Up to now, the search for extraterrestrial life has focused on finding a planet like our own. But could alien biology create its own habitable havens?
Alien worlds may be self-sustaining habitats with very different basic requirements for life than we’ve assumed.
More on alien life
Robin Wordsworth is professor of Earth and planetary sciences at Harvard University. His research focuses on planetary habitability.
We spoke to Robin about what makes a planet habitable, and whether alien life could produce its own conditions necessary for survival.
What do we know about life on worlds beyond Earth?
We only have one example of an inhabited planet in the Universe right now – Earth – so all we know about life elsewhere is an extrapolation.
One school of thought says we should focus on Earth-like planets that can support liquid water and have plate tectonics that allow volatile compounds to cycle between the interior and atmosphere.
We took another approach, exploring whether biology has the capability to generate conditions
for its own habitability, on planets or elsewhere.
Can life generate conditions for its own habitability?
On Earth, life and the planet have been constantly interacting.
This happened when oxygen first emerged and changed the composition of the atmosphere, or land plants appeared on Earth’s surface and covered it, or most recently when humans emerged and caused profound changes to the environment.
There’s no reason to believe this kind of interaction isn’t true of life everywhere it appears.
Why is liquid water essential for life?
There’s an argument that the nature of fundamental interactions between water molecules, and the way water behaves and interacts with other solvents, means the organic chemistry that all known life relies on can be much more complex in water than in other substances.
We assumed liquid water is required, but other forms of life are probably possible.
Is our understanding of extraterrestrial habitability correct?
The current understanding is that life needs a gravity well to exist. In other words, you need to have a planet of a certain size for liquid water to be stable.
We showed that biologically produced materials are quite capable of overcoming these constraints outside of planetary gravity wells.
If life can stabilise water and temperature and obtain access to a source of energy, then there’s no reason it couldn’t persist on its own, without the need for a gravity well.
What did your study involve?
This was a theoretical study. We did calculations to understand the requirements for carbon-based life to exist in a self-sustaining fashion beyond Earth – the need for liquid water sets constraints on temperature and pressure, and the availability of nutrients and energy is also important.
We then analysed what kind of biomaterials could maintain temperature and pressures in a habitable range.
Finally, we looked at how rapidly water and other volatiles would be lost to space – an important consideration for systems without internal gravity.
What kind of biomaterials did you examine?
We looked at the properties of a range of human-made bioplastics, as well as biominerals such as calcite (formed from the shells of many sea creatures), and showed that they could create habitat walls in a vacuum environment.
The kind of life we’re imagining could be free-floating habitats, or organisms that grow on the surfaces of asteroids or icy moons. We demonstrated that such things are possible.
How could this impact future endeavours in space?
One of the key advantages of biology is that it can recycle waste materials, allowing sustainability in a way industrial methods currently can’t.
Once humans need to spend large amounts of time in space, sustainability is going to be crucial.
Biologically generated habitats could be one way to achieve that.
The lessons we learn about sustainability in space also have the potential to improve how we co-exist with the biosphere here on Earth.
Will this work affect the way we look for extraterrestrial life?
We probably should be more broad-minded about potential biosignatures.
If we widen our understanding of the way life can exist in different environments, then we have more avenues to look for potential biospheres and different signatures of life.
We could look for spectroscopic signals caused by photosynthesis outside the conventional habitable zone, or outside planetary gravity wells altogether.
This interview appeared in the January 2025 issue of BBC Sky at Night Magazine.
