Astronomers Reckon There Are 17 Billion Earth-Size Planets in Our Galaxy Alone

Planets, planets, everywhere, and all the distances didn't shrink. Planets, planets, everywhere, nor any warp drive we can think (of). 


Artist's conception of Kepler-10b, the first confirmed terrestrial planet outside of our solar system, the discovery of which was announced in 2010 (NASA)

In 1750, British astronomer Thomas Wright published a book An Original Theory or New Hypothesis of the Universe with a diagram showing the stars of our Milky Way, each surrounded by orbiting planets. 

But there was no evidence to support his view. "At that point in history," Samuel Arbesman writes in his book The Half-Life of Facts, "this notion was nothing more than a hope, and a somewhat sacrilegious one at that. It was no more than a logical deduction derived from the Copernican notion that our place in the universe need not be a particularly privileged one."

For more than 200 years, the idea that there were planets beyond our bubble (known as exoplanets) remained conjecture. As recently as 1980, the documentary Cosmos showed Carl Sagan speaking to a classroom full of children explaining how we might someday detect exoplanets' orbits. "By the time that you people are as old as I am," he says, "we should know for all the nearest stars whether they have planets going around them or not. We might know dozens or even hundreds of other planetary systems."

"That will happen in your lifetime," he continues, "and it'll be the first time in the history of the world that anybody found out, really, if there are planets around the other stars." No surprise, he was right. In 1995 astronomers found a planet orbiting a sun-like star, Pegasi 51, the first star-orbiting planet confirmed beyond our system. "Humanity," Arbesman writes, "in the course of a single issue of Nature, overhauled its view of the universe."

Of course, we do have some practice. It wasn't until the 20th century that we knew for sure that our own galaxy was only one among more than a hundred billion. The patch of space in which we happened to evolve has been getting progressively less special for quite some time. 

But not even Sagan could have imagined how common planets would turn out to be by the early 21st century. Discoveries of other exoplanets have piled up -- a trickle at first, and an outright deluge since NASA's Kepler mission launched in 2009. In 2010, Kepler scientists announced the discovery of Kepler-10b, the first known terrestrial planet outside our system. Today, the count of Kepler discoveries has reached 2,740 candidates, and 105 have been confirmed.

Based on Kepler's data, astronomers are beginning to be able to extrapolate just how many planets there might be spinning around the galaxy's 100 billion stars, and the number they've arrived at is staggering: 17 billion -- and that's just the count for Earth-size planets in tight orbit around their home star, the kinds of planets Kepler is particularly adept at finding. And, as MIT planetary scientist Sara Seager told Ross Andersen, for certain kinds of stars, we've seen that one planet usually indicates that there are more waiting to be found whose orbits we have not yet had a chance to observe.

So, in just two decades (i.e. since yesterday in astronomical terms), we've gone from no certainty that there was even one planet beyond our solar system, to knowing that there are at least 105 and having pretty good reason to believe there are billions and billions more, a galaxy just crawling with them, like ants on a picnic's discarded strawberry.

Does this mean there is other life out there, as some back-of-the-envelope calculations would have you believe? No, not even vaguely. Sure, it raises the likelihood that there will be Earth-like planets in that magical habitable zone around a star, but for any increase in possible habitats, we have a corresponding problem of Fermi's paradox, which asks, if there are so many places life could spring up, and so much time to do it (Earth is a relatively young planet), where are they? Certainly with a few extra hundred million years to evolve *someone* could figure out interstellar travel. The fact of endless planets can just as easily bolster an argument for our loneliness as it does one for life's ubiquity.

The truth is that for now, more planets means exactly that: more planets. More places with unknown landscapes to dream of seeing, a whole galaxy that looks something like what Thomas Wright drew in London more than 250 years ago:


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Rebecca J. Rosen is a senior editor at The Atlantic, where she oversees the Business Channel. She was previously an associate editor at The Wilson Quarterly.

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