Terraforming Planets Under a Red Sun
Matthew S. Williams
Matthew S. Williams is a space journalist and science communicator who writes for Universe Today and Interesting Engineering. In his spare time, he makes podcasts (Stories from Space) and writes hard science-fiction novels (the Formist Series). In the fall of 2022, he began teaching about the history and future of human space exploration at the Kepler Space Institute. His articles have appeared in Phys.org, Popular Mechanics, Business Insider, HeroX, Science Alert, Real Clear Science, and Gizmodo. He lives on Vancouver Island with his wife and family.
Introduction
In the past fifteen years, the number of known extrasolar planets has grown exponentially. As of January 2022, 4,884 planets have been confirmed in 3,659 planetary systems, with an additional 7,958 candidates awaiting confirmation. With so many planets available for study, the focus of exoplanet research is shifting from the process of discovery to characterization. Basically, scientists are no longer just searching for exoplanets—they’re also searching for signs of life!
Exoplanet-hunting missions have also turned up several promising Earth analogs, or as they are more commonly known, “Earthlike planets.” This term is used to classify rocky planets that orbit within the star’s habitable zone (HZ), have dense atmospheres, and plenty of liquid water on their surfaces. In addition to being good candidates for finding life, Earthlike exoplanets might also make good homes for humanity.
There’s just one catch: most of the nearest rocky exoplanets are found in M-type (red dwarf) star systems. In fact, within forty light-years of Earth, there are twelve red dwarf stars that have at least one “potentially habitable” planet in orbit around them. Among them are Proxima Centauri (our closest stellar neighbor) Ross 128, Gliese 1061, Teegarden’s Star, and Trappist-1.
These M-type stars are smaller, cooler, and dimmer than the Sun. They are also known to be quite active in comparison, which means they emit a lot of flares. But from what we know about the planets that orbit these stars, there’s a good chance some (if not all) of them possess the basic requirements for habitability. Even if they are not likely to have indigenous life, they may still be “transiently habitable.”
Compared to Earth, M-type red dwarf stars have extremely long lifespans. Whereas G-type stars (like our Sun) are estimated to remain in their main sequence phase (the main phase of their life cycle) for about ten billion years, red dwarfs may live for up to ten trillion! This also means that they evolve more slowly and take longer (about one billion years) to enter their main sequence phase. This could have a drastic effect on the evolution of rocky planets that orbit within their HZs.
“Transiently habitable” refers to exoplanets that have stable atmospheres, water on their surfaces, and plenty of oxygen that is abiotic (not the result of biological processes) in nature. Scientists speculate that this may be the case for rocky planets that orbit close to their red dwarf suns. Others, interestingly enough, may have the key ingredient for life as we know it (water), but too much of it!
Then there’s the issue of gravity. When it comes right down to it, scientists don’t know how much is needed for Earth-based organisms (especially humans) to thrive, or what the long-term effects of living in a low-gravity environment really are. While studies have been performed on the effects of microgravity (like the NASA Twins Study), it is unclear if humans can live, thrive, and reproduce on celestial bodies (planets, moons, and asteroids) where the gravity is a fraction of Earth-normal.
With some hard work and elbow grease, these planets could someday become new worlds for humanity. Before that can happen, though, several challenges, caveats, and unknowns need to be addressed. Even if we had a solution for getting to the nearest stars in a modest amount of time (decades instead of centuries or millennia), there’s a lot of work that would need to be done at the other end.
Terraforming 101
The word terraforming is derived from the Latin words for “Earth” (terra) and “shaping” (formatura). It is the process where a planetary environment is altered to make it more Earthlike and suitable for terrestrial organisms. This could involve modifying a planet’s atmosphere, orbit, temperature range, surface topography, ecology, or all of the above. One thing that cannot be modified directly is gravity, although large rotating structures on a low-gravity world might be able to provide a higher effective gravity.
The term was coined by American science fiction writer Jack Stewart Williamson in a story published in the 1942 edition of Astounding Science Fiction, titled “Collision Orbit.” This is the first known example of the term being used in print, though there are examples of the concept appearing in earlier works of science fiction.
By the late 1950s and the dawn of the Space Age, the concept of terraforming became popularized as interest grew in space exploration and colonization. Henceforth, the idea was presented by science fiction writers (who were often scientists) as a serious matter. Examples include Robert A. Heinlein, Arthur C. Clarke, Poul Anderson, Isaac Asimov, and Kim Stanley Robinson.
Terraforming also became the subject of many theoretical studies during this same time. As our knowledge of Earth and the solar system grew, so did the scientific research on space exploration and ecological engineering.
Scientific Proposals
In March 1961, famed astronomer and science communicator Carl Sagan published “The Planet Venus,” an article in the journal Science where he proposed that Venus could be transformed by seeding its atmosphere with algae. Sagan theorized that this would convert the abundant water, nitrogen, and carbon dioxide into organic compounds and reduce Venus’s greenhouse effect. Unfortunately, this proposal was deemed impractical by the subsequent discovery that Venus’s atmosphere has little hydrogen or water.
In 1973, Sagan published another article in the journal Icarus titled “Planetary Engineering on Mars.” Sagan proposed two scenarios for transforming the Red Planet: covering the polar ice caps with low-albedo materials or planting dark plants in the area. This would result in the ice caps absorbing more heat from the Sun and melting to release all of the water and “dry ice”—frozen carbon dioxide (CO2)—locked within.
