I’m asking this question because I understand that red dwarf stars are the most common type of star, and our sun is yellow dwarf. As a layperson the first thing I have to wonder is if the sun emits a special type of light or some other thing that makes life more likely around a yellow dwarf than a red dwarf.
Just happened to see a program today, How The Universe Works. A couple things about red dwarfs stuck with me. Their lifespan is amazing, compared to our sun, which will have around 10 billion years of life. Red dwarves have lifespans of trillions of years. They burn their fuel much more slowly, and their surface temperature is about half that of our sun.
I would imagine the “Goldilocks” zone is much smaller, due to the reduced energy output. Whether the different light spectrum would have any effect on the chance of life originating on any planet circling a red dwarf, who knows?
Astronomers Spot Most Earth-Like Planet Yet and yep, it’s orbiting a red dwarf.
The sun is considered a yellow star simply because it’s somewhat hotter than a red star and so more visible light in emitted in those frequencies. There’s nothing special about the light or the heat. However, since it’s hotter earth-like planets can orbit at farther distances.
At this point, finding a small planet so close to a star is at the boundaries of what we can make out. The planets found therefore tend to be much larger than the earth or are larger fractions of their star’s mass or some other factor that allows them to stand out. They are a totally non-representative sampling of all planets. Until our techniques grow much better we can’t know whether red dwarfs have planetary systems in comparable numbers or positioning.
I doubt that the different distribution of energy over the spectrum matters very much, but red dwarfs simply put out a lot less energy per unit time. A planet would need to be a lot closer in to be warm enough for life. Whether that would necessarily lead to any any other issues relevant to life, I am not sure. Tidal effects of being close to a star, and excessive exposure to ionizing types of radiation from the star might raise problems.
Most red dwarfs also happen to be flare stars, and can suddenly brighten to several times their usual output. So it is also possible that a planet alternately freezes and boils on a daily basis.
I think there’s a good chance, yes. The basis of any kind of life has probably got to be photosynthesis, i.e. doing chemistry with light from the sun. The problem is that photons with wavelengths longer than visible light don’t have enough energy to break a chemical bond (and, quantum mechanics being the way it is, you can’t “add together” a whole bunch of low energy photons to create one higher-energy event that could break a bond). Earth plants, which are pretty good, can photosynthesize out to about 750nm, deep in the red, but it’s hard to imagine any way of doing it in the infrared, where red dwarfs emit much of their energy.
It’s even worse if you think about how life could evolve in the first place: green plants today have highly-optimized “antenna” systems that cleverly extract a bond-breaking amount of energy out of visible photons. But such a system is the result of a billions of years long optimization process. It can’t arise spontaneously – it’s too complicated. So the very first crude photosynthetic action would likely have required ultraviolet photons, which are energetic enough to do chemical reactions without any fancy antenna mechanism. There’s a lot less UV coming from a red dwarf.
Well, except for the fact that, if you’re born under a red star, and then move to a planet orbiting a yellow one, you get superpowers.
What?
I just want to note that red dwarfs are actually misnamed. They aren’t actually red at all. Their temperatures range from 2000K to 4000K. The temperature of the filament of an incandescent lightbulb is in the 2000 to 2500K range. Anyone think 100 W bulbs look red? Or any incandescent bulb not specifically made to be red?
If you were to visit a red dwarf system, you’d think the local sun looked white, since your eyes adapt to the different lighting conditions. If you could compare the light to that of the Sun, it’d just look somewhat yellowish, not red at all.
This web-page gives a good explanation and some cute graphs.
Not all life is based on photosynthesis. There are life forms that leave near deep-sea vents thatlive off of chemical energy stored in sulfur compounds. In fact, this is believed to be how the earliest life forms on Earth lived. There are also living things that live off of the gamma rays from radioactive decay, though these evolved from predecessors that ate other things.
Even if you want photosynthesis, you might still be able to pull it off. As mandala mentioned, red dwarfs are often flare stars, which means that even though their steady UV output is pretty low, it can briefly get quite high. Life on such a world might have evolved to photosynthesize in bursts, before possibly evolving to also take advantage of the visible continuum.
The biggest problem with life around a red dwarf would be the tidal effects mentioned by njtt. Specifically, a planet in a red dwarf’s “Goldilocks zone” would almost certainly be tidally locked, with one side forever facing the star, and thus too hot, and the other side perpetually shadowed, and thus too cold. One possible way around this is to have your habitable world be a moon instead of a planet, and to be locked to its planet, not to the star directly.
