Soon after this first attempt, Drake came up with the celebrated Drake equation, which became the fundamental organizing principle for the new cross-disciplinary field of astrobiology. The equation looked at all the factors determining how many (if any) detectable extraterrestrial societies are out there. Drake multiplied the number of sunlike stars in our galaxy that form each year by a handful of variables: the fraction of those stars that have planets; the number of planets per planetary system where life could exist; the fraction of habitable planets where life actually arises; the fraction of those where intelligence emerges; the fraction of intelligent species that develop interstellar communication; and finally, the average length of time that those communicating civilizations survive.

The only one of those factors that scientists had a clue about at the time was the formation rate of sunlike stars. The rest was pure speculation. On the question of how often life arises under the right conditions, for example, optimists like Carl Sagan thought it would almost always happen. Others suspected that life was actually a rare occurrence. At a gathering convened by Drake early on, opinions as to the number of civilizations in the Milky Way ranged from 1,000 to 100,000,000, but many researchers outside that group thought the true number might be 1—just us. That is an absurdly broad range, so scientists of all stripes set out to nail down the actual figures in Drake’s equation. Astronomers went in search of planets circling other stars. Planetary scientists began sending probes to Mars, Venus, and other places to look for signs that life might have taken hold elsewhere. Biologists tried to unravel the secret of life’s origins. And SETI searchers kept scanning for that long-awaited signal from space.

Over the next couple of decades, there were a few fleeting moments of excitement in the world of SETI. In the 1960s astronomers reported evidence of a planet orbiting nearby Barnard’s Star. In 1976 it was reported that NASA’s Viking landers had detected a hint of biological activity in the soil of Mars (See “Renewed Hope for Life on the Red Planet,” DISCOVER, June 2010). A year later, astronomer Jerry Ehman announced his Wow! signal, so named because it looked so much like an alien message that he jotted “Wow!” on the printout from his radio telescope. None of these leads panned out.

It was not until 1995 that astronomers began to make some progress in cracking the Drake equation. That is when a Swiss team found the first planet orbiting a sunlike star other than our sun. The proof that planets really are out there gave a huge boost to the mood of SETI searchers. Today the tally of such so-called extrasolar planets stands at more than 450, including a few super-Earths, worlds that are several times more massive than ours. The recently launched Kepler space telescope will soon begin finding Earth-mass planets if they exist; given the profusion of planets found so far, nobody doubts that they do.

At the same time, SETI scientists have started feeling more bullish about the next part of Drake’s equation, the fraction of planets that could support life. “Until about 10 years ago, we were only considering planets orbiting sunlike stars as suitable places for life,” Drake says. The most common type of star in the galaxy—small, dim, red stars known as M-dwarfs—outnumber sunlike stars 10 to 1, but they were generally dismissed out of hand. For one thing, such stars tend to emit a lot of flares, making them unreliable sources of sustenance. For another, they are so cool and dim that any habitable planets would have to orbit extremely nearby. At such close range, a planet would probably become gravitationally locked to its star, so that one boiling-hot side would perpetually face the star while the other side, freezing cold, would face out into dark space. “We thought those were showstoppers,” Drake says.

But the theorists have reconsidered. The flaring of M-dwarfs seems to die down over time, and new climate models suggest that even a locked planet could be habitable because its atmosphere would help even out the temperatures. Meanwhile, a project called MEarth, begun in 2008 using off-the-shelf amateur telescopes, has already found a planet orbiting an M-dwarf that is only a bit too large and a bit too hot to be considered Earth-like. Expanding the number of potential target stars obviously raises the SETI odds considerably.

So does the realization that the habitable zone (the region around a star where a planet could have liquid water, essential for life as we know it) is a lot broader than anyone had thought back in 1960. “We now know,” Drake says, “that you could move Venus a lot farther from the sun and it would remain habitable” due to greenhouse warming. “We know that there’s liquid water under the surface of the moons Europa and Enceladus. I’m not saying there could be a technological civilization in the ocean on Europa; you probably need fire for that. But I’m more enthusiastic about SETI than ever.”

It is not just the expanded number of possible targets that has Drake and the other SETI scientists feeling upbeat. It is also the drastic increase in the power of the actual searches for alien signals. Ever since Drake’s first experiment, scientists have had to guess at pretty much every aspect of how extraterrestrials might broadcast, including the thorny issue of which among billions of possible radio frequencies they would use. You would hate to be tuning in to channel 5 when the aliens are happily chattering away on channel 6.

Listening in on many channels at once requires powerful electronics. But since the first SETI experiments, digital electronics have increased exponentially in power while plummeting in cost. That means searchers can look at far more stars, in far more ways, in a given amount of time. “We’re doing 14 orders of magnitude better than Frank Drake did with Project Ozma—and that improvement is going to continue,” Tarter says.

