Anybody out there?

For hundreds of years, people have looked into the night sky and wondered if anyone’s looking back.

But answering the question “Are we alone?” isn’t easy.

It unleashes more questions: How many other planets could support life? If there is other life somewhere out there, is it intelligent? Is it near enough to us that we could ever make contact?

To try to break down such a complex question into manageable parts, scientific thinkers have one main approach: the Drake equation, an old idea that is getting fresh attention now that astronomers are identifying thousands of planets beyond our solar system.

Thought up by astrophysicist Frank Drake in 1961, it takes the big, unanswerable question of whether or not we’re alone and splits it into bite-sized factors that are easier to mull over and quantify. It doesn’t promise an answer, but rather a clearer way to think about the question.

“It has primarily been useful as a discussion starter and discussion organizer rather than a clear outcome when processing in your mind,” says University of Maryland physicist Cole Miller.

The equation is made up of seven variables, each of which represents an unknown quantity. When combined, they produce an estimate of the number of civilizations we might be able to make contact with in our galaxy.

Here’s a breakdown of all the factors: In the Milky Way, stars form at a rate of R*. Some (f_p) of those stars have planets, but a smaller percentage (n_c) of those planets are capable of supporting life, and an even smaller percentage (f_l) of those may actually develop at least basic life . A fraction (f_i) of those planets with any life may develop intelligent life. (Miller describes this as the difference between E.T. or Spock and “a whole of bunch of blue-green slime.”) And of those intelligent life forms, an even smaller fraction (f_c) will have the technology to communicate with us.

This is all useful — and daunting, if you’re rooting for a real-life visit from E.T. — but we’re still not done. In its final factor, the Drake equation accounts for how long (L) these intelligent, technologically advanced civilizations will exist before they self-destruct or otherwise disappear.

So, according to the Drake equation, the number of current civilizations we can communicate with in the galaxy (N) is equal to seven factors multiplied together: N = R* x f_p x n_c x f_l x f_i x f_c x L

Since we don’t have precise values for each of these factors, we need to rely on estimates instead.

In his University of Maryland classes, Miller asks students to suggest numbers for the factors. He then groups the low values together and does the same with the high values.

When the numbers are multiplied, the results inevitably suggest one of two extremes: either that “the galaxy, let alone the Universe, should be teeming with life, or that we alone exist in the Universe,” Miller explains.

The good news is that science is making some modest progress on getting more reliable numbers to plug into the Drake equation. “We are living in an era where, remarkably, we’re getting answers to some of the questions that are factors” in the equation, Miller says.

Thirty years ago, the scientists knew only of the planets orbiting our sun. Today, astronomers are finding new exoplanets — planets orbiting other stars — every month. As of January 2019, there were nearly 4,000 confirmed exoplanets.

But things get murkier for the next factor of the Drake equation: the percentage of planets capable of sustaining life. In a 2013 study, astronomers estimated that 22 percent of stars like our Sun could have planets close to Earth’s size orbiting in that star’s habitable zone: the right distance from the star to possibly have liquid water. In a dissertation posted to the preprint server arXiv in 2015, astronomer Erik Petigura increased that fraction to 26 percent.

Since there are anywhere from 50 billion to 200 billion stars in the Milky Way, Petigura’s most recent approximation suggests there could be anywhere from 18 billion to 52 billion Earth-like planets.

Although Miller describes the Drake equation as a fun and useful way to frame the question of extraterrestrial life, he worries that it constrains the search to what is already familiar to us here on Earth. “What’s necessary for life in the Universe? Does everything in the Earth have to be duplicated? Can you have something like an Earth around a Jupiter, but warmed by effects other than the Sun and have intelligent life?” he asks.

There are certain criteria that all life forms meet on Earth — cells, excretion and death, for example — but if scientists assume that those criteria are universal, they could miss novel forms of life, explains Carol Cleland, a philosopher of science at the University of Colorado Boulder.

For example, she proposes the potential of a “shadow biosphere:” There could be microbes on or outside Earth that have a different chemistry we can’t detect with the techniques so far developed. “It’s highly likely there was more than one origin of life, but whether or not it survived is a question. Microbial life dominated the Earth for several billion years, until suddenly multicellular life developed, but not all microbial life evolved into plants and animals,” Cleland says. Since life modifies its environment, she suggests that scientists try to detect the traces, or shadows, left behind by these other life forms.

Like the drunken man looking for his lost keys under a streetlight because it’s the only place he can see, we might be looking for life only where we can detect it, not where it might actually exist. And if we’re only looking for a form of life that looks and acts like life on Earth, we might never find any other kind.

The Drake equation limits the search to intelligent life that can communicate with radio signals, but Massachusetts Institute of Technology astrophysicist Sara Seager suggests replacing this factor. She explained that for over 400 years, astronomers used telescopes as traditional visual astronomy tools to look for what’s out there, and only after World War II did we have radio astronomy to help look for active signals of life. She thinks looking for gases as passive signs of life may be more fruitful than hoping an alien civilization is emitting radio signals we can detect.

But detecting these life-signaling gases won’t be easy. “The atmospheres of exoplanets are hard to study,” Seager says. “Think of the atmosphere as the skin of the onion as opposed to the onion.” The gases around the planet would be a much thinner layer compared to the rest of the planet. Plus, the big gaseous star near the planet might overwhelm the planet’s own gaseous signals.

Although Cleland thinks it’s extremely likely that microbes are living on other planets in the galaxy, and fairly likely that multicellular organisms are on exoplanets, too, she’s pessimistic about finding other technologically advanced civilizations. “We are a living lesson that highly technological civilizations destroy themselves. Once destroyed and the environment is made uninhabitable, it’s hard to come back,” she says.