I hope you’re all ready for a super-blog. We’ve got a lot to cover in this introduction to star type + the perils of life around a red dwarf star, so let’s jump right in.
The stars in the sky all look pretty much the same from Earth. If we can see them at all, it’s as white pinpricks of varying sizes. They don’t seem very colorful. They don’t seem to change much. But this impression is very misleading.
In reality, there are dozens of types of stars with distinct colors, lifespans, and behaviors.
The color of a star plays a major role in determining the color palette and lighting of the planet orbiting it. Their skies will be different colors, and their vegetation will develop different pigments to best take advantage of their energy. This will affect the mood of any humans living on the planet, as well as how the planet’s native life perceives us.
The star’s lifespan, too, plays a major role. Our own Sun is roughly at the halfway point in its time as a habitable host star for Earth: in about five billion years, it will expand into a red giant, sterilizing the surface of our planet. Some stars may age and die so fast that life has fewer chances to evolve intelligence, while others may live up to 1,000 times longer than our own Sun and still be habitable.
The answers to these questions will have obvious importance for any life forms living around them, human or otherwise.
What about stellar behaviors? Do stars really behave?
Why, yes. All stars change as they get older. Some change faster, and in different ways, than others. Many orbit each other in binary or multi-star systems. Astronomers now believe that most stars are part of at least loosely orbiting binary star systems.
Some stars, called variable stars, drastically vary the amount of heat and light they put out over the course of minutes, hours, years, or millennia. These drastic changes can be predictable, or can occur without warning.
These are thought by astronomers not to be the best places for life to evolve complexity or intelligence for obvious, apocalyptic reasons, so we’re not discussing them first. But if you’re interested in a planet with a recurring and/or predictable apocalypse, you can read more about variable stars at the end of this chapter.
Read on and we’ll cover the six types of stars that are thought to be best for fostering life as we know it, with notes about variable and binary stars, their challenges to habitability, and their utility as potentially interesting plot devices.
Class M Stars: Red Dwarfs
Red dwarfs are the most common type of star in the Universe. About 76% main sequence (not newborn or dying) stars in our immediate stellar neighborhood are thought to be red dwarfs. These stars are also the coolest, reddest type of star, and the longest-lived. Because they burn their fuel slowly, they can live for up to 10 trillion years – 1,000 times the lifespan of our Sun.
There are a lot of these stars, and they’re stable homes for planets for a long time. These attributes obviously make red dwarfs appealing as potential homes for either alien life forms or human colonists.
However, there’s a problem. Because red dwarfs produce relatively little light and heat, astronomers think that they might have to orbit dangerously close to their parent stars in order to be in the “habitable zone.” Since all Earth life depends on liquid water as a chemical solvent, NASA astronomers define this zone as “the area around a star where it is not too hot and not too cold for liquid water to exist on the surface of surrounding planets.”
What do we mean by “dangerously close?” There are two major hazards facing planets orbiting red dwarf stars: tidal locking, and stellar radiation.
Tidal Locking & Climate
Tidally locked planets are planets that always show the same face to their parent star. In other words, they have no day/night cycle. Half of the planet experiences eternal day, the other half eternal night. This is most likely to happen in cases where the planet orbits very close to its parent star, as planets in the habitable zone around red dwarfs must. The commonality of red dwarf stars means that most habitable planets in the galaxy might be in this situation.
Scientists aren’t yet sure exactly what tidal locking would do to the climate of a planet with an atmosphere orbiting a red dwarf. They see two main extreme possibilities, with a range of possible options in between.
- A planet with a baking, superheated “day side” and a deeply frozen, subzero “night” side. In this scenario, a narrow “twilight band” between the day and nights sides might be habitable.
- A planet with constant, extreme winds and ocean currents which transfer heat from the day side to the night side. More recent mathematical models show that a planet with atmosphere or oceans would likely experience strong convection currents, as heated air from the day side would constantly rise to the top of the atmosphere planet-wide, while cooler, dense air from the night side of the planet would rush in to replace it.
Since scientists aren’t yet sure what types of planets we’ll find orbiting red dwarfs, there’s room to speculate about environments on the spectrum between the two.
Perhaps your planet has extreme differences between the “day” and “night” climates and ecosystems on the planet, but also strong winds and ocean currents that transfer sufficient heat between the two to make the whole surface habitable – albeit to lifeforms with extreme adaptations, like those we’ll explore later in this book. Charlie Jane Anders’ novel, “The City in the Middle of the Night” explores this type of setting.
