When I write or give public talks about exoplanets—alien worlds orbiting other stars—the most common question I’m asked is, “When will we find another Earth?”
It’s a good question. As we’re learning, space is filled with a great many wildly differing worlds, and it’s natural to wonder if there’s an Earth 2.0 out there, or if they’re all truly, well, alien.
Our Milky Way galaxy harbors hundreds of billions of stars. Given our census of local stars showing that planets occur at least as often as stars there could be trillions of planets in our galaxy alone. Of course, realistically, that doesn’t mean every star has a planet, but rather some don’t have any while others have teeming solar systems.
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Exoplanets come in a dizzying variety of flavors, some incredibly bizarre: planets as big as Jupiter but skimming so close to their host stars’ surfaces that the atmosphere-boiling heat strips away their air, turning them into megacomets; worlds bigger than Earth but smaller than Neptune, which are the most common kind of exoplanet seen despite our solar system not having one; and planets where it might rain molten iron. Oddballs abound.
And, yes, the list includes many Earth-sized worlds. Of the 5,500 or so exoplanets found to date, about 100 are close in size to our home planet. But there’s more to Earth than just size.
If you’re looking for an exact replica—say, with the Earth’s size, mass and composition, with breathable air and drinkable water—then those odds look pretty long. Planetary formation is a stochastic process; a lot of random variables affect how a planet forms and evolves over time. Even small changes can lead to dramatically different planetary evolution,…
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