The first observation to be made here is that it’s likely that if the Sun was indeed red, or reddish, we probably wouldn’t see it that way, assuming we had colour vision. If the visible spectrum of the Sun peaked at the long end, the chances are that we would be relatively less sensitive to that colour, or, like many other mammals, completely lack the ability to see red at all, since it wouldn’t be very useful to us. On the other hand, there’s a different question arising from this, as follows: why is the Sun yellow? Why is it that human colour vision gives sunlight a golden tinge when one would expect that peak at yellow would be toned down for us to give us the impression that the Sun was white?
In fact I’ve never understood why people say sunlight is yellow. To me it’s definitely colourless, and in fact I think it has to be or most outdoor daylight scenes would have a yellow tinge, and they haven’t. I don’t really think I’m unusual in that. In fact I can’t see how any healthy human eye could fail to adjust to the appearance of daylight without coming to perceive it as neutral. Nonetheless, the Sun is a yellow dwarf, which marks the beginning of a mystery which has yet to be solved definitively and is related to the Fermi Paradox.
I’ll just briefly introduce the Hertzsprung-Russell Diagram:
This is probably one of those things not many people know about, but if it isn’t, sorry for boring you. If you plot stars by their absolute brightness and colour, you find that they’re not randomly distributed but tend to fall into particular categories. In particular there’s the Main Sequence, the line on the diagram our Sun is currently on and gradually moving towards the top right hand corner. This may be one reason why there was a major ice age around 700 million years ago, after most of the carbon dioxide had been cleared out of the air via photosynthesis, decreasing the Greenhouse Effect, and it’s also one reason Earth will be uninhabitable an æon from now.
The main spectral types (i.e. “colours”) of stars are classified by a series of letters: W, O, B, A, F, G, K, M, R, N and S. Of these, W, R, N and S are rare. There are thought to be only about fifty W-type stars in this Galaxy of 400 thousand million. They’re also known as Wolf-Rayet stars, hence the name, and are high in heavier elements and often surrounded by luminous clouds of gas. Interesting objects but nothing to do with what I’m talking about today. R- and N-type stars are cool and high in carbon, and S-types have roughly equal quantities of carbon and oxygen in their atmospheres, with zirconium monoxide. All of these are the “freak” stars. The main spectral types are O, B, A, F, G, K, and M, and in colour they are blue-white, blue-white, white, pale yellow, yellow, orange and red respectively. If you want examples of each, here you go: α Camelopardalis (the most distant visible star, actually in a different arm of the Milky Way from us), Rigel (the bright, bluish-white star in Orion), Sirius A and B, Procyon A, the Sun (and also α Centauri A), Arcturus (a fast star from outside the main part of the Milky Way which happens to be passing through our arm nearby at the moment), and of course Betelgeuse. Each spectral type is divided into ten subdivisions, numbered 0-9, and also into seven sizes numbered using Roman numerals, with the smallest at VII. Our Sun is a G2V-type star, or yellow dwarf.
The types of stars as far as age is concerned are called dwarfs, subgiants, giants, supergiants and hypergiants. For some reason there is no concept of an average-sized star. I don’t know why. Although the Sun is a yellow dwarf, it’s actually in the top 10% of stars by mass. You wouldn’t think this by looking at the sky though because many of the stars visible to the naked eye are blue giants. This has the side-effect of many of the stars routinely referred to in science fiction and space opera being the wrong ones. The hottest and most massive stars may form planets but don’t have time for life to evolve very far on them, and it’s also possible that their solar winds drive away the discs of gas and dust which would become planets otherwise, although on the other hand again, they may form small planets a long way away. Hence when ‘Star Trek’ talks about Rigel VII, some explaining needs to be done. Isaac Asimov once calculated that Rigel was visible over a seventh of the volume of the Galaxy, and a star that powerful is not likely to have habitable planets.
Smaller objects of a particular kind are likely to be more common than larger ones. Thus there’s more sand than pebbles on a beach and more pebbles than boulders. This is a general rule, and it applies to stars. Therefore the yellow dwarf, perhaps inappropriately named, we orbit is more massive than nine out of ten other stars, although there are some which are many times as large. Most known stars, including Proxima Centauri which is closest to the Sun, in the neighbourhood are red dwarfs, which are small, cool stars which will outlive the Sun many times over. Eleven of the twenty-seven stars within a dozen light years of the Sun are red dwarfs, and there may be more which are too faint to have been detected yet.
