Aliens under the Ice – Life on Rogue Planets

The wandering planets are planets traveling alone through the universe. They occupy the dark and vast space between the stars. Drifting, in eternal darkness, no light warms their surfaces, and they are exposed to the freezing cold of outer space. They know no seasons, days, or nights that could indicate the passage of time. And yet, the wandering planets could well transport extraterrestrial life to all corners of the galaxy. How would it work?

How does a planet become wandering?

Several very different things are called "wandering planets". For example: brown sub-dwarfs - gas giants that form from collapsing gas clouds and who are the uninteresting little sisters of the brown dwarfs. These are sort of failed stars, and we won't talk about them any further. Much more interesting wandering planets are the terrestrial planets, like the Earth, who got kicked out of their planetary system. Young solar systems are dangerous places, where proto-planets fight to gain mass by ingesting as much material as possible.

Photo by Erik Mclean on Unsplash

In this struggle for domination, they collide with each other or dangerously approach each other's orbits. If a very massive planet moves its orbit towards the sun, it can push smaller planets out of the system. But the fact that a planet has survived the difficulties of training does not mean that it is safe. Planetary systems can be disturbed at any time by overflights of stars or black holes. Almost half of all existing planets could become vagrants. Scientists don't agree on a specific number, but it is possible that there are at least billions of roaming planets in the Milky Way.

Most will share the same fatal fate, as their star disappears day after day, the planet’s surface will cool rapidly to -270 ° C. If they have oceans, they will freeze and become as hard as rock. Their atmospheres will contract to the surface before freezing in turn. However, some of these cold, dark deserts could be life-savers. To understand how, imagine a planet similar to Earth with a similar mass and composition.

If we transported it into interstellar space, how could it still feed life?

According to our current understanding of the nature of life, there is an essential ingredient she needs: liquid water. Water is important because it helps to mix things, both matter and energy, which creates interesting reactions. Like for example life. Our planet therefore needs enough energy to keep part of its oceans in a liquid state. The annoying thing is that about 99.97% of the Earth's energy balance comes from the Sun. So our imaginary wandering Earth has to make do with the remaining 0.03% of energy, which come almost exclusively from its hot center.

 The inner core of the Earth is a huge metal ball, almost as hot as the Sun’s surface, surrounded by an outer core made of molten metals which solidify very, very slowly; releasing a lot of heat. As long as this process continues, our planet will be geologically active with liquid and solid materials moving to bring this energy to the surface where it can be collected as geothermal energy. Although the warm heart of any planet will cool over time, this process takes billions of years, enough time for a form of life to appear and flourish. There is even a scenario that would allow Earth-like planets to have oceans that are not frozen.

 If the planet had a very dense and high pressure hydrogen atmosphere, the gas would not freeze and could trap enough heat escaping the planet to allow the oceans to remain in a liquid state to the surface. There is another way to stay warm: the moons.If a wandering planet is accompanied by one or more sufficiently large moons, these could bring additional energy to the system thanks to tidal forces. These forces stretch and contract the planet a little each day, keeping it warm. But the most likely scenario for a wandering planet carrying life is a planet with subglacial oceans under several layers of ice. This is not a completely absurd idea, the proof is that we already have some in the solar system (on Enceladus, Europe, Ganymede).

How could life support itself at the bottom of a cold and dark ocean? 

On Earth, at the bottom of our oceans, in volcanically active areas plunged in total darkness, there are hydrothermal vents called "black smokers". They spit clouds made of black materials and hot water providing a constant flow of minerals from the Earth’s mantle. Bacteria feed on these minerals and produce organic material that attracts crustaceans: bivalves, snails, fish, cephalopods and tubular worms up to 2m in length. Hydrothermal vents are not only home to an incredibly diverse set of living things, but may well have been the beginnings of life billions of years ago. In the dark abyss of a wandering planet, such chimneys or a comparable volcanic activity could constitute the starting point of a complex ecosystem that one can only imagine.

One of the advantages of an ocean ecosystem on a wandering planet is that the environment is extremely stable. The thick layer of ice protects it from all kinds of extinction events [meteor shower, gamma rays] and as long as the energy of the heart continues to spread, things change very little. The most likely life forms are bacteria and other forms of microorganisms. But over time, more complex extraterrestrial life forms could feed on these little beings and in turn thrive. It is not impossible that a form of intelligent life will emerge in such an environment. If that were the case, she would find herself in a rather strange world: strangled between a thick wall of hard ice like rock and impossible to cross above, and bedrock at the bottom.

 Without any plant to store solar energy, there would be no wood, oil or coal. And even if there were, it is not as if it was possible to discover fire at the bottom of the ocean. Without this energy, metal could never be transformed into useful objects. Our intelligent extraterrestrial friends may well never pass through the ice. They may never know "the outside", and would conclude that their little world constitutes all that exists. Millions of generations would live and die in these abysses, unaware of the vastness of the Universe beyond the ice, until the heart of their planet cools and all life disappears. As the oceans freeze completely, the remains of this civilization and this ecosystem would be trapped in a frozen grave, forever.

 When you think about it, it might be better not to be aware of all of this. But the concept is both disturbing and stimulating. The universe could be filled with life forms trapped on planets that are impossible to leave. Such worlds could frequently pass near the solar system without our being aware of it. Maybe someday, in the distant future, humans will set foot on one of these frozen worlds and try to say "Hello". 

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