I wouldn’t think photosynthesis would occur exactly on another planet as it did on Earth, especially if the sun has a different spectral charactor. I’m sure some biochemical reactions are triggered by infrared light, and give evolution a few billion years, she’ll find them.
Has it been considered that since the red dwarf is emitting heavy in the infrared, and the original atmosphere is composed of carbon dioxide, thus creating intense greenhouse effects, that perhaps the Goldilocks zone would be pushed further out, possibly far enough to relegate tidal locking?
Life seems to have originated here on Earth almost immediately, if such is true for Kepler-186f, then we’d have life there before the planet become locked. This life would evolve to survive any tidal locking seems to me.
Note that Kepler-186f is 0.393 AU from its star, about the same distance as Mercury is from Sol. Mercury isn’t tidally locked, so Kepler-186f may not be tidally locked. Mercury is in a resonance-lock, however, so the days might be very long.
It took about one billion years for life to appear on Earth. It took 3.5 billion years for multicellular life to appear. That’s not immediate.
It is possible in some molecules, to get multiple IR photon absorption in vibrational modes and transfer the energy to an electronic transition, which normally requires a visible or higher frequency photon. But it’s rare, and the efficiency is extremely low – you are basically working against the Second Law, because you need the energy from multiple IR absorption events to coherently focus on one electronic transition.
Could you build a photosynthesizing system nevertheless? Maybe. But then there’s the problem of how you evolve to there. Starting from – what? The advantage of evolving life under a star that has a decent visible/UV flux is that it isn’t hard to start with some basic photochemistry and evolve to become more efficient. Id est, on Earth it seems likely the first use of photochemistry by life used near UV in some crude system, because UV can initiate all kinds of chemical reactions, since UV photons have energy comparable to the electronic transitions necessary. Life could then evolve to make use of the far more plentiful (but lower energy) visible photons. We see this is in the complex “antenna” structure in the photosynthetic reaction center.
But you have to start from somewhere simple before natural selection can get going. You can’t expect a complex antenna system to just assemble itself as the first step. So I think this is a major stumbling block on the road to life around a star that has low UV flux.
Red giants such as Betelgeuse definitely look red.
Astrophysically, red dwarfs are different from our sun in an important way: heat from a red dwarf’s core is transmitted to the surface entirely by convection, whereas in our sun it’s by a combination of convection and radiation. This “stirs” a red dwarf’s core and outer layers, so red dwarfs will never leave the main sequence; they’ll simply burn their fuel for trillions of years and eventually get dimmer and go out.
Yes, from Earth M-class stars look red. That’s because the light level is so low that it doesn’t saturate your cones. Go to the system itself and it will not look red.
Look at the page cited by watchwolf49 above. Ignore the colored circles; as it says, those colors are exaggerated. Instead look at the light curve for the 2500 K star. Despite the fact that its peak is in the infrared, it still has a fair amount of light in the blue part of the spectrum. If you’re close enough to the star to be in the Goldilocks zone, the amount of blue light from the star is going to saturate your blue cones, so the star will look white, just like a 100 W bulb.
One thing that will be different is when the star sets. On Earth, as the Sun sets, it gets very red as it approaches the horizon. This is because it’s shining though a greater amount of atmosphere and more of the blue light is being Rayleigh scattered. On a planet with an M-type star, this effect is going to happen significantly further up in the sky. So it’s only going to look white when it’s fairly high in the sky.
Maybe not quite that long. A few years ago, some fossils were found that might have been multicellular life of some sort from about 2.1 billion years ago. If it’s true, almost certainly an evolutionary dead end, and therefore not one of our ancestors. But still, one of life’s early attempts to go macro.
I read somewhere recently that our sun might not be the goldilocks type of star we assume it is, because yellow dwarf stars have frequent flares that can fry planets, whereas red dwarfs are comparatively calmer. Chronos also mentioned that while a planet in a red dwarf’s habitable zone might be tidally locked, a moon around that planet would experience a day/night cycle.
To the best of my understanding, the only reason Mercury is not tidally-locked is because of its elliptical orbit. As the rotational period slows, tidal friction decreases. Because tides vary greatly throughout Mercury’s year, tidal friction varies. With a Rotation:Revolution ratio of 3:2, tidal stresses remain that would be reduced if the ratio of 1:1. But stresses would temporarily increase if the rotation rate slowed.