Earth is a Goldilocks kind of place: Not too hot, not too cold. Things here are just right. We have a solid rock to stand on, liquid water to sustain us, and an atmosphere to shield us from radiation. Our cozy planet happens to lie just the right distance from the sun, in what astronomers call the habitable zone. But that's not all. On a larger scale, we live in a galaxy that is not too young, not too old. For a few billion years after the Big Bang, there was nothing but hydrogen and helium in the cosmos—nothing to make up terrestrial planets. It took the first few generations of stars to forge heavier elements like oxygen, iron, and uranium, which may power Earth's churning, molten interior. By the time our sun formed 4.5 billion years ago, there was plenty of planet-making material around. But the universe is aging, and astronomers predict it will run out of radioactive uranium, potassium, and thorium, and planets that form later will be as dead as the moon. Within our just-right galaxy, we also live in a just-right spot, about halfway out from the center—not too far in, not too far out. At the core of the Milky Way, the stars are packed together so tightly that they nearly collide with one another, and interstellar radiation would make life—or at least complex life as we know it—impossible. Out at the rim of the galaxy, there aren't enough stars to produce the heavy elements needed for terrestrial planets. Out there, you might get a rocky Mercury, about one-twentieth the size of Earth, but its gravity would be too weak to hold on to an atmosphere. Here in our solar system, in the just-right spot around a just-right star, our Goldilocks planet runs laps around the sun in a nearly perfect circular orbit, always staying 93 million miles from the fire. For decades, astronomers assumed that an orbit like this was essential to habitability. A planet that moved in an oval or ellipse would swing too close to the sun at one end of its orbit and sail into the chilly beyond at the other end. If elliptical orbits prohibit life, it means that astronomers searching for Earth-like planets have fewer candidates to choose from. It also means that Earth is vulnerable. If a wandering star or a rogue black hole were to perturb the orbit of Jupiter, deforming Earth's orbit in turn—an extremely unlikely event, but astronomers estimate there are 10 million rogue black holes in the Milky Way—all life on the planet would be destroyed. Or maybe not. Astronomer Darren Williams and his colleagues at Pennsylvania State University at Erie have been studying elliptical orbits recently, and they think life on Earth can withstand a lot more tumult than scientists previously guessed. They have been running sophisticated computer models of planets in orbits of varying eccentricity circling suns of various sizes. "High eccentricity does not critically compromise planetary habitability," Williams says. Then he drops the astrobiology lingo and translates with a boyish smile: "These planets will still support life."
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EARTH'S ORBIT (the red line above) is a near-perfect circle. But what if the planet took a more eccentric path? Astronomer Darren Williams has run computer simulations of various orbits. In one of the mildest, the planet comes closer to the sun than Venus, then sails to the chilly periphery of Mars. In the most extreme (the dark blue line), Earth careens closer than Mercury, then flies nearly to the asteroid belt. In every case, as the temperature averages show, the planet is habitable.
