Planetary Census: Hundreds of Billions in the Milky Way?

A generation ago he had never detected even a single planet orbiting a distant star. Now the population has exploded

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Galaxy NGC 2683

It wasn’t all that many years ago that you could go to a major astronomy meeting and hear the latest news about galaxies and supernovas and quasars and the Big Bang — but nothing at all about planets orbiting distant stars. That’s hardly a surprise, since until relatively recently, there wasn’t the slightest evidence that even a single one of these exotic worlds existed.

At this year’s winter meeting of the American Astronomical Society in Long Beach, California, by contrast, you couldn’t get away from these so-called exoplanets if you tried. And in contrast to recent meetings, at which observers would crow over finding one new planet here and another there, this burgeoning field has turned into a numbers game, with the population of known or suspected worlds exploding. That planetary baby boom is allowing scientists to start talking about what exoplanets are like in general — a hugely important step in understanding the entire species of the worlds as opposed to just a few members of it.

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Scientists working with the Kepler Mission, for example, stepped up to report that their orbiting telescope, which has been searching for alien worlds since 2009, has identified another 461 planets circling other suns, bringing the probe’s lifetime total to 2,740. The finds aren’t officially confirmed yet, a process that will take years, but, says Natalie Batalha, Kepler’s mission scientist, “It’s likely that 90 percent or more of these are going to turn out to be bona fide planets.”

Not only that: the longer Kepler stares at the sky, the smaller the planets it’s finding — and smaller planets like Earth as opposed to gas giants like Jupiter are the places everyone assumes life has the best chance of taking hold. As of this week, Kepler has spotted a whopping 818 “Super Earths,” planets between 1¼ and two times the size of our own world, and 351 planets that are close to Earth-size.

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Armed with this information, the investigators have gone on to calculate how many such worlds probably exist across the Milky Way. They had to account for the fact that relatively small worlds are hard to spot, that Kepler is almost certainly missing some, and that even when the probe detects a planet, there is enough noise in the data that the software that is supposed to flag these detections and instruct the telescope to investigate them further might not recognize what it’s seeing.

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When you factor in all of these variables, says Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, the results suggest that the galaxy has at least 17 billion planets whose size more or less matches Earth’s. But even that’s a lowball estimate, he says, because Kepler has been in orbit only long enough to find these smaller worlds in relatively close-in orbits, like Mercury’s. Those are easier to see because the closer a planet is to its parent star, the more sharply inclined its orbit can be and still pass in front of the star, leading to the slight dimming of light that tells Kepler they’re there at all. A planet in a distant orbit needs to be inclined only a very few degrees before it would appear to pass above or below the star, leaving its luminosity unaffected. There are undoubtedly more planets to be found in life-friendly, Earth-like orbits, and over the next few years, Kepler is pretty much guaranteed to find them.

There’s one more factor, however, that boosts the population of likely planets in the Milky Way to a number that seems utterly fantastic. Kepler is looking for planets orbiting only about 150,000 stars. About 3% of those stars are the small, reddish variety known as M-dwarfs. There are from 200 billion to 400 billion stars in the Milky way and M-dwarfs are the most common type by far. Kepler is seeing so few only because they’re extremely dim (the number you can see with the naked eye from Earth is precisely zero).

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Since M-dwarfs are rich with orbiting planets, this means that any number Kepler is reporting is a gross under-sampling of what’s really out there. If, as astronomers Jonathan Swift and John Johnson of Caltech and several of their colleagues have done, you multiply the number of planets known to orbit M-dwarfs by the number of M-dwarfs in the Milky Way, you get an estimate of no fewer than 100 billion planets in the galaxy, and maybe a lot more.

“A hundred billion, two hundred billion, three hundred, something like that,” says Johnson. “We’re just talking orders of magnitude.”

And that, more than anything, is a measure of how profoundly astronomy has changed. People laughed when the late Sen. Everett Dirksen archly — and perhaps apocryphally – said, “A billion here, a billion there, and pretty soon you’re talking about real money.” Now we’re talking about stars and we’re talking about hundreds of billions — and no one’s laughing anymore.

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