Smart aliens might live within 33,000 light-years of Earth. A new study explains why we haven't found them yet.

• The universe has so many galaxies, stars, planets, and moons that many scientists believe intelligent aliens should exist within detectable range of Earth.
• Still, human searches for extraterrestrial intelligence have yet to detect any alien signal or "technosignature."
• A new study suggests this may be because we've searched just 0.00000000000000058% of a "cosmic haystack" in our hunt for an alien "needle."
• There's no guarantee that exhaustive searches would ever find aliens, though.

The cosmos almost screams with the possibility of intelligent alien life.

Hundreds of billions of galaxies drift through the visible universe, each one harboring hundreds of billions of stars, and each of those stars in turn shelters roughly a handful of planets. Even if the trillion-or-so planets in every galaxy aren't habitable, countless water-rich moons orbiting these lifeless worlds might be.

And yet, in spite of these numbers, humans have yet to identify any signals from intelligent aliens. The prescient question that physicist Enrico Fermi posed in 1950 - "where is everybody?" - remains unanswered.

However, an upcoming study in The Astronomical Journal, which we learned about from MIT Technology Review, suggests humanity has barely sampled the skies, and thus has no grounds to be cynical.

According to the paper, all searches for extraterrestrial intelligence, or SETI, have examined barely a swimming pool's worth of water from a figurative ocean of signal space.

"We haven't really looked much," Shubham Kanodia, a graduate student in astronomy who co-wrote the study, said during a NASA "technosignatures" workshop in Houston, Texas on September 26.

The study suggests that somewhere in that ocean of space - right now, within the Milky Way galaxy - intelligent aliens might be saying, "hello, we are here."

But we'd have no way of knowing, at least not yet.

Defining a 'cosmic haystack' in the search for aliens

Over the past 60 years, multiple SETI projects have looked and continue to look for alien signals. Some scan large swaths of the sky for powerful signals, while others target individual star systems for weaker signals.

Yet aside from a few anomaly signals that never repeated (like the "Wow!" detection of 1977), these searches have turned up empty-handed.

Kanodia and his colleagues at Penn State University wanted to know how much of the figurative "cosmic haystack" SETI projects have covered, and to what extent they could improve the hunt for the alien "needle."

The group agrees with famous SETI astronomer Jill Tarter, who said in 2010 that it's silly to conclude intelligent aliens do not exist nearby just because we haven't yet found their beacons. Even if such signals exist and are aimed right at Earth, her thinking goes, we've scanned so little of the sky and may not be looking for the right type of signal, or for long enough, to find them.

"Suppose I tell you there's a cool thing happening in Houston right now," Kanodia said during his NASA talk. "I do not tell you where it is. I do not tell you when it is happening. I do not tell you what it is. Is it in a book store? Is it a music concert? I give you absolutely no priors. It would be a difficult thing to try and find it."

He added: "Houston, we have a problem. We do not know what we're looking for ... and we don't know where to start."

In their study, Kanodia and his colleagues built a mathematical model of what they consider a reasonably sized cosmic haystack.

Their haystack is a sphere of space nearly 33,000 light-years in diameter, centered around Earth. This region captures the Milky Way's bustling core, as well as many giant globular clusters of stars above and below our home galaxy.

They also picked eight dimensions of a search for aliens - factors like signal transmission frequency, bandwidth, power, location, repetition, polarization, and modulation (i.e. complexity) - and defined reasonable limits for each one.

"This leads to a total 8D haystack volume of 6.4 × 10¹¹⁶ m⁵Hz² s/W," the authors wrote.

That is 6.4 followed by 115 zeros - as MIT Technology review described it, "a space of truly gargantuan proportions."

How much of this haystack have we searched?

Kanodia and his colleagues then examined the past 60 years' worth of SETI projects and reconciled them against their haystack.

The researchers determined that humanity's collective search for extraterrestrials adds up to about 0.00000000000000058% of the haystack's volume.

"This is about a bathtub of water in all of Earth's oceans," Kanodia said. "Or about a five-centimeter-by-five-centimeter patch of land on all of Earth's surface area."

Those numbers make humanity's search efforts seem feeble. But Kanodia views it as an opportunity - especially because modern telescopes are getting better at scanning more objects with greater sensitivity and speed. For example, he said, a 150-minute search this year by the Murchison Widefield Array covered a larger percentage of the haystack than any other SETI project in history.

"That's the purpose of this haystack ... to help better-inform future search strategies," Kanodia said.

He also noted that the team's calculations assume there is only one alien civilization within range of Earth, and not any more than that. But more than one may exist relatively close by.

"In the ocean analogy, we do not have to drain the entire ocean to find a fish," he said. "In the Houston analogy, if there were two cool things, you wouldn't have to look as hard."

Still, there's no guarantee that a figurative fish or needle or cool thing is out there at all.

Another group of scientists, this one at Oxford University, recently took a different approach to the question of aliens. Instead of focusing on the likelihood of finding "technosignatures" that could be detected, they examined the likelihood that intelligent alien life exists at all.

The Oxford researchers examined dozens of authoritative studies about variables in the Drake Equation. The team then analyzed the results and calculated a bleak 2-in-5 chance that humans may be entirely alone in the Milky Way galaxy.

There's also a more unsettling possibility: Perhaps aliens do exist nearby but don't want us to find them.