Unfortunately, heat distribution isn’t the only challenge to habitability faced by tidally locked planets.
Like the heat distribution problem these additional challenges open doors for science fiction writers instead of closing them. Because we’ve never studied even a single tidally locked planet orbiting a red dwarf up close, we don’t know how big of an obstacle these challenges will be to life, or how common different scenarios will be. Keep in mind how these challenges raise new possibilities, instead of eliminating them, as we discuss the challenges of stellar radiation.
Stars are surprisingly dangerous to the planets they host. We don’t realize it on Earth because our planet has both a healthy, strong magnetic field, and an ozone layer to deflect cosmic radiation. However, even on Earth, life was not able to colonize land until our ozone layer was created out of oxygen created as a waste product of photosynthetic metabolism. For billions of years, life couldn’t inhabit Earth’s continents without getting its DNA shredded by stellar radiation.
Planets orbiting red dwarfs might receive much more dangerous stellar radiation than Earth receives from our Sun. The reason is simple: they’re closer to their stars. They’d be more vulnerable to solar flares, the star’s own magnetic field, and other sources of harmful stellar radiation.
One major obstacle is the expanded lifespan of red dwarfs. They live much longer than our Sun, but they also mature more slowly. For the first half a billion years of our Sun’s life, it was too active and volatile to safely host life on Earth. For red dwarf stars, this volatile period lasts 2-3 billion years – half the lifespan of planet Earth so far.
One danger posed by being closer to an active star is that of atmosphere loss. In addition to shredding organic molecules like DNA, solar winds can actually “blow away” a planet’s atmosphere, leaving it without a protective layer of gases to sustain life.
According to Centauri Dreams’ analysis of a series of papers published in Astrophysical Journal Letters by a Princeton Laboratory studying planets around the red dwarf Proxima Centauri b, stronger solar winds may mean that a planet orbiting a red dwarf has to be larger than Earth in order to have sufficient gravity to hold down a substantial atmosphere. These stellar winds could even dry up a planet’s oceans over a few billion years, evaporating them into space.
The good news is, a larger planet may also help with another concern about stars orbiting red dwarfs: the strength of the planet’s magnetic field.
We’ll cover an entire chapter on planet size and magnetic field strength later in this book, but right now what you need to know is this: a planet absolutely has to have a magnetic field to have a habitable surface. Without a planetary magnetic field, stellar and cosmic radiation will fry complex organic molecules such as the ones that make up living things, and even “blow away” the gases that make up the planet’s atmosphere.
Some models suggest that tidally locked planets like those most likely to orbit red dwarfs may lose magnetic field strength because they don’t spin. Earth’s magnetic field is actually created by spinning molten metal in the planet’s core – and if Earth were to stop rotating, this metal could lose some of its spinning momentum and therefore some of its magnetic field strength.
Both of these problems can be solved by making the planet larger, but remember: humans can’t easily adapt to higher gravity. We wouldn’t just need to get stronger to live in 2+ Gs: we’d have shorter life expectancies and age faster due to the strain placed on our heart by pumping blood to our brains.
Human colonists settling a Super Earth (a terrestrial planet 5-10 times the mass of our own – we’ll discuss those in detail later) might need to undergo a radical redesign, including potentially adopting a body plan like that of the komodo dragon, or another life form that doesn’t have to pump blood vertically from its heart to reach its brain.
One reason red dwarfs are sometimes appealing to authors is that life around a red dwarf would look very different from life around a star like our own.
One difference is obvious: the Sun would likely appear larger in the sky due to its close proximity. It may also appear pink- or orange-tinted thanks to its cooler temperature. Living on a planet with a huge, red- or pink-tinted star dominating the sky sounds terrifying and oppressive – but also intriguing, and even mystical.
Red dwarfs produce very little blue light, so the sky may appear white or red instead of blue. Brighter red dwarfs with thick earthlike atmospheres may produce enough blue light for the sky to have a bluish cast, but cooler red dwarfs would have skies appearing white or red.
Visuals of the sky would look like on several potentially habitable planets identified around different stars by the Kepler telescope have been kindly compiled by the Planetary Habitability Lab of the Arecibo Telescope in Puerto Rico.
Your sky may look very much like Earth’s, but color-shifted – or may be red or grey nearly as dark as the blackness of space, with a few stars poking through even during the day, depending on how much atmosphere is present and precisely how hot and close the star is.
