03 March 2017

Habitable niches are better than habitable zones (and, it's nice to write something again!)

I just came out of a great, if brief, conversation with Dr. Catharine Conley, NASA's Planetary Protection Officer. Part of her job involves ensuring that we don't contaminate the rest of the Solar System with Earth organisms. As a result, she spends a lot of her time thinking about how resistant various Earth critters might be to dying, and how well-suited they are to surviving in the various environments they might encounter around the Solar System as they hitch a ride on our robots.

Since I spend my time thinking about planets in other solar systems, I asked her if her job had led her to have opinions on the search for life on exoplanets. Her response was really a new perspective for me - the idea of habitable niches rather than habitable zones. The point was that many solar system bodies have regions that are quite Earth-like, at least in terms of resembling regions of Earth where life has been found, even if they don't resemble Earth as a whole. (A metabolically unique Earth organism she mentioned in her talk was Desulforudis audaxviator, a species of bacterium that subsists off of the byproducts of radioactive decay from the minerals that make up the rocks where it lives - it would be quite happy to set up shop underground somewhere on Mars.) Furthermore, it is critical to think about how an environment would affect microbial life - which were, after all, the earliest forms of life on Earth. Effects on small scales run counter to much of our intuition. Bacteria don't care much about gravity, for example, and could be just as happy burrowed in Pluto or an asteroid as in your gut.

She went so far as to suggest that perhaps even the Kuiper belt might be the the most hospitable region of the solar system, since it was nice and cozy for the first few million years. This is amazing! Dr. Conley is shattering the habitable zone and scattering its pieces across the Solar System.

I'm sure I'll be thinking for a long time about the implications of this habitable niche paradigm on the search for life. On one hand, it is a refreshing rebuttal to exoplanetology's relentless obsession with the habitable zone, which is poorly defined and formulaicly applied to every known exoplanet system we have thus far discovered. It's the equivalent of the ice, desert, and swamp worlds of Star Wars, when these are all just samples of the variety of ecosystems on Earth. On the other hand, it doesn't completely depart from the idea that the habitable niche itself must approximate Earth-like conditions. I should concede that the working definition of the habitable zone is really nobody's fault and is probably the best we can do given the limitations of our data, which can often only guess at the bulk properties of discovered planets. It doesn't stop me from hating it, though. I look forward to a future when we can take a more fine-grained approach to planetary geography.

Going forward, exploring the Solar System will give us a much better picture of the variety of potentially habitable environments that may exist in all planetary systems. Considering how every single exoplanetary system we have discovered so far is categorically different from the Solar System, the number of possibilities multiplies.

She really painted an extremely optimistic picture for life in the Solar System and in the Milky Way. It left me excited, but also puzzled: if there are so many more opportunities for life than we ever conceived, then where is everybody?

To sum it up:

1. Forget the "habitable zone": many SS bodies have niches that are habitable, especially under the surface.
2. Life starts on small scales, and you really have to think what a particular environment would be like for a microbe.

Helpful links:

There are a couple resources that both friendly to a broad audience, and curate (and visualize! accessibly!) data on potentially habitable exoplanets:

The Planetary Habitability Laboratory from the University of Puerto Rico has a nice tool and explanation of the habitable zone:
The Habitable Zone Gallery also has a very brief explanation, and really cool movies of the orbits of known exoplanets that pass through habitable zones:

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