Extremely Thermonuclear Takes: Is There Life on Venus?

Sometimes, when a big news story in the sciences is set to break, there’s what we call an “embargo”. Certain journalists get informed ahead of time about what the news is — for example, the discovery of the Higgs Boson — allowing them to write up their pieces in advance and release a full, comprehensive, and detailed account of what the discovery is and what it means.

This system is there for a reason: to prevent rumours, incomplete information, or inaccurate stories from circulating and then having to be retracted, which is damaging for the overall reputation of science.

Last night, something leaked past the normal embargo, and it’s a pretty exciting story, and I feel like we could all do with some wonderful science-y distractions from everything that’s generally going on in the world, so I’m going to talk about it — on the understanding that this is still early days for this news, and that it will require a lot more investigation before we can be totally confident of what’s going on here.

Nevertheless, here’s the headline: astronomers think they may have found biosignatures — evidence of microbial life — in the atmosphere of Venus.

So let’s unpack this. Essentially, we can use spectroscopy — splitting the light from planets and their atmospheres into its constituent parts — to determine the composition of their atmospheres. Given that every molecule has its own unique “footprint” in terms of light emission and absorption, you can look for these signatures in the atmosphere itself to determine what the composition of that atmosphere is.

One of the major fields in astrobiology, then — the search for extraterrestrial life — is to use these spectroscopic techniques to probe the atmospheres of other planets, looking for signatures that might possibly be produced by life.

For example, one such potential signature that has often been a source for concern is methane. We know that methane can be rapidly destroyed by photochemical processes in the atmospheres of planets, and we also know that it’s produced heavily by lots of life-forms on Earth and other microbes. If methane continually shows up in the atmosphere of a planet, where convenentional chemistry would suggest that in equilibrium there should be very little, if any, then it may be a potential sign of life. There’s an ongoing debate surrounding, for example, measurements of methane on Mars and what significance that they may or may not have. [As well as how reliable the measurements are, of course.] Similarly, you might look for planets with a large, unexplained buildup of oxygen in the atmosphere, which could be evidence of photosynthesis — things like this.

So what you’re looking for is really a process that inorganic chemistry — the standard chemistry of non-living organisms — can’t really explain by itself: that your models of how the planet’s atmosphere should work cannot generate by themselves. In this gap, you have evidence that some other kind of chemistry is potentially going on. The chemistry of life.

In this particular case, a group of researchers have discovered evidence of a chemical called phosphine in the atmosphere of Venus. Phosphine — chemical symbol PH3 — is probably best known to the general public as the gas that Walter White created in the first episode of Breaking Bad to kill a couple of drug dealers: it’s toxic stuff.

The reason that this particular detection is interesting is that we don’t really know of too many ways that phosphine can be made. Here on Earth, some of it does naturally occur through the decay of organic matter. It can also be created in the lab, by for example interacting phosphorous with potassium hydroxide. And we also know that under certain circumstances, when they’re lacking in oxygen, it can be produced by microbes, although we still don’t know the precise mechanism by which they produce the phosphine. It’s considered likely that they probably absorb phosphate minerals, add some hydrogen, and expel phosphine.

A paper from 2019, “Phosphine as a Biosignature Gas in Exoplanet Atmospheres”, assessed the idea of using this gas as a potential sign of life. They noted that this gas can be produced and has been detected in, for example, the atmospheres of stars, and in the atmospheres of gas giant planets like Saturn and Jupiter. Essentially, these planets have high enough temperatures in some locations that the production of phosphine is “thermodynamically favoured”, i.e. happens by natural chemical reactions, and it then migrates to the top of the atmosphere where it has been detected and studied for many years by our telescopes.

But there aren’t too many natural processes by which we would expect this gas to be made on a planet like Venus, which is likely too cold in most regions for its production to be thermodynamically favoured. The point here is that the level of phosphine you actually need to be detectable in an atmosphere has to be pretty high — parts per million, rather than trace levels of parts per billion. This makes it difficult to explain by other mechanisms, such as volcanism, lightning strikes that temporarily produce conditions where it could be created, or delivery to the planet by other mechanisms from other regions (such as, say, meteorite strikes.) These processes would all produce phosphine in far too small an amount to be detected from Earth, according to the authors of the paper. For that reason, they consider the presence of a significant amount of phosphine to be a strong indicator of anoxic life forms — probably microbial life.

So now we come to the results of this new study, which suggest that they have found phosphine in rather large quantities in the atmosphere of Venus. This has been discovered by scientists from MIT, Cardiff, Manchester and other Universities, using the James Clerk Maxwell telescope and the Atacama Large Millimetre Array.

They also think that they’ve discovered this phosphine in a region of the Venusian atmosphere that has often been speculated as being a habitable region, where the temperatures and pressures aren’t too awful. [The surface of Venus is basically a death zone that would likely sterilize most forms of life that we can imagine, so the only place life could persist would be in this upper atmosphere.] One of the study’s authors is quoted as saying: “Astronomers will think of all the ways to justify Phosphine without life and I welcome that. Please do, because we are at the end of our possibilities to show abiotic processes that can make Phosphine.”

