The Conversation contributed the article to Space.com's expert voices.
Do you think there should be many intelligent civilizations in our universe? Frank Drake, an American astronomer who died at the age of 92, came up with an equation to estimate this. The Drake equation, from when he was too naive to be nervous in his career, became famous and bears his name.
Drake is in the company of physicists with equations named after them. Drake's equation doesn't represent a law of nature. It combines some unknown probabilities into an informed estimate.
It's hard to ignore the conclusion that we shouldn't be alone in the universe. Has Drake's equation taught us anything about extraterrestrial life?
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The principles of Drake's equation are easy to understand. In a universe as old as ours, the number of civilizations that are visible by virtue of them broadcasting their presence must equate to the rate at which they arise.
Drake realized that a value on the rate at which civilizations occur can be broken down into more manageable parts.
The rate at which suitable stars are formed is equal to the rate at which planets are formed. The number of planets that are capable of bearing life per system, the fraction of those planets where life gets started, and the fraction of those where life becomes intelligent are all taken into account.
The rate of star formation was the only thing known about Drake's equation when he first came up with it.
In the 1960s, there was no evidence that any other stars had planets, and one in 10 of them might have. Observational discoveries of exoplanets that began in the 1990s and have blossomed this century make us believe that most stars have planets.
Most systems of multiple planets would include one at the right distance from its star to be able to support life, according to common sense. Our solar system has a planet in it. It's possible that Mars was suitable for abundant life in the past.
We now know that planets don't need to be warm to support life. It can happen in the internal ocean of an ice covered body, supported by radioactivity or tides.
There are many possible candidates for the moons of Jupiter and Saturn. When we add moons as being able to host life, the average number of habitable bodies per planetary system could easily exceed one.
The terms towards the right side of the equation are more open to challenge. Life will start anywhere that is suitable given a few million years of playing.
In other words, the fraction of suitable bodies where life actually gets going is equal to one. According to others, there is no proof of life outside of Earth, and that the origin of life could be extremely rare.
Will intelligence evolve once life starts? It has to become multicellular before it gets past theMicrobe stage.
Multicellular life may not be a barrier due to the fact that it started more than once. On Earth, the "right kind" of multicellular life, which continued to evolve, appeared only once and could be very rare, according to others.
Intelligence might give it a competitive advantage over other species. We do not know for certain.
Will intelligent life develop technology to broadcast its existence across space? It may be rare for inhabitants of internal oceans of frozen worlds.
There is a question about where all the aliens are.
Is there an average lifetime of a civilization? Our TV broadcasts made Earth visible from afar in the 1950s, giving us a minimum value of about 70 years.
L may be limited by the collapse of civilization, the near total demise of radio broadcasting in favor of the internet, or a deliberate choice to "go quiet" for fear of alien invaders.
You can play with the numbers. If L is more than 1000 years, N is likely to be more than 100. Drake stated in an interview that his best guess was about 10,000.
We are learning more about exoplanets every year, and are entering an era when we can measure their atmospheric composition to see if life exists. We can hope for a better estimate of how many Earth-like planets there are within the next decade or two.
It won't tell us anything about life in the internal oceans, but we can hope for some insight into the icy moons of Jupiter, Saturn, and Uranus. We could detect signals from aliens.
Frank Drake's equation has stimulated so many lines of research that it will continue to give us a thought- provoking sense of perspective. We should be thankful for that.
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