Dark Planets

What can happen concerning life on a planet without any sunlight? A planet at a favorable radius from its star, but with an atmosphere continually and completely covered with clouds is an easily concocted example. Earth had life long before life could use solar photons for energy, either directly or indirectly via feasting on a food chain starting with solar photons. It is believed that early Earth life was powered by chemical energy.

Chemistry can provide plenty of energy. To give our dark planet the best chance of making something impressive without photons, suppose that it has an abundance of chemical energy. Consider an ocean on the dark planet first. Suppose there are continuous volcanic events somewhere, and the ocean circulates the chemical products everywhere throughout the connected seas. Maybe there is a basalt flood going on somewhere, dumping something like methane and other alkanes into the water, along with ammonia, ferrous iron, and other edible tidbits. Far away from that, some chemotrophs are busy oxidizing these chemicals. It is like a whole ocean as rich as one of Earth’s undersea vents.

There might be a variable density of these creatures, with more of them nearer the principal sources of chemical energy, but not too close because the water temperature is higher there. There can be a wide variety of life in such conditions, as demonstrated by the various microbes and animals which inhabit sea vents. Our life forms are limited by the evolution that can happen in the duration of a sea vent, but if we imagine the dark planet to have recurring basalt flooding, maybe multiple at a time, perhaps caused by asteroid impacts, then evolution might go on for billions of years in a chemical energy-rich environment, leading to a variety of creatures far beyond what we see at a sea vent.

There is a question here on Earth as to whether the organic chemicals forming cells in the creatures inhabiting the vicinity of a sea vent have been contaminated or worse, contributed to by photic life forms living in the upper layers of the seas. This is not a question for the dark planet, as it could not happen there without any sunlight, but more pointedly, what can evolve in a phototroph can evolve in a chemotroph, although maybe not as speedily. DNA mutation is simply a change in DNA, caused by one mutagen or another or just by accidental errors in DNA copying. Where it happens is largely immaterial.

Consider the atmosphere. If the dark planet has continuously producing basalt floods, the atmosphere may also be full of chemical energy sources, such as the smaller alkanes and ammonia. Is it conceivable that an organism could emerge from the ocean and live on land on the dark planet? Breathing would be the same as eating, and the organism would not resemble anything easily imaginable from Earth’s examples.

One advantage that life had on Earth was that photons arriving on the land surface are more abundant than those arriving underwater, as water absorbs some of them. This means it can be an evolutionary advantage for a plant to live closer and closer to the surface, and eventually migrate to living in the shallows and then on land. A DP-plant would have a corresponding disadvantage, as the atmosphere, being a gas, can hold much less of the energetic chemicals. This does not mean that there would be no land life, but that it probably would not have the diversity that proximity to a solar energy source provides here on Earth.

On Earth, we have a nice clean division between plants and animals, as plants are almost uniformly photosynthetic while animals live on plants, or on other animals. On the dark planet, there might be a similar division, between DP-plants, which live on chemical energy in the oceans or in the atmosphere, and DP-animals, which consume DP-plants. Earth plants typically maximize the absorption of sunlight, by having such things as leaves. Sunlight is absorbed by a surface. Chemicals in a fluid have to be absorbed by maximizing the flow of the medium through or past an absorbing surface. One possible arrangement might be a porous DP-plant, though which the ocean continually flows. This would work if the DP-plant were fastened to the seafloor near a constant or almost constant flow of seawater. Any DP-plant which was free-floating would have to have a mechanism for circulating the ocean water past its chemical digestion tract, much like many Earth sea creatures do who dine on microscopic organisms floating in the water.

Without sunlight, vision might not evolve, neither in oceanic life nor in any creatures which manage to live on the land surface. Senses would be restricted to smell, taste, touch, and vibration. Perhaps some will evolve electrical discharge capability, initially for defense or predation, but perhaps later for communication. Earth has evolved creatures with electrical discharge ability, but perhaps none which can reliably detect a discharge. This does not mean that evolution is not capable of it, but instead that there are so many excellent competing senses possible here that it did not emerge.

How far can evolution take life on a dark planet? Suppose that such a planet were formed early in the history of the galaxy, so that life has had maybe ten or eleven billion years to evolve there. Could there be animals which live in packs, communicating with vibrations or electrical signals? These are all short range, and low frequency acoustics might serve for long-range communications. Another sense possible is echolocation, which has only evolved in the sea in mammals on Earth, but could easily be supposed to evolve in whatever DP-animals arise.

There is a stopping point, however, in the march of evolution on a dark planet. One problem is tool-use, and an example is the specific first tool used by primates, fire. There is nothing equivalent in an ocean. Nor are there advantages to developing the limbs needed to use tools, such as a primate’s hands. Thus, evolution can go a very long distance, but not in the direction of intelligence.

If there was such a planet as our dark planet, teeming with life but no intelligence, would it be detectable? With a cloud cover, no evidence would be visible to even a huge telescope. No oxygen is present, which is considered, perhaps prematurely, as the indicator of life. Such a dark planet might be passed over, even by a nearby alien civilization who were hunting for other planets with life. 

Does this make any difference? Could an alien civilization make any use of a dark planet such as the one we have been postulating? If the energy source is continuous basalt flooding caused by asteroid impact, then the question would be, could there be any regions present there which could be visited, even temporarily by an alien landing party? If the basalt flooding were underwater, in a deep part of the ocean, possibly the land surface might be tolerable, even if the atmosphere was extremely toxic. It is an extremely interesting thought exercise to see if there was any reason that an alien civilization would want to visit such a planet, or to establish some sort of colony there. Perhaps this blog will return to the topic to suggest something relevant.