A Universe of Possibilities: Planets of Red Dwarfs
Live fast, shine brightly, die young: some stars are like that. But they are few. So are, cosmically speaking, stars like our own Sun, though it’s taking its “life” more slowly. By far, the most numerous stars in the cosmos are M dwarfs, also dubbed red dwarfs: tiny, dim stars that will never undergo the flash of going supernova or “merely” shedding their outer envelope in the red giant stage before becoming a white dwarf. These stars, some possessing less than a tenth of our Sun’s mass, will continue shining their faint light for hundreds of billions of years, even a trillion years—magnitudes longer than the current age of the universe—getting incrementally brighter and hotter until eventually running out of their thermonuclear fuel.
Is there life to find around red dwarf stars, or are the most numerous stars in the universe—and most of the universe and its long, long future with them—barren?
Red dwarfs glow very faintly, and so their habitable zone—the distance from the central star where a planet gets just enough light to sustain surface liquid water under the right conditions—is very close. Close-in planets may have several weaknesses for habitability: First, they quickly become tidally locked, always turning one hemisphere toward their star, the other bathed in perpetual darkness. Would that be detrimental to life? And second, they’re permanently in peril from stellar flares. Red dwarfs tend to be very active stars, wildly variable, often flaring—threatening to blow away a planet’s atmosphere. Should we discount any chances for life, then? It’s not so simple . . .
Balmy, or Scorched Dry?
The sun never sets; never moves from right above you. You’re standing in the so-called substellar point on a tidally locked world. Are you baking in the constant sunlight? Surrounded by a dry barren landscape? Would you be better off on the nightside, or is it so cold there that atmosphere begins to freeze out, gradually making the planet less and less suitable for life?
Earlier notions of tidally locked planets conformed to this rather bleak view, but recent simulations have disputed it, showing that an atmosphere with a sufficient density—even less dense than Earth’s—could efficiently distribute heat all over the planet with little danger of overheating on the dayside and freezing out in the permanent darkness. Ocean currents could help as well, depending on the ratio and distribution of land and water. If a planet is mostly water-covered, it could transport heat quite efficiently. If, however, it’s mostly land-covered or plate tectonics drives its continents to the nightside (imagine a single Pangea-like continent that happens to “float” there), the dayside and nightside temperatures could grow increasingly different, and water might then become trapped in a thick ice sheet in the permanent night.
The climate is the least of our problems, though, if there is no atmosphere to enable a climate. An atmosphere is a fickle thing. Mars could tell you that: the relatively mild steady flow of particles from our friendly Sun has blown away much of its atmosphere, stripping so much of it that Martian atmospheric pressure doesn’t allow for the stable existence of surface liquid water. A planet orbiting a red dwarf could well suffer the same fate, albeit for different reasons.
Mars is much smaller than the Earth. With its approximately third of Earth’s gravity, it can’t hold onto its atmosphere as well, and even more importantly, it has lost its magnetic field—its protection against the solar wind—long ago. To avoid Mars’ fate and worse, a planet orbiting an active red dwarf would likely need to be more massive and have a strong magnetic field. That sounds simple but may not be . . . Of all the rocky planets and moons in our system, only Earth, Mercury, and Ganymede have their own magnetic fields. Venus doesn’t. Nor does current-day Mars, or any of the number of moons of the giant planets, with the single exception of Ganymede. Why these three worlds? What connects them? Or, framed differently, what sets them apart from the rest?
It’s long been thought that relatively quick rotation is needed to maintain the inner dynamo, but the case of Mercury—revolving around its axis every fifty-nine days—seems to dispel it. It’s not about size, either; both Mercury and Ganymede are much smaller than Earth and also smaller than Mars. In relative terms, the core of Mercury is exceptionally large, but the Ganymedean core is likely relatively small.
In short, we can’t really predict yet whether planets with strong magnetic fields can be expected in the vicinity of red dwarfs. It probably depends on the chemical composition of the planet, the abundance of radioactive heat sources (influenced, in turn, by the composition of the original star-forming cloud), and tidal effects from other planets.
Speaking of other planets and tides, we’ve now come across multiple, very compact planetary systems of low mass stars, such as TRAPPIST-1. Around it, seven planets orbit within a space that would well fit within the orbit of Mercury. It’s likely that at least some are locked in orbital resonance with each other, completing their revolutions around the star in such a way that they periodically exert greater gravitational influence on each other. Jupiter’s moons Io, Europa, and Ganymede do that, and it’s part of why the inner Io is so squashed by tides that it’s all dry and very volcanically active, and why Europa and Ganymede can maintain huge liquid oceans underneath their icy shells.
