Do we have a planetary bias in understanding where life might persist? It is very natural for us to do this. After all, we are on the same page. But planets may not be necessary for life, and a pair of scientists from Scotland and the US are challenging us to rethink that notion.

We focus on planets as habitats for life because they provide the conditions necessary for life to exist. Liquid water, the right temperature and pressure to keep it liquid, and protection from harmful radiation are the basic requirements of photosynthetic life.

But what if other environments, including those provided by the organisms themselves, could also meet these needs? A new study published in the journal Astrobiology Researchers state that ecosystems can create and maintain the conditions necessary for their own survival without the need for a planet.

The title of the article is “Self-Sustaining Habitats in Extraterrestrial Environments.” The authors are Robin Wordsworth, Professor of Earth and Planetary Sciences at Harvard, and Charles Cockell, Professor of Astrobiology at the School of Physics and Astronomy at the University of Edinburgh.

“Standard definitions of habitability suggest that life requires planetary gravity wells to stabilize liquid water and regulate surface temperature,” they write. “The consequences of relaxing this assumption are considered here.”

Wordsworth and Cockell write that biologically created barriers and structures can simulate planetary conditions that allow planetless life. They can transmit light for photosynthesis while blocking UV light. They can also prevent volatile substances from being lost in a vacuum and maintain the temperature and pressure necessary to keep water liquid.

“Biologically generated barriers that can transmit visible radiation, block ultraviolet rays, and maintain temperature gradients of 25-100 K and a pressure difference of 10 kPa against the vacuum of space can create habitable conditions in the Solar System between 1 and 5 AU.” declared write.

“To understand the limits of life beyond Earth, we can start by looking at why our planet is a good habitat in the first place,” the authors write.

Earth provides not only liquid water and protection against radiation. It is an entire system of layers of complexity that interact with each other. The planet’s surface is open to an easily accessible energy source from the Sun that drives the entire biosphere. The elements we consider essential for life are sometimes present, albeit in limited quantities: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. They circulate through the biosphere through volcanism and plate tectonics and become available again.

The Earth is also oxidized in the atmosphere and at the surface, and replenished in other regions such as sediments and the deep subsurface. This allows “redox gradients to be exploited for metabolic purposes,” the authors explain. Such conditions no longer exist. Astrobiology targets the frozen moons of the solar system because of their warm, salty oceans. So, are there nutrient cycles?

Low-mass objects in the outer Solar System have sufficient surface area, but the Sun’s energy is weak. They are unlikely to be able to retain their atmosphere, so the correct liquid water pressure and temperature are not available. They are also not protected from UV radiation and cosmic rays.

“To survive beyond Earth,” the authors write, “every living organism must modify or adapt to its environment sufficiently to overcome these challenges.”

Biological materials on Earth can already do this, the authors write. It is plausible that ecosystems can create the conditions necessary for their own survival, and if photosynthetic life can do this in the vacuum of space, then so can we. This will be a great advantage for human space exploration.

It starts with water, and when it comes to liquid water, scientists refer to its triple point. The triple point is a thermodynamic reference point that describes phase transitions and how water behaves under different pressures and temperatures.

“The minimum pressure required to support liquid water is the triple point: 611.6 Pa at 0°C (273 K),” the researchers wrote. This number increases to several kPa in the range of 15 to 25 Celsius.

Cyanobacteria can grow at 10 kPa airspace pressure provided light, temperature and pH are in the correct ranges. The question is; Do any living things we know of create walls that can support 10 kPa?

“Internal pressure drops on the order of 10 kPa are readily supported by biological materials and are in fact common for macroscopic organisms on Earth,” the authors write. “The increase in blood pressure from the head to the feet of a 1.5 m tall person is approximately 15 kPa.”

Seaweeds can also withstand internal bubble pressure of 15-25 kPa and release CO2.₂ during photosynthesis. When it comes to liquid water, temperature is the next criterion. The Earth maintains its temperature thanks to the atmospheric greenhouse effect. But small rocky masses, for example, are unlikely to repeat this.

