Copyright © Karl Dahlke, 2023
A red dwarf is a star that is substantially smaller than our sun, having a mass of about 0.07 M☉, and a radius of about 0.1 R☉, that is, about one tenth the size of our sun. Since it is smaller, it burns slower, and cooler, and its surface is red hot, not white hot. It is a small red star, thus a red dwarf.
We believe most of the stars in our galaxy are red dwarfs, but they are too faint to see. They are on average one ten thousandth as bright as the sun. We see the bright white stars in the night sky, but red dwarfs could be far more common.
Can a planet orbiting a red dwarf support life? It has to be in the habitable zone, not too hot and not too cold. Since the star is cooler, the life-bearing planet is closer to its star than we are from the sun. Close orbits, such as these, become tidally locked. In other words, one side of the planet always faces its star. Isaac Newton explained why this is so in his masterpiece, Principia Mathematica, in 1680. I'll skip the details for now. We see this in our own moon; one side always faces the earth. Year after year, whenever you look at the moon, you see the same craters, the same mountains, it never changes. In the same way, one side of this hypothetical planet always faces its red dwarf star. One side bakes and the other side chills.
The night side gets frightfully cold, perhaps cold enough to freeze out the atmosphere. We have miles of glaciers at our south pole, this planet might have miles of frozen air covering its night side, leaving no air to breathe. That's not good. A thicker atmosphere, with ever-constant winds, might be capable of moving some of the heat around, at least to keep the air from freezing. It would still be cold on the night side, colder than anywhere on earth, but perhaps the air would still be air. we need this if life is to survive.
All life on earth comes from the sun. Animals might eat other animals, but eventually some animals eat plants, and plants get their energy from the sun. With no light on the night side, nothing can grow. There are no trees, bushes, grasses, or shrubs, and no animals, just an entire hemisphere of sand and rock and ice. Indeed, might all the water be locked up as ice on the night side? Perhaps the air doesn't freeze, but surely the water does. We hope that some of the water remains on the day side, or condenses out as rain just across the night side, where it is cooler, then flows in rivers back to the day side. We have to make a few assumptions if the planet is going to support life.
The front third of the planet is broiling hot - too hot for life. Plants and animals exist, and thrive, in a ring around the terminator, where the star strikes the planet at a shallow angle, maintaining a mild climate. All the plants and trees and leaves lean toward the star, which is always in the same place in the sky. It is a big red ball that never moves. At the front of the planet, the star is directly overhead, baking the surface like the heating element of an oven, but near the terminator it is low in the sky, and it feels good, as winds blow across the land.
Life could exist just across the terminator, just barely on the night side. This is because of refraction of red light, as we see just before sunrise and just after sunset. Red light bends around the planet and illuminates a thin ring on the night side, just enough for plants to grow. If you were standing in this thin band, you would see red light on the horizon, but no sun. Plants would be meager, growing in this dim red light, but they would find a way. Still, humans and other active animals will find more food on the day side of the terminator. That's where all the cities are, in my vision of this world.
You can play all day in the red sun and never get sunburned. The smaller star isn't hot enough to produce ultraviolet light. There isn't a lot of blue or violet light either. The sky is not bright blue like ours; there is a tinge of dark blue around the red star, and that's it. Away from the red star, you might see a few bright stars against the black sky, even though it is perpetually daytime. There isn't enough blue to wash them out.
The eyes of the animals might not see blue at all; there isn't enough blue for those retinal cones to evolve. Perhaps we would all see shades of red orange yellow and green, and nothing more.
On the night side, the sky could appear like ours, if the timeframe is the present. However, far in the future, the white stars will be gone, whence the night sky consists of tiny red dots, red dwarfs, just like the one we are circling - red stars that live for trillions of years. There may be a few white stars, brighter than the rest, that are newly formed, but there won't be very many.
The interesting question for me is: do animals on this tidally locked planet sleep? Here on earth, every animal has a resting period, usually at night, when it is inactive. (Some animals, like owls, are active at night and inactive during the day.) It makes sense to sleep during your inactive period. That saves energy. But next to a red dwarf, it is day all the time, there is no inactive period. Is there any need for sleep?
You might point out that every animal on earth goes insane if it doesn't sleep. Even flies, with brains the size of a grain of sand, must sleep. Fair enough. However, sleep could have evolved first, as a method of energy conservation, and then repair mechanisms became part of sleep later. Now they are intertwined, and we can't tease them apart. Perhaps the brain could repair itself while it is awake, assuming sleep never evolved in the first place. If, however, sleep is physiologically necessary, animals on this planet could be like ducks and dolphins, where half the brain can sleep while the other half remains awake. The animal is never completely asleep the way we are. Or, they might sleep whenever they feel like it, without a rigid schedule.
There's plenty of time for life to evolve around a red dwarf; even intelligent life, and technological civilizations too. Our sun shines for 10 billion years, but a red dwarf can shine for 10 trillion years. That's a thousand times as long. Plenty of time for evolution to try and try and try again. Keep spinning that wheel until intelligence evolves.
I worry about our sun sterilizing the earth at the end of its life in a hot fury, which is just a billion years hence. It's one of those silly things I obsess about. But the people who live near a red dwarf can put their feet up and relax, as They have 10 trillion years before their star runs out of fuel. Plenty of time to move to another star or make a new plan. Try to imagine 10 trillion years; it boggles the mind.
Copyright © Karl Dahlke, 2023