The illumination of sunlight falling across the land would be red-tinted, which can read as either “danger, warning” or “warm, cozy” to humans, depending on our mood and the precise shade. The landscape may begin to feel monochromatic to humans, who are unlikely to see the colors blue or green anywhere outside of the Raleigh scattering of the sky.
To human eyes, these would be landscapes of reds, oranges, yellows, pinks, browns, and blacks. Some botanists believe that photosynthetic life around these stars may appear black to humans, as the algae or leaves try hard to absorb all available light from the dim spectrum.
Animals on these planets would likely have a completely different visual spectrum than our own. Evolving under different wavelengths, they may be able to see in the infrared, differentiate between shades of red, orange, and yellow in ways that we can’t, and they may be unable to see the color blue at all just as we are unable to see ultraviolet.
We’ll cover more about different visual spectrums and emotional color associations in alien life in a later chapter.
Exercise 3: Life Around Red Dwarfs
To demonstrate how knowing more lands you with more possibilities, not fewer, let’s do a quick exercise. Take a look at the following three sets of scenarios that you could play with for human explorers or indigenous life on a red dwarf planet:
- High solar winds. The planet’s oceans and atmosphere are disappearing, if they’re not gone already. Stellar radiation kills any life that lives above ground without a thick protective shell. Life may still exist in the oceans, underground, or in a barely recognizable mineral-like form, or the planet may have been rendered airless and barren long ago. Remains of a rich ecosystem, or one that never got off the ground, may be found.
- Medium solar winds. The planet’s oceans and atmosphere are slowly dwindling, and more and more stellar radiation is reaching the surface. Human settlers or native life forms may just be noticing that something is wrong, or the process may have happened too slowly for life forms to remember the paradisal climate that existed before.
This might yield startling and terrifying finds for human or indigenous paleontologists, who might turn up evidence that the planet is getting slowly but surely less hospitable.
- Low solar winds. The planet’s gravity and magnetic field are strong enough to resist the solar winds. You can be really optimistic and postulate that an Earth-sized planet could achieve this, or consider whether any native life and human colonists would need to have flat, low-to-the-ground body plans like those of the komodo dragon to minimize strain on their circulatory systems and brains.
Now, choose one of those scenarios and combine it with the following possibilities regarding weather and climate:
- High convection. The planet’s surface is constantly buffeted by hurricane-force winds, and it always has been. Life may have been forced to evolve in caves, or in the fast-moving but slightly less violent ocean currents. This planet may be more friendly to flying creatures thanks to the strong winds, but any life form living here would need to have extraordinary reaction time and toughness to cope with life in the open air.
A journey to the night side of the planet might not mean certain death, and the night side may even have its own liquid water and native ecosystems. But you might face plummeting temperatures as you travel.
- Medium convection. The planet experiences constant high winds and swift ocean currents, but these may not be as severe as in the high-convection model. Daily weather variance can occur, and there may be a wider range of surface life.
The day side will be hotter than the night side, both of which may experience extreme temperatures at their centers. But both may manage to host life, and the “twilight zones” between the two may be downright pleasant.
- Low convections. Good news: no hurricane-force winds. Bad news: likely no native life. The atmosphere may have frozen or been blown off by solar winds millenia ago. Now, one side of your planet is an airless desert, the other side an airless snowball.
If your planet used to have an atmosphere that conducted heat sufficiently to maintain liquid water anywhere on the planet, life may still exist underground on the day side, or locked under ice on the night side. Life living on the night side may thrive on geothermal heat from undersea volcanic vents. The existence of either type of life on such a dead world may surprise human explorers.
Exercise 4: Now, Paint a Picture
Combine what we’ve learned here about the possible colors and climates of stars orbiting red dwarfs, and write a detailed, descriptive scene taking place on one.
Remember to take into account:
- What colors of light you have available – reds, oranges, yellows, pinks, browns, and blacks.
- What your atmospheric choices will mean for the appearance of the sky. A thin atmosphere means dark grey to dark red sky, while a thicker atmosphere means a white, blue, or pink sky depending on precisely how cool the star is.
- What landscapes you are likely to have to work with based on the choices you’ve made about the availability of water and heat. More water makes possible the existence of oceans, lakes, and trees, while drier climates can support only grasses, deserts, and shrubs.
To answer further questions you might have about the specific types of red dwarf stars and planets that can orbit them, I recommend the Worldbuilding Stack Exchange as a great place to go with your questions.
Stay tuned for next week, when we’ll discuss a far more hospitable alien star type – and jump into some questions that will open up dozens of doors for the design of alien life.