The Numbers Are Astronomical

By Seth Shostak, Senior Astronomer

A press conference today laid bare some new results from NASA's Kepler space telescope. They're astounding.

It turns out that about 22 percent of all Sun-like stars boast a planet that's at the right orbital distance to sustain liquid water on its surface. In other words, one in five of such stars has an Earth-size orbiting world in the so-called "habitable zone."

This result, announced by astronomers Erik Petigura, Andrew Howard, and Geoff Marcy (from the universities of California and Hawaii), significantly plumps the odds that there's life elsewhere in the cosmos. That's because the prevalence of the messy, organic chemistry we gentrify with the term "life" is proportional to the amount of real estate available for it to arise and flourish. The argument is straightforward and unassailable, unless you're convinced that biology is zapped into existence with lightning bolts hurled by the gods.

Before the Kepler mission, no one knew what fraction of stars would have hospitable planets. So this work is both important and encouraging. Why encouraging? Well, consider the numbers that tumble from this result - the quantitative consequences.

We'll do the simple math for you. However, and just in case arithmetic makes your eyeballs glaze over like a holiday ham, we'll make sure the bottom line of this article gives you the bottom line. It's a result you'll find useful for striking up conversation with people who are overly impressed with their own importance.

Here are the stats. As noted, 22 percent of all Sun-like stars will sport a habitable, Earth-size world. Since stars similar to Sol (so-called G and K stars) make up 20 percent of the roughly 200 billion stars of the Milky Way, they account for 9 billion planets able to support life. That's the contribution to the planetary population from cousins of the Sun. 

However, three-quarters of the Galaxy's stellar complement is comprised of so-called "red dwarfs" - dim, puny stars rather smaller and much dimmer than the Sun. A recent analysis of Kepler data by Harvard astronomers Courtney Dressing and David Charbonneau implies that 16 percent of red dwarfs sport a planet in the habitable zone. Do the multiplication, and you can throw another 24 billion candidates for life into the galactic barrel.

We've accounted for 95 percent of all the stars in the Milky Way. The other 5 percent are big, bright stars - the kind that dominate the night sky, but are lamentably both rare and short-lived. If biology's your thing, you can forget those guys.

So now we have the skinny for our galaxy: There are at least 33 billion habitable worlds. At least. (We haven't considered large moons that could be paved with protoplasm, such as the fictional satellite Pandora in the movie "Avatar"). If they exist, they'll simply swell this already-impressive crowd.

However, and before cutting to the chase, a brief, historic note: In 1961, a dozen scientists gathered in West Virginia to bat around the idea of intelligent life in space. The agenda of this small meeting was a simple equation, constructed by the organizer, astronomer Frank Drake. The Drake Equation has since become a staple of astronomy textbooks everywhere, as it neatly summarizes the parameters that determine how many aliens might populate space. 

One of the factors in this famous formulation is the average number of Earth-like worlds in a random solar system. In 1961 - which was long before planets around other stars had been found - the meeting participants didn't dare to offer an official opinion on this number. But they did vaguely speculate that most stars might have such life-friendly locales. Today, their optimism seems both justified by the data, and remarkably insightful. They were either prescient, darn smart, or secretly clued in by aliens. I vote door number two.

So check out the Milky Way next time you're outside the glare of city lights, and ruminate on the thought that at least 33 billion habitable planets are somewhere up there. But that's just the local population. We can't see the entire universe, but the fraction we can see is studded with roughly 150 billion other galaxies; each with its own complement of habitable worlds. So the number of life-friendly planets that are currently in the part of the cosmos we can possibly observe is five thousand billion billion.

That's a big number. It's bigger than the number of cells in all the people of Earth. 

You may note that we've generally reckoned these figures to two decimal places, although that accuracy isn't really justified. For example, you could argue about the definition of "habitable zone," and plenty of people do. Different zone limits would shift these totals by maybe a factor of two or three, one way or the other. But when you're talking five thousand billion billion, all that really counts is the number of zeroes (in this case, 21). 

It's a real morale booster for those who are searching for extraterrestrial biology. Look at it this way: In the Powerball lottery, your chances of winning the jackpot on a single ticket are about one in 175 thousand. If Earth is the only planet with life in the visible universe, then our planet has done the equivalent of winning the jackpot four times in a row. Not just four wins. Four wins in a row. That would, I aver, qualify our home world as a miracle.

So here's that promised bottom line. Unless you're convinced that our watery planet, one of hundreds of billions floating in a non-descript galaxy similar to a hundred billion other galaxies, is somehow more worthy than all the rest, you should expect not merely an occasional Chewbacca or Klingon hanging out in space. The universe is far more likely to be a teeming shore of life, and biology as much a part of nature as rocks and rain.

Are you special? Sure you are. But it's a good bet that you're not alone.