There are big questions in science that scientists care about, and then there are big questions that everyone (well Prospect readers, at least) care about. The big two in the "everyone" category are probably: what is consciousness (and how did the life that eventually became conscious get started in the first place?) And what was the spark that turned a chemical mush into life?
The answers to both questions, of course, are in all the world’s sacred books and I have to admit that so far we scientists haven’t provided anything much more convincing. The recent discovery of the gas phosphine in the atmosphere of Venus, however, may be a major step forward to at least answering the question about the origin of life.
An equation that measured alien life
The first person to think systematically about the chance of extraterrestrial life was the astronomer Frank Drake in the early sixties. Equations are usually a way of mathematicising something we know about the world, but Drake dreamt up an equation that illustrated just how little we knew back then about the origin of life.Drake’s equation estimated the number of intelligent alien civilisations in the galaxy. Drake started with the number of stars in the galaxy, which is roughly 300 billion. He then multiplied this by a large number of factors, two of the most important being the probability that a star has planets and—if the star does—the probability that one of the planets lies in the habitable zone in which water can exist in liquid form.
In the early sixties, apart from the number of stars in the galaxy, almost all the factors were completely unknown. We didn’t know whether any star apart from our own had planets, and the uncertainties in all the terms in the equation made it plausible that at the time, either every star in the galaxy had life around it, or that none of them did (apart from our own star, of course.)
We now know much more about most of the terms in Drake’s equation. We know that most of the stars in the galaxy have planets, and almost every month it seems there is an announcement of some Earth-like planet in a star’s habitable zone. We also know that Drake was much too conservative in his assumptions about life. The discovery of extremophiles—bacteria existing well out the normal temperature range of liquid water—shows that life is possible well outside a star’s habitable zone. Microbes have also been discovered existing kilometres deep under the Earth’s surface. Life, once it gets started, is clearly resilient and seems to be able to colonize virtually anywhere.
The resilience of life
The other planets in the solar system also seem much more hospitable as places for life than they did a generation ago. Four billion years ago, Mars probably had suitable conditions for life and it still has large amounts of frozen water: it is easy to imagine some of the extremophiles on Earth surviving there. Some of the outer moons have underground oceans, and on the surface of Titan, the largest of Saturn’s moons, there are even lakes of simple organic compounds. Even Pluto, way out in the frozen outskirts of the solar system, has been suggested as a possible location for life.One of the few planets in the solar system that has not been recently been suggested as a location for life is Venus. For a brief period, scientists (and science fiction writers) did speculate that there might be life there, hiding below the planet’s clouds, but we then discovered that Venus is the victim of a runaway greenhouse effect, with a crushing carbon dioxide-heavy atmosphere. No life, at least as we know it, could possibly survive on its surface.
But the one crucial factor in Drake’s equation that we still don’t know much about is the probability that life starts in the first place. Given that the chemical building blocks of life exist, what is the probability that these building blocks assemble into some basic form of life? Once—and if—that happens, it seems reasonable to assume that evolution will take over and eventually produce the smorgasbord of life we see around us.
One huge cosmic fluke
But this factor suffers from the one-in-a-trillion problem. We know of only one planet on which life has arisen. Does life exist here because of some huge cosmic fluke, or is life actually very common in the universe? There are roughly one trillion planets in the galaxy. We are here, of course, so we know that the probability of life arising on a planet must be at least one in a trillion. But since we have only ourselves as an example, we don’t know whether the probability of life occurring is actually one in a trillion or whether it is as routine as dirty laundry.But here, too, there has been one big advance. The two families of big molecules that make up life on Earth are the proteins and the nucleic acids, RNA and DNA. The building blocks for these molecules are amino acids (for the proteins) and nucleobases (for the nucleic acids). The building blocks themselves are quite big molecules and in Drake’s time it wasn’t clear whether these molecules would form easily from smaller molecules. But both amino acids and nucleobases have now been found in meteorites, and other related molecules are now routinely detected in interstellar space. The galaxy therefore now seems a much more benevolent place for life than it did a generation ago: we now know that there are a trillion planets and the building blocks for life everywhere. But we still don’t know the chance of life arising on one of these planets—that still remains the one-in-a-trillion problem.
What the Venus discovery tells us
The discovery of phosphine in the atmosphere of Venus, which was announced last week, may provide the solution. Although Venus is designated the "hell planet," in the sixties Carl Sagan suggested that one place where life might survive is somewhere high in the planet’s atmosphere, where the pressure and density are not that different from those in our own.Several years ago, an international team of scientists, led by Professor Jane Greaves—who I should say is a colleague and friend of mine—set out to test Sagan’s idea by looking for the gas phosphine in Venus’s atmosphere. Phosphine is a useful biomarker, a signature that life is present on a planet. The phosphine on Earth is produced by microbes and is a biomarker because it is rapidly destroyed by reactions with other chemicals in the atmosphere, which is true on Venus as well. Therefore, if any phosphine is in the atmosphere, it must have been produced by recent biological activity.
Jane and her team, I suspect slightly to their surprise, did detect phosphine on Venus. They then spent years trying to think of every other possible chemical or geological way it might have been produced. In the set of papers they released last week, they were careful not to say they had discovered life on Venus and that it might still turn out that the phosphine was produced by some currently unknown chemical or geological process. They speculate that if life is the explanation, the life probably originated on the surface before the runaway greenhouse effect and the microbial life then gradually moved upwards as conditions worsened on the surface.
They have, therefore, not discovered life in hell. But they have discovered the first solid evidence for life outside the Earth. The significance of this for me is that, if the evidence survives—it looks convincing to me but it is still a big if—it solves the one-in-a-trillion problem. If life did start independently on two planets in a single planetary system, which is otherwise nondescript in every way, it must be common everywhere.
