We love to think that life is not just an insulated incident in the universe. Earth is swarming with 9 million species, including those clever enough to understand that intelligent life might occur elsewhere.
A string of discoveries has fuelled the idea that alien life is plentiful. A growing catalogue of exoplanets, many of which orbit in the habitable zone of their home stars, suggests that there is plenty of real estate for life.
Subsurface oceans on icy moons in the outer solar system are evidence of life-friendly conditions. The discovery of phosphine in Venus “toxic atmosphere suggests that life can thrive even in the most hostile places.
Given all this, it is easy to imagine intelligent life developing on a planet or one of the 100 billion stars in our galaxy. So simple that, given the vastness of the visible universe, we assume that there must be other technological civilizations out there.
But we haven’t heard from them yet. In the absence of evidence from space, when astronomers turn their focus back to Earth as the sole example of intelligent life, we will reconsider this question. And if we cannot find anything that is consistent with what biologists have been whispering about for a while, then the expectation of hearing about alien civilizations will fizzle out.
For decades, SETI researchers have swept the sky with radio telescopes hoping to discover messages from technological civilizations. Despite its fruitlessness, the effort has never been lacking in optimism.
When estimating the number of intelligent civilizations capable of transmitting or receiving radio signals from outside the Milky Way, we use a formula in 1961 by astronomer Frank Drake. The formula multiplies seven variables: the initial rate of star formation in the galaxy, the proportion of stars with orbiting planets, and what proportion of these planets are habitable. Thanks to the Kepler space telescope, which has discovered thousands of exoplanets, we now know that some stars harbor planets of which many could evolve life.
This means we can use solid numbers for several of Drake’s equations. But the calculations also include other biological variables which, aside from the example of Earth, we…