“Therefore, a biologically engineered habitat should achieve the same effect using solid-state physics,” the authors write.

Energy input and output must be balanced, and some organisms on Earth have evolved to maintain this balance.

“For example, Saharan silver ants have evolved the ability to increase both their near-infrared surface reflectance and their thermal emissions, allowing them to survive at higher ambient temperatures than other known arthropods,” Wordsworth and Cockell write. This allows them to survive by foraging in the heat of the day when predators have to stay out of the sun.

People have created silica aerogels with extremely low density and thermal conductivity. Although they have no direct biological equivalents, the authors write that “many organisms that produce complex silica structures exist in nature.”

In fact, some diatoms can produce silica structures by manipulating smaller silica particles than those used in our manufacturing processes. Aerogels made from organic materials have similar properties to artificial ones.

“Given this, it is likely that highly insulating materials can be produced artificially from biogenic raw materials or even directly by living organisms,” the authors write.

The authors predict that such structures could maintain the temperature and pressure necessary to support liquid water.

“As can be seen, it is possible to maintain an internal temperature of 288 K for a wide range of orbital distances,” they explain. “This calculation assumes a free-floating habitat, but similar considerations apply to habitats on the surface of an asteroid, moon or planet.”

Another problem is volatile loss. A living space that cannot hold its own atmosphere cannot maintain the temperature and pressure required for liquid water.

“All materials have some permeability to atoms and small molecules, and over long periods of time the vacuum of space essentially becomes a stable sink for volatile species,” the authors explain.

This can be solved with the same barriers that support pressure and temperature. “Control of volatiles may be most easily achieved by the same part of the habitat wall that is responsible for maintaining the pressure difference necessary to stabilize liquid water,” the authors write.

The authors also take into account the effect of UV radiation. Radiation can be deadly, but there are examples of life on Earth that have evolved to understand this.

“However, it is readily blocked by compounds such as amorphous silica and reduced iron, which attenuate UV radiation in silicified biofilms and stromatolites without blocking the visible radiation required for photosynthesis today,” they write.

The availability of solar energy for photosynthesis is probably not a major obstacle in most parts of the Solar System. The authors note that arctic algae grow under the ice in extremely low light.

Just like on Earth, a certain type of nutrient cycling is required. “An additional factor in the long term is the ability of the closed-loop ecosystem to recycle waste products such as refractory organic matter and maintain internal redox gradients,” the authors explain.

Extreme heat in the Earth’s interior does this, but without these extremes, “a completely closed ecosystem in space would need some internal compartmentalization to create chemical gradients and specialized biota that could break down recalcitrant waste,” they write.

In their paper, the authors also consider other factors, such as cell size and factors that limit the size of single-celled organisms and larger, more complex organisms. They concluded that the existence of completely autonomous habitats cannot be ruled out.

“However, a fully autonomous system capable of regeneration and growth is apparently not limited by any physical or chemical constraints and is therefore interesting to consider a little further,” they write.

This is possible as long as the system can regenerate its walls. The authors note that extant photosynthetic life is already capable of producing amorphous silica and organic polymers. These materials can serve as walls and may at least suggest that there is a way organisms can evolve to form habitat walls.

“Just as plant cells regenerate their own walls at the micrometer scale, a more autonomous habitat could produce its own wall material,” they explain.

If life exists elsewhere, we tend to think that it followed the same evolutionary path as on Earth, but this may not be true. “Since the evolution of life elsewhere can follow very different paths than on Earth, habitats with unusual but potentially detectable biosignatures may also exist outside of conventional habitats around other stars,” the authors write.

The authors ask the question: “Can the biological structures we discuss here evolve naturally without intelligent intervention?” They argue that non-intelligent life could provide all the necessary conditions for survival in an extraterrestrial environment.

They conclude: “Life on Earth has not yet done this, but has undoubtedly adapted to ever-expanding environmental conditions over time.” “Exploring the plausibility of different pathways for the evolution of life under alternative planetary boundary conditions will be an interesting topic for future research.”