Some 75% to 80% of the stars in our galaxy are red dwarfs, which are low mass stars, ranging from 40% or 50% of the sun’s mass, down to perhaps 7.5%, where the lights go out. It so happens that as mass decreases, the pressure in the core decreases, making the fusion fire more dim. After the mass gets low enough, it cannot ignite. Red dwarfs are smaller, but the amount of energy produced drops faster than the mass, and faster than the surface area. This means that the temperature drops, as that is a measure of the energy output per area. Often the relationship is expressed in the other direction, that as the temperature drops, less energy is emitted. But physics laws go both ways.
The sun’s surface temperature is about 6000 degrees C, but a red dwarf might have temperatures as low as 2000 degrees C. This means that most of the energy output is not in the visible spectrum, like the sun’s output, but down in the infrared. They are called red dwarfs because we can’t see infrared, and if you look at one in a telescope, red is the only thing left to see, and there is not much of red. If we had snake eyes, they might be called infrared dwarfs.
Plenty of red dwarfs have planets. Having planets is the default case, and maybe there are very few stars that don’t have any. What do you do with all the leftover dust and gas when the star collapses sufficiently and ignites? All that fusion energy turns into output light, and light has pressure, and slows or stops the collapsing of the residual gas. Whatever didn’t get into the star at ignition time is a candidate for condensing in orbit, and making planets, moons, asteroids, comets, and all the other interesting stuff that inhabits a solar system.
There seems to be a lot of angular momentum around the galaxy, which is spinning around anyway, and any condensing cloud gets its share. Angular momentum is definitely a problem for condensing gas, meaning more stuff is held out in orbits. So, between angular momentum keeping things out by centrifugal force, and solar wind and light pressure pushing things outward, most stars are likely to have planets. Even red dwarfs should have planets, although the share of mass that goes into a red dwarf’s planets is likely to be much less, as the cloud that forms the red dwarf is smaller to begin with. This has been borne out observationally, with not many Jupiters flying around red dwarfs, but lots of smaller stuff, Neptune-sized for instance. The ratio of planetary mass to the mass of the star depends on the amount of angular momentum in the cloud. Some clouds have more angular momentum, others have less. After the condensing happens, the first group has more mass stuck out still rotating and therefore will have more planets. There is a distribution.
There may well be planets in the habitable zone of a red dwarf. Habitable zone is an astronomer’s term, meaning that if you stood on the planet with a bucket of water, the bucket of water would not freeze or boil. Perhaps you would have to stand in a particular place to have this luck. On planets too close to their star, tidal effects make the rotation of the planet slow down so it matches the orbital period. This is call phase-locking, as one hemisphere of the planet is locked into a position always facing the sun. So, maybe habitable means that your bucket would do just fine on the light side, or on the dark side. With non-locked planets, this definition is not so ambiguous, as you would just average what happens as your planet spins around for a day, or if you liked the concept of averaging, for a year as well.
Unfortunately, habitable is a word in the English language, and non-astronomers may interpret the word to mean life could survive there. Perhaps it could, but there are many other factors than average temperature or the proper amount of solar energy impinging on the planet to determine whether life could survive there. This factors could have to do with radiation, with chemistry or the distribution of elements, with wind velocity, tectonic stability, and others.
One thing that habitable does not mean is that intelligent life could form there, and build starships to come and visit us. Maybe life could form there, as there is a good chance that life forms as chemotrophs, little organisms that live on chemical energy. Chemotrophs and creatures that dine on them might form in some nook on an ocean on a red dwarf in the habitable zone. Oceans can exist and maintain themselves on a planet in the habitable zone, just like the water in your bucket. However, without all those high-energy photons, the chemotrophs who are striving for something better are not going to evolve chlorophyll. In other words, chlorophyll is likely a Great Filter for red dwarfs, even if it is not for higher power and mass stars.
Intelligent life requires energy, and chlorophyll is the way to transform large amounts of stellar photons to chemical bonds, and chemical bonds are a great way to store that energy and allow its concentration. Thinking takes a lot of energy, and to provide that energy, there has to be a mechanism that puts it in one accessible place, like a plant or an animal. Is it conceivable that chemotrophs could accomplish this? Perhaps if there was a massive source of the chemicals that fed that life, and it was concentrated as well. Where would it come from? Perhaps some more speculation will come up with a planet that is reasonable and has the chemical sources, but so far, there are none.
This means no life of much interest to aliens on red dwarf planets. They could go to any planet in the habitable zone with impunity. But would they want to stay there and colonize it? Because the habitable zone is so close to the star that phase-locking happens, the question is, would it pay to colonize a planet with a hot side and a cool side? If the planet was on the outer edge of the habitable zone, only the spot closest to the star would have pleasant temperatures. A colony could be set up there, but much of the planet would be off bounds. Similarly with a planet on the inner edge. This means that there is a spot in perpetual night which is habitable, temperature-wise, but the rest of the planet would not be. In the center of the habitable zone, a strip near the boundary between the day side and the night side would be right.
On Earth, there is much more heating at the equator than at the pole which has the most night. That differential heating leads to atmospheric instability, and is a driver of the winds. With a planet where the heating was constant, circulation would be inevitably set up, and winds might exist which interfere with any colonization. Venus has a very slow rotation, and high winds. Clearly, mountain chains and the location of oceans play a great factor in wind direction and speed. Nevertheless, it seems quite likely that many phase-locked planets around red dwarfs would have interesting winds.
Thus, there are likely reduced opportunities for colonization, because planets near the edges of habitable zone have only limited spots for colonization, and planets near the center of the habitable zone have only a strip for colonization, perhaps not even extending to the poles. The aliens would do their cost-benefit analysis for colonization, and having a little portion of a world with the same expense to perform the colonization does not look well compared to a planet around a better star, where much of the planet would be used. This means that a red dwarf planet would certainly not be a sweet spot planet, and might not even be a penumbra planet. It might be a last recourse planet if there were no other opportunities. It certainly could serve as a source of materials. There would be no fossil fuels, since there was no natural vegetation to form them, but other sources of energy might be there.
Before committing to mining a red dwarf planet however, the aliens could do a calculation exercise to see what happens to the normal tectonic behavior of a planet if it becomes phase-locked in its early history. Would the same separation of minerals occur? Mining does not have to be done on a habitable planet if performed by robots, so there may be many other planets, further out that the habitable zone, that might be mined. These would be very cold, and the aliens have not shared with us any information on a cold temperature limit for mining robots. Again, this may not be preferable if other options exist. All in all, red dwarfs may be the most populous star type in the galaxy, but that does not mean that alien ships would be going there. A little more thinking will be necessary to decide if they should be included in a starship hunt, but it is not initially very promising.
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