Kate Becker
Kate Becker

How many intelligent civilizations are thriving, right now, in the Milky Way?

Spoiler alert: I'm not going to answer this question by the end of this column. Today, it is unanswered and unanswerable.

But back in the 1960s, pioneering SETI (Search for Extraterrestrial Intelligence) researcher Frank Drake proposed an equation that lays out the path to answering it. His formula, called the Drake equation, goes like this:

The number of civilizations in our galaxy with which we might be able to communicate equals our galaxy's average rate of star formation multiplied by the fraction of those stars that host planets, multiplied by the average number of habitable planets per planet-hosting star, multiplied by the fraction of such planets where life actually arises, multiplied by the fraction of those planets where such life evolves to become intelligent, multiplied by the fraction of such civilizations that develop the technology for interplanetary communication, multiplied by the length of time that civilization sends out detectable signals.


Despite its imposing length, the Drake equation still isn't perfect. After all, what about planets where intelligent civilizations arise twice, three times or more? What about civilizations that intentionally maintain radio silence so as to keep their presence a cosmic secret? But let's put those quibbles aside. After all, I don't see the Drake equation as an answer to the deep question of our cosmic isolation: Instead, it is an inventory of ignorance, a roll call of all that we don't know about the universe and our place in it.

And yet, in the decades since Drake first penned the equation, we've begun to fill in one of the most critical terms: the fraction of stars that host planets. Now, a team of astronomers analyzing data from the Kepler Space Telescope has estimated that our galaxy could hold more than 100 billion planets -- an average of one per star, though they aren't necessarily divvied up that way.

The astronomers scrutinized a star called Kepler 32, a garden-variety M dwarf hosting five suspected planets. Dimmer and longer-lived than our sun, M dwarfs are the most common stars in the Milky Way: They account for about three-quarters of our stellar census. Though Kepler 32 is remarkable mainly for its ordinariness, it does have one striking feature: The Kepler telescope happens to view it "edge on," meaning that as each planet swings in front of the star, temporarily blocking a bit of starlight, Kepler sees the full impact of the apparent dimming.

This arrangement gives astronomers unusual insight into this planetary system and, by proxy, the tens of billions of similar M dwarf systems that throng the Milky Way. By combining the data on Kepler 32 and its planets with information on the other M dwarf systems observed by Kepler and calculating the odds of spotting such planets passing in front of their stars, the astronomers were able to estimate that pesky Drake equation factor, the fraction of stars that host planets. The result: Our solar system contains about one planet per star, for a total of 100 billion planets.

If that number rings a bell, it happens to be the same estimate that an independent group of astronomers came up with early in 2012, using a totally different planet detection method called gravitational microlensing. Both estimates are just statistical educated guesses, of course -- we won't be hand-counting exoplanets any time soon -- but they bring us just a little closer to closing the gap of our ignorance and answering, once and for all, the question we've pondered for so very long: Are we alone?
