Everybody knows that for life to thrive on any world, you need water, warmth, and something to eat. It’s like a habitability mantra. But, what other factors affect habitability? What if you relaxed the conditions conducive to life? Would it still exist? If so, what would it be?
Those are interesting questions that arise as new worlds continue to be discovered around other stars. Astrobiology (the science of life on other worlds) has a general (and conservative) assumption that Earth-like environments are the best places to search. The problem is that Earth is the only place that fits that definition—at the moment. We know of approximately 6,000 exoplanets (and the number is growing) out there. Only a few come close to the Earth-like definition, which sets artificial limits on where we think life could exist.
If we widen the definition of habitability, will that expand the places we can look? What other factors should scientists consider as they search for life in the cosmos?
A recent paper titled “Self-sustaining Living Habitats in Extreme Environments”, by Harvard scientist Robin Wordsworth and Professor Charles Cockell, University of Edinburgh, examines the possibilities of specific types of organisms arising on worlds where habitability might not fit the “standard definition.” In particular, they examine the viability of photosynthetic-based simple life forms in space or on other worlds. “Our idea is to probe the limits for habitability of non-sentient life. We were able to show that there are no physical limitations on simple forms of life existing outside of planetary gravity wells, which was not a result we expected initially,” Wordsworth wrote in an email.
There’s a lot to unpack in the team’s paper, but the TL:DR summary says that life CAN exist in a variety of situations, provided certain parameters are met. And, they don’t have to be strictly Earth-like. But for the best chances, those organisms need to be photosynthetic and live in a place where sunlight from the system’s star can get through.
We only have to look at the other worlds of the Solar System to see that the standard definition isn’t going to fly for them. Venus, for example, can’t support any life on its surface. But, recent findings (and disagreements about) phosphine and warm layers in its atmosphere suggest that it could have habitable spots high above the surface. There’s no evidence that it exists in those clouds. But, they may provide a set of conditions for certain kinds of life—and those conditions don’t fit the Earthlike definition.
A composite image of the planet Venus as seen by the Japanese probe Akatsuki. The clouds of Venus could have environmental conditions conducive to microbial life. Credit: JAXA/Institute of Space and Astronautical Science
Scientists also suggest Titan, Enceladus, and Europa as possibly habitable havens for life. Again, nothing’s been found at any of them. However, it’s possible that at least Enceladus and Europa could have safe harbors for certain kinds of life. Not Earthlike, to be sure, since those forms probably wouldn’t survive there.
So, the authors ask, how much complexity do you need for life to sustain itself beyond Earth? That leads to a far more interesting question: what’s the minimum physical structure that could sustain habitable conditions on another world? Could non-sentient organisms exist in and modify different conditions?
To answer those questions, the authors looked at various parameters, including planetary habitability, atmospheric pressure, temperature, volatile loss (from the surface and atmosphere, which also involves looking at the gravity well), radiation, free energy, and nutrients, scale and location, and maintenance and growth. All of these factors affect the rise of life and its ongoing evolution. They considered simple photosynthetic forms (that is, those that depend on photosynthesis) as a test case. That’s because, as Wordsworth points out, a solar radiation energy source is key. “When solar radiation is the energy source, life can flourish and spread over a much larger area, until its growth is limited by other things, such as availability of essential nutrients or raw materials,” he pointed out.
A schematic of the key similarities and differences between habitable worlds and a micro-organism when looking at life habitability. Courtesy Robinson and Cockell.
That reliance on solar energy is important. However, it plays much less of a role in places like Europa or Enceladus. Those two worlds do have internal energy sources or chemical energy sources, but those do not allow for photosynthesis to occur. If life exists under their ice shells, it won’t be basking in the sunlight. That’s because those surfaces are not transparent enough to allow sunlight to pass. It would have to depend on the central energy sources. That pretty much limits the areas where life can flourish. That’s not to say that it won’t exist there. It will occur under more limited circumstances than simple photosynthetic organisms arising with energy input from the star.
As a result of their research, Wordsworth and Cockell argue that non-sentient life can flourish under the proper conditions at other worlds. They found no limitations to it surviving in self-contained ecosystems elsewhere, provided those ecosystems can regulate their habitability internally. In other words, life—particularly simple forms of it—can exist under conditions that aren’t always Earthlike.
One other outcome of the Wordsworth-Cockell research points out benefits for other fields of study. For example, life support for humans in space. That would allow for the use of biotechnology in medicine, food, habitat construction, and spacecraft propulsion. Essentially, we could create biologically generated habitats for environments such as the Moon or Mars.
In addition, the idea that such simple life can exist in a wider variety of environments could push astrobiology to get past the idea that only Earth-like places should be the “holy Grail” of the search for life. Of course, once you assume that other places with more extreme environments can support life, you need to figure out ways to detect it. Such detections require new strategies that depend on where you’re searching and what you’re searching for.
Finally, we need to look at how much the living beings on our planet have shaped its habitability. We also need to understand what the initial conditions were that shaped life here. Then, scientists can apply that information in the hunt for life in other places. That leads to further speculation about how we could (if we wanted to), shape the biospheres of other worlds. Obviously, Mars comes to mind. That’s terraforming, and scientists continue to examine that possibility.
Self-sustaining Living Habitats in Extreme Environments (PDF)