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Are There Habitable Planets Beyond Our Solar System

   


     Ever since people first tilted their gaze up toward the heavens, they have wondered about the possibility of other worlds like ours orbiting distant suns. Until very recently, such questions were left to the realm of speculation. Today, thanks to telescopes like the Kepler space observatory and increasingly advanced surveys from ground-based technology, we know that the galaxy is swarming with planets. But are any of them habitable? Do any of them resemble our own?


    The question of habitability is a tricky one, and the odds of any individual planet possessing Earth-like properties are rather low. That said, the numbers are in our favor. Kepler recently confirmed the discovery of its 1,000th exoplanet. Some astronomers now estimate that there is one exoplanet for every star, on average. That means there are billions and billions of planets in our universe! Many of these planets, though, are nothing close to habitable. The first exoplanets that astronomers found orbited impossibly close to their suns, tidally locked, exposing one side to scorching heat and radiation and the other side to permanent night. In contrast, Earth orbits the Sun in the so- called Goldilocks Zone: not so close that all liquid water boils away, but not so far that it is perpetually frozen in ice. What’s water got to do with the existence of other planets? The capacity to harbor liquid water is the key characteristic that astronomers look for in the search for habitable alien worlds, due to water’s paramount importance to life on our own planet. But liquid water and a planet’s average orbital distance are but two of several key factors. For instance, the class of star that serves as the sun is important:


    Habitability requires a sun that emits the right type of radiation and is likely to live long enough to allow life to evolve. A stable orbit is also important, ensuring thatthe planet’s climate doesn’t fluctuate wildly. The mass of the planet—massive enough so that it’s capable of generating and holding onto an atmosphere, but not so massive that the atmosphere is oppressively dense—is also critical. While astronomers have not yet confirmed the presence of habitable exoplanets, all signs currently point to the affirmative. Scientists reviewing data from the Kepler observatory recently discovered eight planets, roughly the size of Earth, in their respective sun’s Goldilocks Zone. Other candidates exist, from as nearby as 40 light-years to thousands of light-years distant, some orbiting superclose to colder suns, and some much larger than Earth; socalled super-Earths range in size from two to 10 Earth masses. We seem on the verge of discovering a planet that might not only be capable of supporting life, but could hypothetically support life. Whether these habitable planets already support life-forms and whether those lifeforms are intelligent—well, that’s a whole other mystery.


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