Is Solid Land Necessary for Biology?
And if life could arise on an ocean world, how far could it evolve?
Exoplanets come in a variety of types, and so-called “water worlds”—planets with at least ten percent of their total mass consisting of water, and no land exposed to the atmosphere—appear to be among the most common. At last week’s Goldschmidt Conference in Boston, Li Zeng from Harvard University and co-authors claimed, based on their modeling of the evolution and growth of planets, that exoplanets with diameters about two to four times greater than Earth’s are likely to have surfaces covered by oceans, rather than being a mix of land and water as on our own planet.
While those super-Earth water worlds are probably uninhabitable due to very high surface temperature, smaller, cooler water worlds could be habitable, according to another recent paper by Ramses Ramirez from the Tokyo Institute of Technology and Amit Levi from the Harvard-Smithsonian Center for Astrophysics. They point out that subpolar sea ice may enhance the habitability of water worlds by moderating their climate.
This begs the question: Assuming life can originate on a water world, say, at a hydrothermal vent at the bottom of the ocean, how far could it evolve? Could we expect intelligent life, or even technologically advanced life, on a world with no exposed land area?
Octopi and other cephalopods evolved in the oceans of our planet, so there is no obvious reason why such intelligent animals could not exist on an alien water world. Whales and dolphins, on the other hand, evolved from land animals, so we wouldn’t expect to find their analogs on exoplanets covered by oceans.
What about technologically advanced life? Fire can’t exist underwater, and fire is thought to have been essential for humans to develop technology. There would be no controllable electricity underwater, either, and without electricity, it’s difficult to imagine what kind of technology could exist on a water world.
However, as William Bains and I speculate in our book The Cosmic Zoo: Complex Life on Many Worlds, perhaps smart ocean creatures could use thermal vents on an alien planet’s seafloor to supply concentrated heat energy. Such energy wouldn’t be as portable or controllable as fire, but the underwater civilization could farm fish without fire, or perhaps use other resources to make tools and technology. As an example, long strands of kelp could be ideal for making rope.
And maybe we shouldn’t give up too quickly on the idea of electricity on ocean planets. In fact, some fish on Earth use electricity, if only to stun prey. I’m not convinced it’s impossible for a technologically advanced species to evolve in a world with no solid surface. The evolutionary path may seem harder to us, but maybe only because we’re land animals. After all, it wasn’t easy for marine species on Earth to conquer the land billions of years ago by adapting to its many challenges, from lack of water and nutrients to increased radiation.