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Reid: In 1960, a young astronomer named Frank Drake sent radio waves into space, hoping to make contact with an alien world. His signal was never answered, but it was the first modern search for extraterrestrial life. 20 years later, Drake became one of the first members of the Search for Extraterrestrial Intelligence, or SETI Institute, and they’ve been combing the stars ever since.

Okay, so... where are my aliens? Could there be other creatures sending signals for us to find? Or could we look forever but never find anything, because we’re the only ones here? Only a year after his first experiment, Frank Drake proposed the Drake Equation to estimate the number of alien civilizations in our galaxy sending signals we can detect. But we don’t yet know enough about the universe to solve the Drake Equation.

In the meantime, it is a useful way to think about the odds that there are other intelligent species out there. To figure out how many aliens live in our neighborhood, Drake had to first decide on what factors contribute to the birth of intelligent life. He eventually decided on seven.

The first variable was R*, the number of stars born each year that could support life. Life probably won’t evolve around stars that are too cold and dim, that are unstable, or that burn out too quickly. The best habitable stars seem to be stars like our sun, which have been stable and bright for billions of years. In the decades since the Drake equation was developed, we’ve learned that about 1 of these stars forms in our galaxy every year.

Drake then considered the fraction of life-friendly stars in our galaxy that have planets, f_p, and the average number of planets in those solar systems with environments that could support life, or n_e. In other words, planets with liquid water and an oxygen-rich atmosphere. By measuring the brightness of different corners of the sky and using statistics, astronomers have figured out that there are between 100 and 400 billion stars that could support life in the Milky Way Galaxy.

With data from the Kepler Space Telescope, which was built to study Earth-like planets outside our solar system, they can go even further and estimate the number of habitable planets in our galaxy between 40-49 billion.

Now, here’s where things get tricky. While we have rough estimates of the first three variables in the Drake Equation, the last four are a little more complicated. Drake’s fourth variable, f_l, was the fraction of those habitable planets with life. The next variable is the fraction of those planets with intelligent life, f_i and the fraction of those intelligent civilizations that are broadcasting signals into space, f_c. Finally, there’s L: the length those civilizations have been broadcasting. Even signals traveling at light speed can only go so fast, so if a distant planet hasn’t been broadcasting very long, there’s a chance the signal may not have had time to reach us here on Earth.

According to Drake, by multiplying those seven terms together, you get N, the number of alien worlds broadcasting messages we can detect from Earth right now. The problem is, we’ve never found life on a planet besides Earth, so we have no idea what fraction of the billions of planets in our galaxy have life, let alone intelligent life. So, until we find ET, we won’t know what those last four variables should be.

And even some of the numbers we DO have are based on life as we know it here on Earth. There could be creatures that thrive in toxic atmospheres or that can’t stand water, which would totally throw off our guesses. But that’s okay, because the Drake Equation isn’t designed to have an answer — instead, it’s a tool to help us think about how alien life might exist.

It helps us think about what life off-Earth could be like, and our odds of finding it under different conditions. Like in a paper published in May of this year in the journal Astrobiology. In the paper, two researchers rewrote the Drake Equation so that instead of asking, “What are the odds we can find intelligent aliens right now?” it asked, “What are the odds humanity is the only advanced civilization to have ever existed?” They considered only two variables: the things we know, and the things we don’t know.

Their first variable, the number of habitable planets in a given corner of the universe, takes into account what astronomers have learned over the past 50 years. Their second variable, the odds of finding an intelligent civilization on one of those planets, covers everything we still don’t know. Using this version of the equation, the researchers concluded that humans aren’t the first intelligent species, unless the probability of intelligent life evolving is less than one in 10^24.

That’s a 1 with 24 zeroes after it, or a trillion trillion. While we don’t actually know the odds of intelligent life evolving — they could be much higher or much lower than a trillion trillion — their analysis is another way of helping us consider one of humanity’s most basic questions: Are we alone?

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