Red dwarfs are long-lived main sequence stars cooler (and therefore lower mass and less luminous) than the sun. Red dwarfs have spectral types K or M, where the cooler M type spectra are dominated by simple molecules such as TiO, VO, and H2O. In the disk of our Galaxy, KM dwarfs are very numerous, roughly 70% of stars.
Until recently, solar-like stars of spectral types F and G were the ones scrutinized most carefully for planets, in no small part because such stars are intrinsically brighter, and telescopes always need more photons. But with the recent explosion of planet-hunting, especially via the transit method, many KM dwarfs have now been surveyed, and many planetary systems have been uncovered. Extrapolating from transit-method discoveries, the Milky Way contains at least 1.1 planets per star – a quarter trillion to half a trillion planets. Around 10 billion of those are earth sized and in the habitable zone between the freezing and boiling points of water. Most of those (~70%) orbit KM dwarfs.
The habitable zone (HZ) is a simple concept that gets complex fast. Let’s start simple.
For the definition of a habitable zone, we seek the range of distances from a star where the surface temperature of a planet is between the freezing and boiling points of water (on the questionable assumption that liquid water is essential to life). To find the equilibrium temperature of an airless planet, first we note that the only possible energy exchange is via radiation. The planet is not touching anything, physically, so neither convection nor conduction are valid heat transport mechanisms. The planet absorbs energy from its star. The planet emits energy back to space via blackbody radiation. The two processes balance.
energy incoming = energy outgoing
Expanding that to symbols, incoming energy is the starlight flux multiplied by the cross-sectional area of the (assumed spherical) planet. Outgoing energy is the surface area of the sphere times the blackbody flux.
In that case, there are only two variables left: the star-planet distance d, and the luminosity of the star L. This luminosity depends in turn on the star’s mass and how old it is, primarily. Fortunately, a star does not brighten by more than a factor of two during the phase we call main sequence, which is also the longest and most stable portion of a star’s life journey.
And with that table data, we can make a plot of distance versus star luminosity. The habitable zone under these simplifying assumptions is shown in gray. Closer to the star (to the left) is steam. Farther from the star (to the right) is solid water ice.
For spice, in the figure above, I add a few example multi-planet systems. The letters are in discovery order, and “a” is reserved for the star.
Note that, for the solar system, Venus is in the HZ under our assumptions. So our assumptions are wrong, because we know that Venus is quite the baking oven of a planet.
Is it clouds? If we put in Venus’s actual albedo, 0.76, it shifts the HZ to smaller radii, the opposite direction we need to make Venus bake. It’s not clouds.
What we failed to consider at all is the greenhouse effect. Atmospheres make surface temperatures warmer, 30 degrees C in the case of earth and more like 400 C in the case of Venus. What we learn from this is that we need to know the planet properties before deciding on whether its surface is at approximately the right temperature for life. If a planet is too distant from its star and therefore too cool, we can always “fix” that by throwing more atmosphere on.
The other direction does not work. If a planet is too hot by virtue of being too close to its star, it really will become uninhabitable. Its high temperature will drive its atmosphere away in short order (meaning, less than a billion years), leading to a bare rock planet.
On to KM dwarfs in particular. What are the speculations regarding habitability?
- Low-mass stars have long pre-main-sequence phases where they start very luminous and gradually dim. The time between the active planet-forming phase and arrival at the stable main sequence gets long (only 30 million years for 1 solar mass, but more like 1 billion years for a star of 0.2 solar masses). That means one would need to “pre-load” a planet with a thick atmosphere such that the atmosphere would erode during pre-main sequence to a habitable level after a billion years. The planet would be too hot for habitability in this interim.
- Tidal locking. Planets hugging close to their stars will evolve to a 1:1 spin-orbit ratio, where one face points perpetually to the star. If the planets have oceans or atmospheres, this will drive a global circulation where hot gases expand and rise at the subsolar point, then push toward the night side, where the gases cool and descend. (c.f., Yang, Cowan, & Abbot 2013, ApJ 771, L45) This situation is quite weird, for sure, and at least somewhat negative in terms of its impact on habitability.
- Starspots. M dwarfs can have a lot of their surface covered in starspots in weatherlike patterns that can decrease their luminosities by 40% for years at a time. For earthlike planets, that will drive extreme climate oscillations.
- Flares. M dwarfs are magnetic, like the sun. This magnetism gradually fades over the billions of years, but until it does, magnetic activity is about the same as the sun. Like the sun, M dwarfs have flare events that produce bursts of hard ultraviolet and soft X rays. The flares are about the same strength as the sun’s, but the M dwarfs are 10,000 times dimmer overall, so these radiation storms are 10,000 times more dramatic in terms of the amount of energy deposited on the planet. Also, life will have to be protected or hardened against radiation.
- So many planets! KM stars are so numerous, and preferentially produce (roughly) earth-sized planets, that statistics is definitely in favor of KM dwarfs habitability.
- Large moons of even larger, but tidally-locked, planets would tidally lock with their planet, not the star, and thus would retain “normal” day night cycles.
- KM stars are extremely stable for a very long time in terms of overall luminosity (and after their magnetically-stormy youth has passed). A planet that arrives at a habitable state 2 billions after formation will stay that way for a very long time.
- Tidal locking depends on a lot of factors, and may not go to completion for rocky worlds. For example, Mercury settled into a 3:2 spin-orbit coupling, not a 1:1 spin-orbit coupling.
- A smaller HZ is probably not much of an issue because the planetary systems are also more petite.
- The shift of a red dwarf’s light to the infrared does not seem to me to be a problem, though it is true that classical photosynthesis requires visible-wavelength light. There is still plenty of starshine to provide an energy source for life, and many metabolic pathways for capturing lower-energy photons.
- If life exists in gas giants or in ice moon subsurface oceans, none of these considerations matter much, since the life will evolve very protected from the influence of its parent star.
I should emphasize that all these points are educated guessing. We still have only one example of life (our own planet). Lacking other examples, we have a very poor understanding of what is truly essential for the development of life.
KM dwarfs are excellent places to search for life. The minute we build starships, we should stuff a few geo-astro-biologists on board and find out more. That’s the final good thing about KM dwarfs, the local stellar neighborhood is packed with them, making them the nearest and most easily-visited systems.
*Initially prepared for the SpoCon conference. Article to be edited after the conference itself.