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Bad Omen

15 September 2020

Phosphine detected in the atmosphere of Venus. ESO / M. Kornmesser / L. Calçada & NASA / JPL / Caltech
Phosphine detected in the atmosphere of Venus.

Astronomers have detected the presence of phosphine in the atmosphere of Venus.1 This could indicate the presence of life since some living organisms are known to produce phosphine. There is a reason to be cautious since the chemical can also be made in other ways. This isn’t conclusive proof of life on Venus, but it is certainly exciting news and worthy of further investigation. But let us suppose for a moment that it is produced by life on Venus. If that’s the case, it could be terrible news for life on Earth.

Whether life is abundant in the universe remains one of the great cosmic mysteries. From the evidence we have, we know all the necessary ingredients for life are common. Most stars have planets, and countless worlds orbit within the habitable zone of their star. The building blocks for life as we know it are also found throughout the cosmos. We have found amino acids in meteorites, and can detect them in interstellar clouds of gas and dust. But as for life itself, we have yet have found conclusive evidence.

We have also found no evidence of intelligent life beyond Earth. Given that life could be cosmically abundant, why not? This is known as the Fermi Paradox and has given rise to many ideas. One is that the brilliant aliens are keeping us isolated in a kind of cosmic zoo. They won’t reveal themselves until we prove ourselves worthy, which could be a very long time given the state of our planet.

Life may play a necessary role in keeping a planet habitable. Credit: Chopra and Lineweaver
Life may play a necessary role in keeping a planet habitable. Credit: Chopra and Lineweaver

The Zoo Hypothesis sounds a bit like circular reasoning. The fact that we don’t see aliens is held as evidence they are out there. A better idea is something known as the Great Filter. First proposed by Robin Hanson, the argument is that while life often arises in the universe, there is some stage in the evolution of life that is extremely difficult. It could be that single-cell organisms are easy, but multi-cellular ones are difficult, so life rarely gets beyond bacteria. It could be that the Cambrian explosion of diverse life rarely happens. It could also be that young civilizations usually destroy themselves, which is why a benevolent alien species hasn’t visited us.

So what does that have to do with Venusian life and the future of Earth? If cellular life exists on Venus, then it implies that it exists almost everywhere. Venus is a hot, inhospitable world that has no liquid water and rains sulfuric acid. If life can evolve in the atmosphere of Venus, then places like Mars, Europa, and Titan almost certainly have life. That means the great filter doesn’t happen at an early stage.

The great filter must happen much later. While Earth has seen many periods of mass extinctions, life has always rebounded. When it does, it tends to increase its diversity pretty quickly. Intelligent life also seems to have lots of opportunities. If we primates hadn’t risen to civilization, it’s likely that some other branch of the evolutionary tree would have. If there is life on Venus, then a wet temperate world such as Earth will almost certainly give rise to an intelligent species.

But we don’t see intelligent life scattered about the universe. That would likely mean that the great filter occurs after the rise of civilization. Here on Earth, we now have the technological power to transform our environment. But our technology has also created pollution and global warming problems that threaten to drive us to extinction. We could be our own great filter.

It could also be that there is no great filter, and Fermi’s paradox has another solution. But as we search for life on neighboring worlds, perhaps we should also look a bit closer at our world’s long-term prospects.


  1. Jane S. Greaves, et al. “Phosphine gas in the cloud decks of Venus.” Nature Astronomy Vol. 4, no. 9, (2020). ↩︎