Tidal heating from orbital resonance could be so strong that it would leave a planet dry and akin to Io or Venus—a scorched volcanic hell—yet another nail in the coffin of habitable planets in red dwarf systems, it seems. No magnetic field can protect against tidal forces.
Still, if we avoid orbital resonance between planets leading to strong tidal heating, there’s “only” the question of protection against stellar wind and eruptions.
But flares might not be as much of a problem as originally thought. A recent survey showed that most eruptions on small stars occur near their poles—while planets typically orbit in a plane around the star’s equator. An M dwarf’s planet would still need to withstand being very close to the star, but superflares and coronal mass ejections stripping its atmosphere may not be such a great risk. Furthermore, if a planet can survive the last four or five billion years of greater stellar activity or acquire a secondary atmosphere, for instance through volcanic outgassing, it enters a much calmer period. Older red dwarfs, such as Barnard’s Star, appear to flare less, and have a less damaging UV and X-ray flux, than during their long, wild youth.
We could also solve the atmosphere protection problem another way, though: by letting our world be protected by the strong, far-reaching magnetic field of a giant planet. Let’s talk exomoons.
So far, we don’t know any exomoons for sure—there are two uncertain detections—but if other planetary systems resemble ours at least vaguely, we can expect them to host a variety of moons, some of them perhaps habitable. Could they also be present in systems of red dwarfs?
Giant planets in our system orbit far away from the Sun, but we know of a few hundred “warm/hot Jupiters,” gas giants very close to their stars. Though these planets form farther away, where volatile compounds can condense and persist, they can migrate in the protoplanetary disk toward their star early on. But are they present around red dwarf stars?
They seem to be rare (as massive planets of low mass stars generally are), but a handful are known. While their proximity to the central star may make it hard to keep a moon (or, if it orbits the planet close enough to avoid disruption by the star’s gravitational pull), it appears that it’s possible, and that habitable moons of red dwarf planets may exist. Outside of the conventional habitable zone, moons could also be kept warm enough for liquid water and life by tidal heating alone—either letting moons such as Europa or Enceladus host subsurface oceans, or perhaps even allowing surface liquid water on larger and warmer exomoons. Unfortunately, we have practically no chance of ever remotely detecting life on a moon with an ocean locked under ice.
If the moon had its own magnetic field too, it could boost the protection given by the planet’s magnetosphere. Unfortunately, the habitability of exomoons in M dwarfs’ habitable zones doesn’t look promising—but the matter is far from resolved. Perhaps, one day, we might discover a moon like Asimov’s Erythro from Nemesis—whether hosting life or not, that will remain to be explored.
Sights to Behold
Imagine for a moment that you have traveled to a planet orbiting a red dwarf. You have landed on the dayside near the terminator. You can just about make out the brightly colored auroras dancing behind it in the eternal night. You can see the lights even in the permanent daytime sky—so powerful they are, due to the planet’s strong magnetic field and the intense stellar wind. Or you might not see much—a thick cloud cover might obscure your view of the star and auroras as well (but you should be glad the clouds are there, since they help cool the dayside and distribute heat more evenly).
In any case, you’re not here just for these sights; you came for the life. The first thing that startles you is the peculiar shape of the trees, looking as if someone had cut off more than half their crowns. Botanist Frank Landis depicts an image of how plant life on a red dwarf world might look: Since light on a tidally locked world would always come from the same angle, plants would have their leaves turned permanently toward it. Imagine trees with all of their foliage turned one way, as if you cut most of the tree’s crown. The steady insolation and wind patterns, in combination, might create spectacular sights alien to our Earth-accustomed eyes.
What about the color of the plants’ leaves? A popular image paints them as black in order to absorb as much of the faint sunlight as possible.
Using only visible light, plants on a red dwarf’s planet would have limited energy available—enough to sustain a less abundant biosphere than here on Earth, and potentially generating fewer chemical signatures of life for us to detect remotely.