Criteria for habitability used to include the proviso that a star not be too small for a planet. Objects that orbit near massive other objects have locked rotation and constantly present the same face to their primaries or companions. For instance, we see Oceanus Procellarum, Mare Tranquilitatis and the other maria but we never see the far side of our satellite because it always faces us and therefore has a day lasting a whole month. It used to be thought that Mercury constantly faced the Sun and therefore was in constant sunlight on one side and constant darkness on the other, but in the 1960s this was found not to be so, although it does rotate only very slowly. A star less than seventy percent of the mass of the Sun would not be enough to warm Earth sufficiently at this distance for life as we know it to exist on the surface, and this planet would have to orbit so closely that it would, in fact, be like this, and for this reason it was long thought that life was impossible around smaller stars. However, this is only true to a certain extent. Although the orange, K-type stars in question do reach a point where Earth-like planets would have locked rotation of this kind and would therefore be completely uninhabitable, there’s a further point where the radiation from the star becomes weak enough for it not to heat the planet beyond bearable temperatures in its twilight zone. Such planets are called “eyeball planets” because of their appearance:
The above is an artist’s impression of such a planet forty light years away in the constellation of Aquarius. It could conceivably have an ocean of water and habitable temperatures on its sunlit side with ice towards the terminator (the line between day and night) and of course a frozen solid dark side.
Not all red dwarfs are suitable stars for life-bearing planets because many of them tend to be flare stars. Barnard’s Star, Proxima, Wolf 359 and one of the UV Ceti binary system are all in this category. A flare star is a generally faint star which suddenly increases dramatically in visible brightness and other electromagnetic radiation, probably due to the release of energy in its magnetic field, making the conditions on its planets quite unstable. These are like solar flares but because the scale of the star is much smaller and the stars are much dimmer, they make a proportionately bigger difference to their output. This means they may not be so suitable after all. They can also become covered in many more sunspots than the Sun, which would reduce the radiation a lot, by up to forty percent. However, on the plus side they don’t suffer the catastrophic coronal mass ejections that the Sun occasionally does, meaning that planets are more likely to be able to hang on to their atmospheres, and it isn’t clear that red dwarfs are generally flare stars either. Some of them may be quite stable.
There are, as I’ve said, likely to be more orange dwarfs than yellow dwarfs, and more red dwarfs than orange or yellow dwarfs. Orange dwarfs, i.e. K-type stars, spend longer on the Main Sequence and evolve more slowly than yellow G-type dwarfs such as our Sun, so in a way the question could equally be, why isn’t the Sun orange?
Ten percent of stars are yellow dwarfs, but three-quarters are red dwarfs. If they were the same age as the Sun, only one in seven would need to be stable for them to have an equal chance of having habitable zones on their planets, all other things being equal (which they may well not be). However, red dwarfs stay in their current state for several times the length of the whole career of the Sun, whether or not it’s on the Main Sequence, and the smaller ones will last hundreds of times longer. Therefore it’s possible to use something similar to the Doomsday Argument here.
We evolved on a planet circling a yellow dwarf star, about eighty percent of the way through the time when it would remain suitable for life to be maintained here. The fact that it’s that late should already provide food for thought. However, there are seven and a half times as many red dwarfs as there are yellow dwarfs and they also last much longer, say ten times on average as a very conservative estimate, but in any case Barnard’s Star, for example, is twice as old as the Sun. Therefore, given that nothing else is relevant, and science relies on not making stuff up without a good reason, the probability that we would have come into existence during the Main Sequence lifetime of a yellow dwarf is only two percent. It also compares poorly with the orange dwarf scenario, though not quite as badly – it’s maybe about twice as likely that we would’ve evolved around a K-type star and the captured rotation problem cuts it down a bit.
Hence: why isn’t the Sun red? Why have we appeared in a system which is less common than the kind considered most likely to give rise to habitable planets? Also, why have we appeared so early in the history of the Universe? Barnard’s Star has had twice as long to have intelligent life forms evolve on one of its planets, assuming any are suitable. Maybe the answer to the Fermi paradox (where are all the aliens?) is that they haven’t evolved yet, and there will one day be many of them, but right now we are freakishly early and have appeared under a freakishly yellow star.
The principle of mediocrity evolved in connection with our growing perception that there was nothing special about who, where or when we are. China’s name for itself is 中國, the “Middle Kingdom”. Jain cosmology placed India at the centre of the flat Earth and Earth at the centre of an impressively large Universe. We refer to the sea between Europe and Afrika as the Mediterranean. Likewise, we used to think Earth was the centre of the Universe and that everything revolved around it. And so on. Then many of us were dislodged from thinking of ourselves as special in that way, and in order to come to a clear understanding of physical reality, and perhaps also psychological, we now realise that it often helps for us not to think of ourselves as in any way central. If we apply this to our appearance on this planet, we apparently ought to assume that we aren’t unusual, which suggests various things. For instance, since life appeared here after a very short period of time, it’s often thought that life must be present in all conditions where it can arise at all. As far as the evolution of intelligence and technology are concerned, on the one hand if we’re not special, one can expect aliens to be all over the Universe, but on the other, most of the time life has been on this planet it was in the form of microörganisms alone, so looking at the entire history of life so far, the way in which we’re not unusual may be in the sense that most life in the Universe is extremely simple compared to humans and also microscopic. The trouble is that we are a single example. All we know is that we’re here, and the fact that our Sun is yellow may not mean that in principle intelligent life can only evolve in yellow dwarf systems, but that it’s just unlikely in general, and we just happen to have a yellow dwarf Sun.
