Whose Galaxy Is It?

As astronomers discover a universe flush with potentially life-supporting planets, it seems more and more likely that we are not alone.

by Caleb Scharf Published Winter 2014-15
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Illustrations by Davide Bonazzi

For the past six years, a van-sized NASA spacecraft called Kepler has been orbiting the sun, following the same elliptical path as Earth but trailing a few million miles behind us. This two-thousand-pound hunk of aluminum and electronics, carrying one of the most sensitive telescopes ever built, has been drifting off our tail, rather than orbiting us the way many space observatories do, so that it can enjoy an unobstructed view of the heavens. Its mission is to spot previously unknown planets orbiting distant stars — an extraordinarily difficult task for the simple reason that planets are small and dark, while stars are big and luminous. Kepler pulls this off by sensing a planet when it passes in front of its star, momentarily blocking a tiny amount of light, like a moth flitting across a porch lamp. Many astronomers, including me, believe that this planet-hunting expedition is providing us clues to one of the greatest mysteries that humans have ever pondered: whether extraterrestrial life exists.

The possibility that we are not alone in the universe, and that other intelligent creatures are strolling around on their own Earth-like planets somewhere, has been a subject of serious speculation among astronomers since at least the sixteenth century. That is when Nicolaus Copernicus triggered an intellectual revolution by suggesting that Earth is not the center of the universe. His insight inspired jaw-dropping discoveries by Galileo Galilei and others who realized, over the following centuries, that the sun is merely another star, that the stars we see in the nighttime sky are a tiny fraction of those that exist, and that the universe has no center. By the late eighteenth century, many scientists subscribed to a view that remains popular among both laypeople and astronomers today: that given the immensity of this decentralized cosmos, life must exist somewhere else. The alternative — that our sun, among the unfathomably large number of stars out there, is the only one supporting life — seemed faintly ludicrous.

Until very recently, though, we lacked the technology to see if other Earth-like planets were indeed orbiting any of the hundreds of billions of stars surrounding us. This effectively stalled scientific progress in learning whether we are unique, obscuring the true nature of our existence. No amount of philosophical reasoning could overcome this ignorance.

That veil may be lifting. In the past year alone, astronomers have identified nearly one thousand planets in other star systems — or exoplanets — based primarily on Kepler’s observations. These discoveries have more than doubled the number of known exoplanets, bringing the total to about 1,900. This number is expected to increase steadily over the next couple of years as scientists pore over Kepler’s vast trove of data.

Both the number and variety of exoplanets found so far have astounded even seasoned scientists. Whereas the first exoplanets discovered back in the mid-1990s were massive gaseous beasts with a passing resemblance to Jupiter — these behemoths were the easiest to spot — we’ve since identified exoplanets of nearly every conceivable size and character. Some are orbiting stars so closely that their surface temperatures are likely to be more than a thousand degrees, causing heavy iron-rich rock to gasify, forming clouds and then condensing as metallic raindrops. Others are likely to be icy snowballs plunged into permanent winters so deep that their atmospheres have frozen and fallen to the ground. Still others follow orbital patterns that suggest they may have atmospheric conditions that resemble Earth’s. In fact, based on our observations to date, we can extrapolate that roughly one out of every two stars in our Milky Way galaxy has an Earth-sized planet orbiting it, and that almost 30 percent of these planets orbit their stars at distances that could perhaps allow for surface environments with liquid water — that marvelous enabler of known biochemistry. This suggests that there are tens of billions of such worlds in the Milky Way alone.

The idea that our galaxy is teeming with other Earths is thrilling just to think about. But we can now do more than that. Today, many astronomers, biologists, chemists, physicists, geologists, and others are coming together to share data, ideas, and analytic techniques in hopes of answering questions that just a generation ago were not considered valid lines of scientific inquiry — questions like: How much life is out there? If life exists elsewhere in the universe, what form might it take? Is it likely to follow the same rules as life on Earth? Might it be reflecting intelligently on its own existence as we are?

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