One of the most fundamental questions in the search for extra-terrestrial life is also the most basic. In the words of Enrico Fermi: “But where is everybody?” Scientists from many disciplines have considered the question of where and how intelligent life might arise elsewhere in the universe. A new study suggests that complex alien life — the sort of life we could one day meet and potentially communicate with — might be present on far fewer worlds than previously thought, due to the toxic presence of so much CO2 on certain planets that are otherwise within the Habitable Zone (HZ) of their respective stars.
The argument, broadly stated, is that the CO2 levels required to maintain liquid water on a planet towards the outer edge of its star’s HZ might be too high for complex life to survive on its surface.
Life on the Early Earth
I am not a biologist, but I’m lucky enough to know one. Jessica Hall agreed to step out of ET retirement and fact-check this bit.
Before we dive into the meat of this paper, there are a few useful things to know about the emergence of life on Earth. It’s incredibly old. The first indisputable evidence of life on Earth is three billion years old. Evidence may exist for life as far back as 4.2B years ago, but these finds are still contested. Regardless, life arose on Earth relatively early in the planet’s existence.
For most of our planet’s lifespan, however, the life it supported was simple, unicellular organisms without a nucleus. Multi-cellular life only appears in the fossil record 600 million years ago. If we use the 3B point (3.5B and 3.8B have also been proposed as dates for the earliest life), that means that our planet has supported life for about 66 percent of its own existence. For 80 percent of that time, life on Earth was strictly single-celled.
Discussions of why and how organisms on Earth made the leap from single-celled to multi-celled are of great interests to scientists in multiple disciplines. In many cases, dramatic shifts in the type of life living on Earth are tied to changes in its climate, atmosphere, and active geological processes. The Great Oxygenation Event, in which oxygen replaced methane, was one such transformative event that forever reshaped the kind of life that could live on this planet (and incidentally, caused an ice age known as the Huronian glaciation that may have nearly frozen the planet solid).
The idea that atmospheric gas concentrations could impact the types of life that live on a planet, in other words, is well-supported in our own research into Earth. What these researchers have noted in particular is that virtually all life on Earth is spectacularly ill-adapted to living with the high levels of CO2 required in the atmosphere in order to render a planet habitable if said planet is on the edge of its star’s habitable zone. Other greenhouses gases besides CO2 could theoretically work, but CO2 is what you expect to get if you have an oxygen atmosphere in the first place — and oxygen, it turns out, is the gas we would pretty much expect most complex life to require. From the paper:
The metabolic oxidation of organic matter with O2 produces significantly more free energy than any other plausible respiratory process, and O2 is the only high-potential oxidant sufficiently stable to accumulate within planetary atmospheres (Catling et al. 2005). As a result, it is likely that the centrality of molecular O2 in the emergence and expansion of a complex biosphere on Earth is a general phenomenon (Catling et al. 2005).
The researchers performed sophisticated calculations to estimate the relative likely abundance of CO2 on various planets detected around their host stars. In many cases, the levels of carbon dioxide on these planets would be multiple orders of magnitude higher than any life form known on Earth is capable of tolerating. The alternative proposed chemistries they considered failed to satisfy all of the requirements currently thought necessary for complex life to evolve.
The end result of this may be that the habitable zone for complex life to evolve could be much smaller than previously thought. This would not preclude the evolution of simpler life using alternative chemistries, and the authors readily acknowledge that no, we won’t know for certain until we’ve advanced far beyond our current level of technology. But it does suggest that the habitable zone of a star — typically defined as the temperature range that allows liquid water to be present on the surface — may not be a particularly useful metric. It may be that while simple life can still survive on such worlds, more complex life forms require conditions that are far more rare in the universe.
Feature Image: ESO/M. Kornmesser CC BY 4.0
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