With his dimpled cheeks, handsome face, and wardrobe of quiet collared shirts, Williams looks like a man who might draw his circles round. It was his mentor, renowned geoscientist Jim Kasting, who first defined the "habitable zone" in which planets could support life. The idea had been floating around since the 1960s, but in the early 1990s, Kasting used computer modeling to determine the zone's exact dimensions: between 79 million and 140 million miles from a star (farther out for hotter stars, closer in for cooler stars). Outside that narrow path, Kasting argued, planets will overheat or freeze. At the time, astronomers knew only of planets with fairly circular orbits. But when the first extrasolar planets were discovered in 1995, some of their orbits were highly elliptical. Williams decided to see how life would fare in this unknown territory—and if his mentor's formula would hold. He teamed up with Penn State colleague David Pollard, a paleoclimatologist who has developed a respected computer model he uses to study Earth's ancient climate. The model, known as GENESIS2, is made up of 70,000 lines of computer code that mimic Earth's atmosphere, oceans, ice sheets, and a host of other factors, including the shape of its orbit. To push Earth into an oval orbit, all Pollard had to do was plug in a new number. If an orbit is perfectly circular, in the model it is said to have an eccentricity of 0; a straight line has an eccentricity of 1. Earth's orbit is very close to the former—0.0167. Pollard and Williams decided to stretch it toward the other extreme. They ran models for eccentricities of 0.1, 0.3, 0.4, and 0.7. In each case, they kept the average distance of the orbit the same: Earth still made one lap of the sun in 365 days. They let each simulation run for 30 theoretical years and then looked to see what Earth's climate was like in the brave new orbits. The least eccentric orbit—0.1—kept the planet inside the habitable zone all year long; not surprisingly, there was barely any change in climate. At higher eccentricities, though, things got interesting. As astronomer Johannes Kepler explained in 1609, the more elliptical a planet's path, the closer it gets to the sun at one end of its orbit (known as perihelion), and the farther from the sun it goes at the other end (known as aphelion). At an eccentricity of 0.3, the planet's orbit would pass inside the orbital path of Venus at perihelion and fly within 20 million miles of Mars at aphelion. In Pollard's model, though, even when Earth drew closer to the sun than Venus, it didn't develop a Venus-like climate. "Water has a very high heat capacity," Williams says, "so the large amount of water on Earth is slow to warm up." And the heat wouldn't last long. As Kepler also explained, planets on eccentric orbits travel fastest at perihelion, accelerating furiously. "Well before the oceans start boiling," Williams says, "the planet is racing away." At the other end of an eccentric orbit, Earth slows down again. But here the climate model takes a strange and welcome turn. The planet absorbs so much heat during its brief trip à around the sun, Williams explains, that its coldest months out by Mars are still warmer than winter months on a circular orbit: The average global temperature is 73 degrees Fahrenheit, versus 58 degrees on Earth now. It's not a perfectly regulated system: Some parts of the African, South American, and Australian interiors heat up to 140 degrees at perihelion. But the extreme temperatures only last a month or two. Erie, Pennsylvania, where Williams lives with his wife and two children, is nearly as temperate and cozy in a 0.3 orbit as it is on a circular one. On a 0.4 orbit, the annual mean temperature jumps to 86 degrees, and larger landmasses become insufferably hot. But again, Williams says, "This is a habitable planet."
In the Zone
AT ANY SCALE, Earth sits squarely in the planetary comfort zone—the narrow margin in space and time where the right kind of star can give rise to the right kind of planet with the right conditions for life. Most scientists agree that the following criteria apply to higher life-forms. Single-celled organisms are extremely adaptable and may be able to survive in harsher climes.
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Local Zoning LawsThe habitable zone around a star is defined by the distance at which water on a planet's surface can remain liquid. In our solar system, the zone's inner limit is just outside the orbit of Venus; its outer edge is near Mars. Whether the Red Planet is inside the zone is still a matter of debate.
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Galatic Zoning LawsNear a galaxy's core and in its spiral arms, the stars are so dense that they may give off too much radiation and cause too many gravity-perturbing collisions to support life. Stars too far from the center may contain too few metals to make planets massive enough to hold on to an atmosphere. The sun sits right in between these extremes.
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Universal Zoning LawsAt the very largest scale, life depends more on time than space. Right after the Big Bang, only helium and hydrogen existed. It took 6 billion years for the heavy elements to form that are needed for life-supporting planets. Several billion years from now, some of those elements—uranium 235, for instance—will begin to run out.
