What are the odds that the universe is home to advanced alien civilizations? Astronomer Frank Drake came up with an equation to help scientists ponder those odds back in 1961. Most of the variables of the equation are as yet unsolved, but by turning the equation on its head, and incorporating the latest data from the Kepler telescope, a pair of astronomers has come up with an interesting estimate.

In the *New York Times*, study author Adam Frank describes the research in an opinion piece titled, “Yes, There Have Been Aliens.”

Drake’s famous equation measures the odds that advanced civilizations are out there and broadcasting their presence. It comes down to **(R*)** the number of stars born each year that **(f _{p})** form planets that

**(n**can support life. Out of those that

_{e})*can*support life, how many

**(f**actually do evolve life, and out of those, how many

_{l})**(f**evolve intelligent species that

_{i})**(f**are technologically advanced enough to release detectable signals into space? And then, finally, how long

_{c})**(L)**do those civilizations survive to broadcast those signals?

Multiplying all those variables together would give a rough estimate of humanity’s chances of making contact with an alien civilization.

The Drake Equation estimates the number of technologically advanced civilizations we can communicate with

N = the number of civilizations in our galaxy that we can communicate with

R*=average rate for the formation of stars that are suitable for life

f_{p}= the fraction of those stars that have planets

n_{e}=the fraction of those planets with an environment suitable for life

f_{l}=the fraction of those planets where life actually appears

f_{i}=the fraction of planets with life that evolves to be intelligent

f_{c}=the fraction of planets with intelligent life that develop technologies that release detectable signs into space

L=length of time those signals are released

Scientists still don’t know what numbers to plug into most of the variables, but they’re getting closer. They know how many stars are out there, and thanks to the planet-hunting Kepler space telescope, they know that the number of planets is much larger than scientists could have hoped for back in the 60s. Kepler has discovered 2,300 planets so far, leading to the estimate that every star in the sky probably has at least one planet circling it.

The pair also estimates that 20 to 25 percent of planets are likely orbiting in their stars’ habitable zones–the area around a star that’s not so close that all a planet’s water would boil off the surface, and not so far that it would be a frozen wasteland.

That takes care of variables **R***, **f _{p}**, and

**n**, with four unknowns left. Then the researchers restructured the question to get rid of another variable,

_{e}**L**. In study author Adam Frank’s own words:

Instead of asking how many civilizations currently exist, we asked what the probability is that ours is the only technological civilization that has ever appeared. By asking this question, we could bypass the factor about the average lifetime of a civilization. This left us with only three unknown factors, which we combined into one “biotechnical” probability: the likelihood of the creation of life, intelligent life and technological capacity.

…[W]hat our calculation revealed is that even if this probability is assumed to be extremely low, the odds that we are not the first technological civilization are actually high. Specifically, unless the probability for evolving a civilization on a habitable-zone planet is

less than one in 10 billion trillion, then we are not the first.

Frank goes on to explain that pessimistic scientists have estimated that there is only a one in 10 billion chance per planet that an advanced communicative species would evolve. But even with those slim odds, more than a trillion advanced civilizations could have evolved over the history of the universe.

While the calculations don’t say much about whether advanced alien civilizations currently exist, Frank writes, they “almost certainly existed at some point in cosmic history.”

### Sebastien Clarke

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