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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