Back in the days of Project Ozma, radio was the only efficient method of interstellar conversation anyone could imagine. The subsequent development of powerful lasers, however, suggested another way to transmit signals between the stars. Physicists at the National Ignition Facility at Lawrence Livermore National Laboratory have built lasers that put out a beam with a petawatt, or 10¹⁵ watts, of power. “A nanosecond pulse from one of these lasers, reflected off a 10-meter telescope mirror like the one in the Keck telescope, would outshine all of the light from a star by a thousandfold,” Drake says. “If ETs are sending those signals out, we have phototubes that could detect them.”

Even a modest telescope can detect millions of stars, which means it could also pick up any flashes of light that outshine those stars. Drake has conducted a large optical SETI, or OSETI, search, looking for spots of light in the sky that flicker in some meaningful way; he is seeking funding to start up a new hunt at Lick Observatory near San Jose, California. Paul Horowitz, a Harvard physicist and another SETI veteran, has been working with optical wavelengths for more than a decade and has an active OSETI project going on now. He is—of course—more enthusiastic than ever. “Charlie Townes [a pioneer in developing the laser] gave a talk several years ago,” Horowitz says, “and pointed out that if we could make lasers just an order or two of magnitude more powerful than they are today, we could create pulses that would be naked-eye visible” from a faraway planet.

Aliens would have to be only slightly more advanced than we are to have done that, yet we do not see naked-eye flashes. “You could see that as discouraging,” Horowitz says. “But I question why they would bust their asses to transmit a prodigious amount of power to make themselves visible when they could crank it down to something we could detect in maybe 20 years.” He suggests that alien civilizations might aim their messages only at civilizations more advanced than ours.

Shostak too doubts that any aliens are trying to contact us specifically. “They don’t know we exist,” he says. “I say that unequivocally. They know there’s some sort of biology here, but that’s been true for billions of years.” Only in the last 100 of those years have we developed radio transmitters. “They’re not going to waste energy signaling blue-green algae.” Instead, he thinks they might have a long list of planets that show some evidence of life and might ping each one for a millisecond or so every few days—a simple, low-effort project to see if anyone at the other end is paying attention. “If there were a bright flash tonight and another three weeks from now,” he says, “we’d study the heck out of it—and then we’d probably build a huge receiver to try and learn more.”

The overriding problem with SETI searches is that even if aliens are out there, we have no clue how they might think, what they might choose to do, or how they might do it. SETI searchers, Tarter notes, have been looking for only a limited class of signals from a very small number of stars. “Maybe we haven’t looked at enough stars or enough frequencies,” she says. “Maybe we’re not looking for the right technology because we haven’t invented it yet.”

To the SETI community, such thoughts are inspiring rather than deflating. People like Tarter and Drake consider crazy-sounding new ideas all the time; they think of it as part of their job. Back in the 1960s, for example, the physicist Freeman Dyson suggested that advanced civilizations would build shells around their suns to trap and utilize every bit of energy. Some astronomers promptly went ahead and searched for Dyson spheres, which would leak telltale infrared radiation. In his new book, The Eerie Silence, astrobiologist Paul Davies proposes looking for evidence of ETs hidden in the DNA of microbes right here on Earth. “An alien expedition to Earth might have used biotechnology to assist with mineral processing, agriculture, or environmental projects,” he writes in an essay in The Wall Street Journal. “If they modified the genomes of some terrestrial organisms for this purpose, or created their own microorganisms from scratch, the legacy of this tampering might endure to this day, hidden in the biological record.”

Some SETI ideas are so far-out that they are nearly impossible to test. The only sensible approach, Shostak suggests, is to start with the science we understand today and assume that we will be able to explore a whole lot more later. “Imagine it’s 1492,” he says. “Would you tell Columbus not to bother—that if he just waits 500 years he’ll be able to cross the ocean in six hours eating bad food?”

For now, that attitude mostly translates to bigger, better, faster, and smarter versions of the radio SETI project Frank Drake initiated in 1960. The latest tool is LOFAR, an array of 44 low-frequency radio detector stations that are being brought online across western Europe, 36 of them in the Netherlands. The relatively small, inexpensive antennas will be linked together to rival the power of the world’s biggest observatories, and part of LOFAR’s observing program will be dedicated to SETI.

A similar approach is at work at the Allen Telescope Array, a collection of 42 twenty-foot dishes located in Hat Creek, California, funded by Microsoft cofounder Paul Allen. The array began operating in 2007, using low-cost electronics to combine the input from the many radio antennas and to comb through the resulting signal, simultaneously doing conventional radio astronomy and scanning for signals from ET. Tarter, who helps manage the project, is seeking funds to increase the number of dishes to 350.

“We shouldn’t even think about getting discouraged at this point,” Shostak says, with customary verve. Horowitz amplifies that thought: “Imagine how foolish you would feel if you didn’t try only because someone said you’re a lunatic.”