How Common is Life, and Are We Unique?

In a new paper published in Nature Astronomy, Sara Seager from MIT and co-authors tested whether life as we know it is viable in an atmosphere very different from Earth’s—one dominated by hydrogen. They showed that some common terrestrial microorganisms, such as the bacterium E. coli and the yeast Saccharomyces cerevisiae, can still grow in an oxygen-free atmosphere consisting of hydrogen, helium, or a mixture of nitrogen and carbon dioxide.

This is not a major surprise by itself. But the authors argue that it will expand the number of places where life might exist, as there are plausible scenarios where even rocky planets like our own can have an atmosphere consisting mostly of hydrogen.

In principle that is correct, but there are several caveats. I doubt that very many of these worlds exist, for a couple of reasons. First, the scenarios for a terrestrial rocky planet with a hydrogen atmosphere seem to be rather contrived. Hydrogen will escape quickly from an atmosphere like ours, because the gravitational force of a terrestrial planet is too weak to hold on to it (unlike large “gas giant” planets such as Jupiter and Saturn). The most likely scenario for a rocky planet with a hydrogen atmosphere, in my view, is a so-called rogue planet that got ejected from its solar system.

Even when a hydrogen atmosphere is retained, the hydrogen would surely be a great food source for microbial life. This is the case on Earth, with the most basic metabolic pathway (methanogenesis) using up hydrogen and carbon dioxide to produce methane and water. Since hydrogen outgassing rates are very low—at least on planets like Earth—any microbes would be expected to consume it faster than it’s produced. So the presence of hydrogen in an exoplanet’s atmosphere would be more an anti-biosignature: evidence that life is not present. Still, given the zillions of planets in our Universe, there may be a significant number of the kind of world Seager imagines that are habitable and host microbial life. Only microbial life, however. For more complex life you would need a chemical like oxygen to provide the required amount of energy.

Might we assume, then, that when oxygen became available, Earth underwent the “typical“ evolution of life expected to occur on a habitable planet circling an average G dwarf star like our sun? As it turns out, maybe not.

Timo Reinhold and colleagues from the Max-Planck Institute in Göttingen, Germany, examined 369 Sun-like stars and found that most of them are much more variable in their energy output than our own local star. The Sun seems to be exceptionally stable—which is good for life on Earth, since our star is less prone to emit solar flares that would cause radiation damage to living things on the surface. But is the Sun really exceptional? Or is it perhaps in a period of low activity? The study by Reinhold et al. cannot tell.

If our Sun is unusual compared to other G dwarf stars, it may have a bearing on the Fermi Paradox, which calls into question why we haven’t already found technologically advanced extraterrestrials. It would say that our setting here on Earth is special, although by no means unique. And even if life is common on hydrogen-atmosphere planets (of the kind Seager considers), complex life would be unlikely—unless we’re only glimpsing a particular moment, early in the planet’s history, before life moved on to more powerful metabolic pathways.

So what does all this mean? Many questions and possibilities, and few constraints. We’re still at the beginning of understanding life, its origin, evolution, and place in the cosmos. And the best way to advance our understanding is to find life on another planet.

Dirk Schulze-Makuch | | READ MORE

Dirk Schulze-Makuch is a Professor at the Technical University Berlin, Germany, and an Adjunct Professor at Arizona State University and Washington State University. He has published eight books and nearly 200 scientific papers related to astrobiology and planetary habitability. His latest books are The Cosmic Zoo: Complex Life on Many Worlds and the 3rd edition of Life in the Universe: Expectations and Constraints.

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