In their paper, the authors cover other factors like cell size and the factors that limit the size of unicellular organisms and larger, more complex organisms. They conclude that fully autonomous living habitats can’t be ruled out. “Nonetheless, a fully autonomous system capable of regeneration and growth is apparently not prohibited by any physical or chemical constraints and is therefore interesting to consider a little further.”

It’s possible as long as the system can regenerate its walls. The authors point out that existing photosynthetic life can already produce amorphous silica and organic polymers. These materials could serve as walls and at least show that there’s a pathway where organisms could evolve to create habitat walls. “A more autonomous living habitat would be able to grow its own wall material, just as plant cells regenerate their own walls on the micrometre scale.”

We tend to think that if life exists elsewhere, it follows the same evolutionary pathway as it did here on Earth, but that may not be true. “Because the evolution of life elsewhere may have followed very different pathways from on Earth, living habitats could also exist outside traditional habitable environments around other stars, where they would have unusual but potentially detectable biosignatures.”