So, what to make of this? I think, before we get too excited, we should remember that the process of doing good science requires lots of things before we can definitively say that a discovery has been made. I’m reminded of, for example, the experiment that thought it had discovered faster-than-light neutrinos. What they did was not actually to announce that they had discovered something that broke the light speed limit, but instead invited other scientists in to exploring this unusual result to see if they could explain it by another means. Eventually, it was explained by other means, and special relativity survived, which is a good job too. The point being that before your work is thoroughly reviewed, checked, and you have several lines of evidence for your claim, it’s too early to jump to any particularly radical conclusions about their being microbial life on Venus.

So what are the options? Well. Perhaps the measurement is incorrect, although I think that’s probably the least likely explanation.

More likely, there might be some new phenomenon in the atmosphere of Venus or other rocky planets that could produce phosphine. It is a relatively simple molecule — PH3 — which can be produced by many different chemical reactions. The authors claim that they don’t know of any abiotic pathway — i.e. a pathway without life — that could produce this much phosphine — the mechanisms they considered could only produce around 1/10,000th of the phosphine that they detected, although again, these calculations are obviously going to be subject to a lot of scrutiny now.

On that, I’m really not qualified to comment, but it’s of course also possible that there is plenty that we don’t know about Venus, and other possible mechanisms may be discovered that could produce this kind of false alarm. The scientists themselves say that “Either we’ve discovered life, or our understanding of rocky planets is severely lacking.”

I have to admit that I’m nowhere near qualified to tell you which of these two options is more likely. It wouldn’t stun me if, now that this news is out there, others come up with different possible mechanisms for the production of phosphine in Venus’ atmosphere which actually don’t end up requiring life.

Certainly, you need some pretty hefty additional confirmation to be absolutely sure that this is proof that microbial life exists in Venus’ atmosphere. The scientists hope to do this going forward; studying the signal in more detail to see if there are diurnal or seasonal variations in the production of phosphine, which again might possibly indicate a biosignature. It would also make an extremely strong case for us to launch more probes at Venus to detect the source of this signal and see if we can possibly get more evidence about what’s really going on here.

So, what to make of this potential discovery? I think it’s still worth taking it with a great deal of scientific reticence. Life in the Venusian atmosphere is a big claim, and it’s still quite possible that what we’ve actually discovered is just some interesting chemical or physical process that we weren’t aware could happen on a planet like Venus which has allowed this phosphine to build up. So before we start praising our microbial overlords just yet, we’re due a little bit of caution here, and I will want to see all the questions that they come up with at the press conference to determine this in advance.

On the other hand, it’s an incredibly tantalising prospect. So, given it’s been such an awful year, let’s allow ourselves now to speculate wildly about what this could mean.

I’ve always been convinced that we are not alone in the Universe. For me, this comes down to some pretty simple maths. You have two numbers — one, the truly vast number of potentially habitable planets in the Universe — and two, the tiny probability of life developing on a planet while it’s habitable.

The idea to me that these numbers exactly cancel out when you multiply them to give you 1 — the idea that we would be the only planet that had ever formed life in the Universe — seems to me to be hopelessly naive and quite bizarre. Why would that be the case? It simply doesn’t make sense.

For example, a paper by Frank and Sullivan back in 2016 tried to estimate how unlikely technological civilizations would have to be, given a habitable planet, for us to be the only one that had ever developed in the Universe. They settled upon the idea that the lower bound would be one in a 10²⁴ — i.e. if you have a habitable planet, you have to have odds of less than one in a million billion billion for a technological civilization to develop on it for us to be the only one that’s ever developed. That seems quite unlikely to me.

There’s a big range of probability density, of course, that there are no other technological civilizations within our galaxy, but there are some in other galaxies in the Universe. That may well be the most likely outcome that is most consistent with my expectations and our observations to date.

We know that life can form under certain circumstances, because we exist. We know that the circumstances that produced us have likely been reproduced countless times across the Universe at different points in history. So any idea that we’re all there is, frankly, would strike me as evidence of some kind of divine intervention.

That’s one perspective, and it’s justified by the law of large numbers — heck, forming life could be rare enough that it’s only happened once in our galaxy, and yet there would still be potentially billions of other galaxies where life could have formed or could form. Essentially, the only “upper bound” I’m setting with a belief that life is probably out there, somewhere other than Earth is that the formation of life is not next-to-impossible, but simply very, very, very unlikely. Maybe it takes a billion habitable planets a billion years of random chemical reactions before you have that rare coincidence that allows life to form. When you only have one data point, one instance when something happened, you can only really say that it’s evidently not impossible.

Life on Venus, if detected, changes the whole ballgame there. If, as the scientists suggest, life formed many billions of years ago when the planet was habitable, and now exists only as vestiges in that atmosphere (with the surface totally inhospitable, as we’ve said), then this gives us the thing that we’ve always hoped for in astrobiology: another data point. What’s more, we’d be saying that of the planets in our solar system, life has formed on at least two of them at one point.

