousy is president and primary-propulsion senior scientist at Icarus Interstellar, a U.S. non-profit dedicated to achieving interstellar flight by 2100. A physicist, he believes that conventional chemical rockets—the kind that got us to the moon—won’t be much use for travel to Alpha Centauri. “Alternative schemes are a lot more exciting,” such as nuclear fission and fusion, or another, more powerful option, “matter-antimatter annihilation.” Matter and antimatter have the opposite electric charge; when they come together, they destroy each other and produce energy. Antimatter isn’t easy to come by. “We only create it in minute quantities,” Obousy says, at facilities like the Large Hadron Collider (LHC) particle accelerator in Switzerland: about one-trillionth of a gram per year, certainly not enough for a mission. But the LHC is an experiment, not an antimatter factory. Maybe creating a facility dedicated to antimatter could make starship fuel more cheaply.
Another idea is to use solar sails to propel a spacecraft by sunlight. In this scheme, a power station would be built near the sun, which would beam energy to the starship. The ship would then ride the beam of energy out into space. Solar sail-powered ships have the advantage of not needing to carry fuel, but building a base station at the sun doesn’t sound feasible—at least, not in the near future.
Then there’s an idea better known in Star Trek: the warp drive, which Harold “Sonny” White, advanced propulsion theme lead at NASA’s Johnson Space Center, is developing. Nothing can move faster than light, not even the universe’s most powerful starship. But space itself, we know, can expand at the speed of light, or faster. White aims to harness the universe’s expansion, creating a “bubble” that opens in front of the starship and pinches closed behind it. A starship would essentially surf through space-time, while obeying the faster-than-light law inside the bubble, enabling interstellar-transit times on the order of months, rather than centuries. In his NASA lab, he’s trying to create a microscopic warp bubble to perturb space-time by one part in 10 million, proof of concept it could be done.
These days, when even conventional missions have a hard time getting funding, building an interstellar ship is far-fetched. Even so, interest does occasionally crop up, especially after major new planetary discoveries: with some initial funding from the U.S. Defense Advanced Research Projects Agency, a small initiative called the 100 Year Starship Project is being led by former astronaut Mae Jemison to look into how we might reach nearby stars. If we can’t send humans or sophisticated probes to Alpha Centauri, maybe we could send thousands of tiny bots loaded with nanotechnology, not much different than the phones we carry in our pockets. “As we flesh out Alpha Centauri, I guarantee people will stop laughing and make it happen,” Fischer says. For a probe to get there at 10 per cent the speed of light would take 40 years, she notes. “Some of us could be around to see it.”
For now, using telescopes to explore Alpha Centauri and the billions of planets that populate our galaxy—which is, itself, just one of billions of galaxies in the universe—it seems more certain than ever that our Earth is special, but not unique. “It’s almost like it’s impossible to look at a star and not find a planet anymore,” says Jason Rowe, a Kepler scientist from Mississauga, Ont., who’s based at the NASA Ames Research Center. “When you want to ask about the implications for life out there, your mind just wanders toward, it’s got to be true.”