The final simulation showed just how far the boundaries of life can be pushed. This time, Williams threw the planet into an orbit with an eccentricity of 0.7, sending it closer to the sun than Mercury at its perihelion and well beyond Mars at its aphelion. In all, it would spend only 75 days of the year in the habitable zone. Could such a world be habitable? Well, yes, but only if you cheat a little. Before they ran the simulation, Pollard and Williams reduced the sun's luminosity by 29 percent. They knew, by then, that planets with eccentric orbits get hotter than planets with circular orbits, even if their average distance from the sun is the same. Widening the eccentric orbit would have made the planet more habitable, but the GENESIS2 model has a 365-day year hardwired into it. So the researchers took another tack: They dimmed the sun just enough so that the overall heat the planet received would be the same as for our Earth. Any changes in climate could then be attributed to the highly eccentric orbit. Even with a dimmer sun, life on a 0.7 orbit isn't exactly what we would call comfortable. In Erie, Pennsylvania, summer temperatures spike to 140 degrees Fahrenheit, and the sun looks twice as large in the sky. It doesn't rain for months, and the evaporation rate is so high that Lake Erie dries up altogether. Six months later, in the chilly winter beyond the orbit of Mars, the sun shrinks to half its usual size in the sky. The oceans have stored up so much heat during the summer that temperatures still stay mostly above freezing. "It never snows in Erie, Pennsylvania—something people around here would be thrilled about," Williams says. "But we'd have to migrate with those summer temperatures so high." Most likely, we wouldn't come back. In a 0.7 orbit, the Arctic Ocean melts, Pollard says, "and anywhere on its shores—Norway for instance—wouldn't be such a bad place to live." By contrast, central Africa in the summer is a stovetop with temperatures near boiling—if there were any water to boil. Higher life forms probably could not live there, Williams says. But microbes have been shown to withstand temperatures of 230 degrees, and nowhere on this vastly changed Earth does it get that hot. The oceans get hotter, but not so hot that they boil away. Life is certainly different on this Earth—but it's still life.
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Heavy EccentricityOn Earth's familiar, circular orbit, the seasons are determined by the planet's tilted axis. When the Northern Hemisphere leans toward the sun, it's summer there; when it leans away, it's winter. On an eccentric orbit, the distance to the sun makes all the difference. The maps below show how temperatures would vary worldwide, over the course of a year, if Earth's orbit had a mild eccentricity of 0.3 (top) or a high eccentricity of 0.7 (bottom). Note how temperatures rise and fall much more dramatically on land than on sea. The oceans act as giant planetary temperature regulators: They absorb massive amounts of heat at the solar end of the orbit, then slowly release it as the planet swings into frigid space. On the mildly eccentric orbit, the planet passes closest to the sun in February, but the oceans continue to absorb heat in the weeks that follow. The hottest months are March and April, when temperatures in Africa rise above 120 degrees Fahrenheit. Winter temperatures reach their lowest point in August and September, when the planet swings out toward Mars. Yet even the Arctic never cools down below 32 degrees, because the oceans are still releasing their pent-up heat. On a highly eccentric orbit, the distances and temperature swings are far more extreme. Here the planet comes closest to the sun in early March, bringing continental temperatures near the equator all the way to the boiling point. That heat, retained by the oceans and atmosphere, keeps much of the planet sweltering until it's hurtling out toward the asteroid belt. The Arctic Ocean would melt in this scenario, offering prime beachfront real estate.
"The bottom line is that this planet is habitable," Williams says, beaming. Even his mentor, Kasting, agrees: "Planetary habitability is not that hard to achieve." Tinker with the planet a bit, and the possibilities for life get even better. A bigger ocean, for instance, or a thick, insulating atmosphere like Venus's, would help smooth out the temperature extremes on eccentric orbits. We may already have such rocks in our sights. In the past seven years, more than 100 extrasolar planets have been detected through a method known as radial velocity. Astronomers can't actually see these planets, only a telltale wobble in the stars that the planets are orbiting. But the amplitude and timing of the wobble can reveal a planet's size as well as the shape of its orbit. One star, 16 Cygni B, has a planet with an eccentric orbit of 0.67; another star, HD222582, has a planet with an orbit of 0.71. Both these stars are brighter than our sun, but their planets have a wider orbit than Earth, so they pass straight through the habitable zone. The planets are gas giants like Jupiter and thus less likely to harbor life. But according to Williams's climate calculations, if they have large rocky moons, those moons could be habitable. Here Kasting sounds a note of caution: "It's going to be very hard to detect those moons if they exist," he says, and the total population of planets in eccentric orbits may be small. Solar systems with elliptical orbits tend to be less stable than systems with circular orbits: Their planets can cross one another's path and bang into each other. When astronomers get better at detecting planets, Kasting suspects, they will find a host of Earths out there, running circular orbits inside his habitable zone. Still, he says, Williams's work is "one more reason to be optimistic" that we can find another Earth—even if it is a bit more eccentric.