If you want to talk about Bayesian reasoning — where new information updates your prior beliefs on a subject — I mean, holy hell, does that update your prior belief on how likely life is to form. If life has formed separately on two planets within our solar system, that would surely mean that the probability of forming life on a habitable planet is pretty damn high. Either that, or the old idea of panspermia — that life forms throughout the solar system came from a single source and were “seeded” from outside our solar system, perhaps by a shower of meteorites that contained life or something like that, would seem to me at least to be quite a bit more likely now. [Then, of course, you only need to explain a single origin of life in the Universe again — from whatever region of space it came to us from!] Saying “the alien life on Venus may simply be our distant cousins” is too irresistible an opportunity to pass up.

But if we did conclude that life managed to form separately on two planets in our solar system, given the sheer number of planets that are in the habitable zone and likely to have been habitable at one point throughout the Universe, then you would expect the Universe to be simply teeming with life. It’s always been tantalising to note that, as far as we can tell, with our one existing data point, life formed on Earth very shortly after Earth first became habitable, where the kind of conditions that life might form on Earth took hold. If it’s a low probability event, you might have expected to have to wait quite a while for it to happen, but it seems to have begun almost immediately.

Again, if we find evidence that life formed on Venus, too, when it was habitable, that totally flips the script. Suddenly, it seems that life has a very high probability to form on habitable planets.

And then, the so-called Fermi Paradox comes back to haunt us again — where the hell is everybody???

I’ll remind everyone of the Drake Equation — okay, not really an equation, more a way of trying to determine what factors might influence our ability to detect life in the Universe.

Remember, it’s:


mean rate of star formation


fraction of stars that have planets


mean number of planets that could support life per star with planets


fraction of life-supporting planets that develop life


fraction of planets with life where life develops intelligence


fraction of intelligent civilizations that develop communication


mean length of time that civilizations can communicate

Now we have pretty decent constraints and bounds on the astrophysical side there. We know that stars form quite often; we know that there are plenty of exoplanets, planets are very common in the Universe; we are finding more and more evidence for planets that seem like they could support life, or could one day support life: they may well be abundant even within our own galaxy.

Again, as I say, if this Venus discovery turns out to be independently-developed life, then we suddenly have another quite high upper bound to set here. It would appear, then, that the fraction of life-supporting planets that develop life must be pretty high. Otherwise, it would be a hell of a coincidence for it to happen twice in our solar system.

At that point, I think we would actually find it difficult to explain why we haven’t seen any evidence for other technological civilizations within our own galaxy. We would have to assume that the great filter which reduces that number must lie somewhere else — maybe developing intelligence is very difficult (after all, we have no evidence that the life on Venus, if it exists, was ever actually intelligent: it could have been microbes all along.) Perhaps intelligent civilizations don’t want to communicate with us — as I’d joke with my friend Georgie, we’re basically just the trash of the Universe that no-one can be bothered to speak to until we become more civilized — or maybe it’s just that communications occur in a different way, or are far less common than we think. Or, perhaps, chillingly, as Fermi himself worried about, it’s very rare for technological civilizations to survive for that long.

Do technological civilizations inevitably discover some technology that wipes them out? Is that barrier — between being able to make things like nuclear weapons, nanotechnology, whatever it may be — and actually colonising the rest of the Galaxy much too high for the average civilization to vault? And here we’re really into the fun zone, going back into our series on existential risks, speculating about the end of the world, and all the things that you know I’d happily do for hours if you let me get carried away on the subject.

So let’s get back down to Earth — which, we now discover, there’s a tantalising possibility (albeit only a possibility) is one of several planets in our solar system to harbour life. All we have, at this stage, realistically, is a clue that something unusual and unexplained is going on up there on Venus. We’ve all seen plenty of these exciting scientific stories and we know that quite often, there is a more reasonable and boring explanation for what is going on, even though initially the findings can seem remarkable.

So I want to pour a little cold water on the excitement, given that it’s still very, very early days with this story. We need a hell of a lot of extra research to be done to confirm such a huge revelation without any doubt. But at the same time, I really don’t. At the very least, it feels like 2020, which has been a terrible year for most of us and a particularly bad one for me, owes us aliens.

And, if it were to be true, the philosophical implications of this are truly mindblowing.

Cosmology and astronomy has been a relentless sequence of events that made us realise that we are not the centre of everything. First, we realised that we were not the centre of the solar system; then, that the pinpricks of light in the sky were Suns, like our Sun; then, that the smudges of nebulae in the sky were whole galaxies, just like our own Milky Way. Every discovery has made us realise just how small we are — and how vast, unknown, complex, and fascinating the rest of the Universe is. Every time we thought we were in some way special or unique, we turned out to be wrong, shaken out of our solipsism by observations amongst the stars. Would it really be such a shock for us to realise that we are not even the only living beings in the Universe? The only intelligent life?

If this turns out to be true, and we — poor, foolish, misguided species — somehow manage to stumble and stagger through the minefield of the 21st century and on into the future, this would be a pivotal discovery in our history: the moment we realised that we were not alone as living beings in the Universe. And if you can’t let yourself get a little bit excited about that prospect, even if there’s a long way to go yet to actually prove it… what can you get excited about?