However, exotic photosynthesis could well resemble what Earth bacteria are able to do, using far-red light or even infrared light from geothermal radiation to photosynthesize. Earth microbes can grow under simulated M dwarf spectra, using far-red light for oxygenic photosynthesis. Would there be enough evolutionary incentive for this ability to spread widely if the visual light is weak, but reliable, never going away completely and never changing in seasons, only getting partly obscured by clouds?
Speaking of light—depending on how thick the atmosphere is, whether there is any ozone layer (it might be more easily destroyed under the M dwarf light spectrum and flares), and how strong the magnetic field is, life would need to evolve ways to shield itself against radiation damage or cope with it. Aquatic life has it relatively easy—all it needs is to submerge a bit deeper. Land-based life, though, would probably use a variety of repair measures, going into dormant states and sporting shielding layers of dead tissue or excreted material. The UV and X-ray flux would be a steady limitation, while flares would be only occasional, but all the more destructive.
Still overlooking the shadow-zone landscape, you happen to notice the thick bark of the contorted trees, probably enabling them to survive minor flare events even if their leaves and shoots do not. You wonder what adaptations they might have for greater flares: perhaps sturdy seeds or deep-hidden tubers, so that when most of the surface life withers and dies, it can spring up again?
Once you wander off to the ocean and start exploring it, you find that it lacks many of the light-driven communities, such as coral reefs known from Earth; the faint, red sunlight doesn’t penetrate deep enough to enable their existence. But although photosynthesis is limited to the uppermost layer, life still thrives in the deep. Luckily, this particular planet is quite geologically active, providing plenty of hot vents for life independent of the red star’s light. How long is it going to remain active enough to support a biosphere? For M dwarf planets, the star’s lifetime is not a limiting factor, but the planet’s geophysics eventually will be.
Another sight to behold would be other planets in the sky. Compact systems, such as TRAPPIST-1, may host multiple planets in the habitable zone, getting close enough to each other to appear approximately as large in the sky as the Moon viewed from Earth. Imagine seeing another world, perhaps with continents and oceans, regularly move through your sky. What incentive would that be for early spacefaring, and for thoughts about life on other planets?
But they might not resemble Earth very much. The TRAPPIST-1 planets are less dense, which could mean many things, from little to no iron cores to high water content. If they have little metallic cores, or none at all, they probably lack magnetic fields (unless they’re generated in molten silicates, which is a possibility).
So much yet to discover for scientists—and science fiction authors too! SF is rife with planets of M dwarfs, be they around our closest stellar neighbor Proxima Centauri (such as in Cixin Liu’s The Three-Body Problem or Stephen Baxter’s Proxima) or elsewhere, for instance The City in the Middle of the Night by Charlie Jane Anders or Four Hundred Billion Stars by Paul McAuley. They all might resemble reality in some ways, or maybe none—that remains to be seen. We can probably expect a wide variety of worlds: Some scorched dry, their water and other volatiles irrevocably trapped in a thick ice sheet on the nightside, their atmosphere almost blown away. Some Venus-like, with a thick atmosphere and hot all over, including the dark side. Some ocean-covered “eyeball Earths,” mostly resembling Europa or Ganymede, but with an atmosphere and a fraction of open ocean closest to the star. Or maybe some worlds where the dayside is too hot and dry, the nightside too cold and dry (think an Antarctica-like desert), but life teems around the terminator in the “shadow zone.”
There are so many possible combinations of chemical compositions, radiogenic heat availability, tidal heating, tectonic regimes (largely dependent on the former traits), magnetic fields (ditto), atmospheric compositions (where, in turn, tectonics and volcanism come into play), stellar activity, and other traits important for habitability, that it’s probably going to take a lot of time to figure out whether red dwarfs commonly host habitable planets or not.
Luckily, red dwarf planets are relatively easy to discover due to their relative mass and size to the star and their short orbital period—it’s easier to detect such a planet transiting in front of its star, from our point of view, and to gather enough data to show it’s really there and what its properties might be. Their gravitational pull on the star also enables relatively “easy” detection by monitoring the star’s motion, evidenced by the Doppler shift of its starlight.
The James Webb Space Telescope, currently slated to be launched in three weeks on December 22, 2021, could help us unravel the mystery of red dwarf planets’ habitability by observing their atmospheres. Systems like Proxima Centauri or TRAPPIST-1 are among prime targets for the telescope (and other telescopes such as ARIEL). Until then, we know so much and so little at the same time. There is a whole universe of almost boundless possibilities—and so many stories waiting to be told, set on these strange, strange worlds.