Fifty-two years ago, the young radio astronomer Frank Drake wrote an equation that organized many factors, almost all of them unknown, that determine the abundance of civilizations that we could potentially detect with a radio telescope. The Drake Equation has survived all these years, and appears in astronomy textbooks. I would type up the equation here, but I’m unsure how to make the little subscripty thingees. Oh, heck, I’ll take a stab at it (the Wikipedia version is much more elegant):

N = R* x Fp x Ne x Fl x Fi x Fc x L

You start with the rate of star formation – a number known for a number of decades (our galaxy has at least 200 billion stars). What fraction have planets? What’s the number of planets in each of those star systems that is potentially habitable? On those “habitable” planets, how often did life actually arise? And then how often did that life evolve into complex organisms that gained the adaptive trait of intelligence? And how many intelligent civilizations develop communications technologies and beam signals into space, either intentionally (because they’re garrulous and want to chat) or unintentionally (because they’ve got big TV antennas broadcasting “I Love Lucy”)? And finally (cue the ominous music in a minor key) how long do intelligent civilizations survive, on average?

That final “L” in the Drake Equation, the longevity of a communicative civilization, has always been rather haunting. [Here’s the Achenblog item on that, inspired by a Krauthammer column.]

Now comes the latest analysis of data from the Kepler Space Telescope, and what’s fascinating about it, to me, is that it can be plunked directly into the Drake Equation. It’s n subscript e in the equation — the number of stars with habitable planets, or, as Drake puts it, in the ecosphere of the star.

When I say “as Drake puts it” I mean as he said to me yesterday when I called him about the news.

“I think all such calculations based on the Kepler data are still very preliminary. Theyr’e still analyzing the data, and there may be problems with the way they do the analysis,” Drake said. But he added, “It’s encouraging, it gives us a number and it’s a number we can stick into the equation.”

Since Drake first wrote out his equation, a number of developments have made him more optimistic that there are plenty of planets out there with intelligent life. First, it’s now clear that most stars have planets — that planets are common. Scientists didn’t know that in 1961.

“It now appears that essentially every star has a planetary system. In the very beginning we thought at best half.”

In the old days, astronomers assumed that binary star systems wouldn’t be able to have planets. Now we’re seeing planets around binary stars. So that’s a big boost for planetary abundance.

Also, Drake and his colleagues used to worry that planets couldn’t be habitable around red dwarf stars, because these planets would have to be close to the parent star to be in the habitable zone, and presumably they’d get locked into a synchronous rotation with one side always fried and the other side freezing. But now it appears such rotations are not the law of nature after all, and, moreover, an atmosphere would serve to distribute heat around the planet even if one side was usually in the dark.

But there’s one factor that makes Drake slightly more pessimistic about detecting intelligence out there: In 1961, human beings had huge TV and radio towers broadcasting to all points of the compass. We’ve switched to cable and satellite TV.

“We’ve been detectable for about 100 years by our radio transmissions, but now what’s happening is that Earth is becoming less visible,” Drake said. “The earth is fast becoming invisible.”

Excerpt:

Roughly one in every five sunlike stars is orbited by a potentially habitable, Earth-size planet, meaning that the universe has abundant real estate that could be congenial to life, according to an analysis of observations by NASA’s Kepler space telescope.

Our Milky Way galaxy alone could harbor billions of rocky worlds where water might be liquid at the surface, according to the report, which was published Monday in the Proceedings of the National Academy of Sciences and discussed at a news conference in California.

If the estimate is correct, the nearest ocean planet might be just 12 light-years away, which, though extremely distant for all practical purposes (such as sending a robotic space probe), is just around the corner in our galactic neighborhood.

“When you look up at the stars in the night sky, how many of them have a planet like the Earth?” asked Erik Petigura, a graduate student at the University of California at Berkeley and the lead author of the paper. “We’re able to start answering this question.”

The best estimate is 22 percent of stars like our own, with an error margin of plus or minus eight percentage points.

Earth-sized planets having the temperature of a cup of tea are common around sunlike stars,” said planet hunter Geoff Marcy, a Berkeley astronomer and a ­co-author of the study. He said the finding “represents one great leap toward the possibility of life, including intelligent life, in the universe.”

Kepler, launched in 2009, is no longer able to search for “exoplanets” — outside our solar system — because it has been unable to point with precision after the failure of a steering mechanism this year. But the telescope amassed more than three years of observations before going on the blink. Kepler mission scientist Natalie Batalha said there is still another full year of data to rummage through.

The telescope’s original mission was to obtain an estimate of the percentage of stars with potentially habitable planets, and this latest analysis comes close to meeting that goal. This is still an extrapolation of data and is not the same thing as taking a careful census of these Earth-size planets directly, said Sara Seager, an astrophysicist at MIT who was not directly involved in the new analysis.

“Earth-size” doesn’t necessarily mean “Earth-like,” Seager noted. But she said this result will boost efforts to build telescopes that could obtain direct imagery of one of these extremely distant worlds.

“Earth-sized planets are not rare, so we’ll know we’ll have stuff to look at,” Seager said. “It’s reassuring for us.”

Jill Tarter, a pioneer in “SETI,” the search for extraterrestrial intelligence, said in an e-mail: “We haven’t yet found Earth 2.0, but these statistics suggest that it should be forthcoming, and soon. When we can point to Earth 2.0 in the sky, it will seem completely natural to ask ‘Does anybody live there?’ and ‘Can we go there?’ I think Earth 2.0 will concretize SETI as nothing else has.”