This next bit is based on this paper: https://arxiv.org/abs/2106.11207.
Kipping proposes the following solutions to the problem: life is rare on red dwarf planets; there is only a short period when complex life can evolve in the vicinity of these stars; suitable planets are rare.
Rarity of life might result from the initial instability of the stars leading to them causing all the water to evaporate from their planets, and assuming that water is essential to life, this would not enable life to evolve there, or survive if it arrived from somewhere else. Also, in our own system Jupiter protects Earth and other planets from cometary and asteroidal impacts and may also have corralled planetesimals into a comfortable distance from the Sun for Earth to form by a regular tug every orbit or every few orbits those objects made. If red dwarfs have nothing like Jupiter to do either, planets may not form at an appropriate distance or any that do form may be constantly pelted with asteroids and the like. Even so, and this is my observation, this could be a temporary phase while the system settles down into a more hospitable state, because the time might come when most of the débris has been cleared by colliding with planets, and after that life could appear, or evolve if it already had. That said, maybe that lies in the future for most red dwarf systems, and they have a lot of future compared to us. However, if life evolved in their systems about a hundredth as often, it would level the playing field and help explain why our Sun isn’t one of them. It would make it more of a coin-toss situation.
As previously observed, red dwarfs may not be particularly hospitable places for life. If flares and/or starspots are as pronounced as they often appear to be, it might simply be that they’re not good candidates for life to survive for long on. For instance, for a long time it was thought that Barnard’s Star was stable, but a flare were observed in 1998 which more than doubled the surface temperature and two more in 2019, the second one being sufficiently powerful to strip away Earth’s atmosphere within about fifteen million years if it occurred that regularly. Barnard’s Star does in fact have at least one planet, orbiting at about the mean distance Mercury does from the Sun, with about thrice Earth’s mass. With the same density, this would make it about 40% greater in diameter. Its existence is disputed, but it may not be in a good place. It also occurs to me, and I don’t know if this is true, that a planet with a much denser atmosphere than Earth’s, such as Venus, may become more habitable by having some of its atmosphere stripped away. The previously mentioned hotter phase early in such a star’s lifespan may also mean that life develops on planets somewhat further out but is then frozen out of existence as the star starts to settle down.
Kipping’s final suggestion is that planets orbiting within the habitable zone of red dwarfs could be rare. It’s easiest to detect planets orbiting brighter and larger stars, and for a red dwarf Barnard’s Star is unusually large, and most red dwarfs are unknown. The smaller stars are of course more common, and it may be that these small stars, which will last the longest, even up to ten thousand æons, don’t have any planets, although I personally don’t see why they wouldn’t have.
Kipping is clearly a very intelligent and well-educted guy, so I hesitate to come up with another explanation for our suffusion of yellow. Nonetheless I do have one, and perhaps strangely it starts with the Australian armed forces.
Some time ago, the Australian Army was having a problem with its uniforms, because many of them didn’t seem to fit very well, so they decided to come up with a small number of standard uniforms of various sizes. They embarked on a project of measuring various young adult males – inside leg, chest, arm length and the like – and found that there simply were no “average”, in the sense of being modal, sizes. Extending this further, it was found that the average Australian is female, thirty-seven years old, weighs 71.1 kg, is 161 centimetres tall, and has Australian parents but ancestry in Britain. This doesn’t sound like it’s a tall order to satisfy. There are, after all, twenty-five million Ozzies, so you’d think that just one of them at least would be exactly like this, but they aren’t. That’s a list of seven variables, so for them to be true of nobody in Australia the mean number of possibilities per variable would have to be about eleven. Another example might be having a fairly short mental list of characteristics for an ideal romantic partner. It may seem like it’s not much to ask, but in fact your chances of finding someone who perfectly satisfies that list diminishes very quickly with each added criterion.
Now apply this to planets with intelligent life on them. The average habitable planet might orbit a red dwarf star, have a mass about twice that of Earth’s, be 25% larger in diameter than this planet, have mainly shallow oceans, a dozen small continents, be closer to the inner edge of the habitable zone than Earth, be about six æons old, have a denser atmosphere than ours with more absolute amount of oxygen (but a smaller percentage). These are mainly the criteria for a “super-habitable” planet, that is, one which is even more suitable for life than Earth. Earth isn’t like this. Our Sun is brighter, larger and hotter, we’re smaller, have deeper oceans (nowadays – shallow seas used to be more common here though), have only half a dozen continents (again that number varies for reasons connected to shallow seas), is slightly colder than average, has a thinner atmosphere and less oxygen. Those are nine variables, some of which are continuous. If there are six hundred million habitable planets in the Galaxy, as has sometimes been estimated, each of them only needs to have about nine possibilities for such a planet to be unique. So perhaps the reason the Sun is yellow is that each habitable planet is an individual and has its own personality. Most of the planets might orbit red dwarfs, but each would be unique in its own way, and taken together there just is no such thing as the average habitable planet.
A Box Of Chocolates 