Are we alone in the Universe?

Written by: Wallace Arthur


Images courtesy of NASA and UnSplash

Are we alone in the Universe?

Wallace Arthur, author of The Biological Universe, sheds some light on one of humanity's most enduring questions.


The short answer to this question is ‘no’. As a scientist, I have little doubt about this, which is odd, because so far we have absolutely no evidence for extraterrestrial life of any kind, anywhere. And this applies to past life as well as present. The putative fossils described in a meteorite found in Antarctica in the 1980s are now recognized as having been produced by chemical, not biological, processes. The sum total of evidence for extraterrestrial life from about 60 years of searching – from the early days of SETI around 1960 to the present – is zero, zilch, nothing. A resounding silence. No replies to our outgoing messages; no incoming signals that have been generated by life-forms. We appear to be a living speck of dust, or a ‘pale blue dot’, as Carl Sagan famously referred to the Earth, in a barren expanse of space littered with lifeless stars and planets.

But that last word – planets – is the key to my odd positivity in the face of the overwhelming lack of evidence. The ancient Greeks knew of six planets: Mercury, Venus, Earth, Mars, Jupiter and Saturn. Then, in the 1700s and 1800s, the number went up to eight, with the discovery of Uranus and Neptune. For more than a century, the number remained the same, apart from the transient recognition of Pluto as a planet, a phase that ended in 2006 with its declassification by the International Astronomical Union, the reason for which we can ignore here, because we have bigger fish to fry.

The planet Jupiter, photographed by the Hubble Space Telescope. It’s lifeless, though some of its moons might not be. The many recently-discovered ‘hot Jupiters’ are also lifeless. Image courtesy of NASA.

In the 1990s, the number of known planets increased again, but in a different way. During that decade, the planets discovered were the first ‘exoplanets’ – those orbiting other stars than our local one, the Sun. By the turn of the millennium, more than 20 exoplanets had been discovered. By 2010, the number had risen to a few hundred, and today it’s a few thousand. As these figures suggest, this increase in human discovery of planets roughly fits an exponential pattern – it just keeps accelerating. When will it end? Actually, it probably never will – prior to our extinction.

The Transiting Exoplanets Survey Satellite, or TESS for short, has discovered Earth-like exoplanets. Image courtesy of NASA.

Here’s the reason why we will keep on discovering exoplanets ‘forever’. Apologies in advance for the astronomical numbers (in both senses of that term). There are about 400 billion stars in our galaxy, the Milky Way. Almost all of these are thought to have planets orbiting them. The number varies, but on a conservative estimate of the average – three – the number of planets in the galaxy is over a trillion. But the number of galaxies in the observable Universe is also over a trillion. So our best guestimate of the total number of planets is more than a trillion trillion, or ten to the 24th power for the mathematicians among you.

That’s an awful lot of planets. Some, such as the huge ‘hot Jupiters’ orbiting within a whisker of their star, are uninhabitable by life as we know it, and probably by life as we don’t. Others, such as those that orbit volatile ‘flare stars’, may be equally uninhabitable. But a small fraction of planets orbit in the habitable zones of quiescent Sun-like stars, and it seems inconceivable that none of these have life. After all, a small fraction of a huge number is a very large number indeed. It’s probably at least a billion trillion, or 10 to the power 20. That many potential Earths and only one of them with life? Nonsense! But as I said at the start, that’s only the short answer. The longer one is here:

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About the Author: Wallace Arthur

Wallace Arthur is an evolutionary biologist who is fascinated by the possibility of evolution occurring on other planets. His first book on this subject was Life Through Time and Space (Harvard 2017), which the Astronomer Royal Sir Arnold Wolfendale described as: ‘brilliant and thought-provoking in every way'. The Biological Universe is the seque...

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