In 1976, two major developments occurred: the first conference session on the subject of terraforming was organized, and the release of a NASA study titled “On the Habitability of Mars: An Approach to Planetary Ecosynthesis.” The report indicated that terraforming Mars would come down to three steps:
1. Introducing greenhouse gases to warm the atmosphere
2. Melting the polar ice caps to release CO2 and water vapor
3. Introducing photosynthetic organisms to convert atmospheric CO2 into oxygen gas
In 1982, planetologist Christopher McKay wrote a paper for the Journal of the British Interplanetary Society (titled “Terraforming Mars”) where he discussed the prospect of creating a self-regulating Martian biosphere. The paper not only included the required methods for doing so, but also the ethics of it. This was the first time that “terraforming” appeared in the title of a published article and subsequently became the preferred term.
In 1984, James Lovelock and Michael Allaby released The Greening of Mars, a fictionalized account of how Mars is terraformed in the future that remains one of the most influential works on the subject of terraforming. It was also the first proposal to introduce chlorofluorocarbons.
In his 1991 study, “Terraforming Venus Quickly,” UK scientist Paul Birch proposed bombarding Venus’s atmosphere with hydrogen and iron aerosol. The resulting chemical reaction would convert atmospheric CO2 into graphite and water. Cyanobacteria could then be introduced, which would use water, CO2, and sunlight to produce hydrocarbons and release oxygen. While the graphite would need to be sequestered and removed, the water would fall to the surface and cover roughly eighty percent of the planet in oceans.
In 1993, Mars Society founder Dr. Robert M. Zubrin and “Terraforming Mars” author Christopher P. McKay of the NASA Ames Research Center wrote “Technological Requirements for Terraforming Mars.” They proposed that orbital mirrors positioned near the poles could sublimate Mars’s polar ice caps (including its CO2 ice) as a first step in terraforming the planet.
In their 1996 study, “The stability of climate on Venus,” Mark Bullock and David H. Grinspoon of the University of Colorado Boulder indicated that Venus’s atmosphere could be transformed by introducing “carbon sinks” in the form of calcium and magnesium. They further proposed that Venus’s own deposits of calcium and magnesium oxides could be used by mining and exposing them to the atmosphere. This process would yield calcium and magnesium carbonates (rocks) and remove CO2 from the atmosphere. It requires vast quantities of calcium and magnesium and would take many millennia.
Since 2000, Geoffrey A. Landis and his colleagues at NASA Glenn Research Center have worked on mission proposals to explore Venus. In 2008, they proposed that floating research stations could be built above the cloud tops. These cities could also terraform Venus’s atmosphere over time and eventually carry humans to the surface.
In 2009, US Dept. of Energy engineer Kenneth Roy presented his concept for a “shell world” in a paper published by the Journal of the British Interplanetary Society. Titled “Shell Worlds—An Approach To Terraforming Moons, Small Planets and Plutoids,” his paper explored the possibility of using a large “material shell” to enclose a celestial body and a portion of its atmosphere. Under the shell, artificial lighting could recreate an Earthlike light spectrum and temperature, and maybe even an Earthlike ecology, completely independent of the star’s light spectrum and intensity.
In 2014, the NASA Institute for Advanced Concepts (NIAC) program and Techshot, Inc. began work on a concept called the “Mars Ecopoiesis Test Bed.” This entailed the creation of sealed biodomes to be built on the surface of Mars where colonies of oxygen-producing cyanobacteria and algae would grow.
While technically not ecological engineering, Eugene Boland (chief scientist of Techshot, Inc.) has stated that it is a step in that direction.
Ecopoiesis is a neologism created by Robert Haynes and refers to the origin of an ecosystem. In the context of terraforming, Haynes defines the term to mean the fabrication of a sustainable ecosystem on a currently lifeless, sterile planet.
As you can see, there is no shortage of proposals for how humanity could make extraterrestrial environments more liveable. First, here’s a look at those that are closest to home.
Terraforming the Solar System
Within the solar system, several possible candidates for terraforming and related methods for ecological transformation exist. The most obvious of these are Venus and Mars, which are rocky bodies like Earth and skirt the inner and outer edges of our Sun’s circumsolar habitable zone (aka the Goldilocks Zone).
This refers to the region around a star where an orbiting planet would experience temperatures warm enough to maintain water on its surface (in liquid form).
However, because of Venus’s runaway greenhouse effect and Mars’s lack of a magnetosphere, their atmospheres are either too thick and hot or too thin and cold to sustain life as we know it. However, both planets could undergo an atmospheric and climatic transformation that would allow Earth-based plants and animals to thrive in their environments—at least in theory.
Beyond these two candidates, there is no shortage of solar bodies that could be made habitable with the right strategies and resources. But whereas Earth’s immediate neighbors are well-suited to terraforming techniques, others are better suited to paraterraforming or similar methods. Let’s begin with the former:
Earth’s Siblings
Currently, Mars is considered the best candidate since it is the most habitable solar body beyond Earth because of its similarity to it. Like Earth, Mars is a terrestrial (or rocky) body composed of silicate minerals and metals differentiated between a metallic core and silicate mantle and crust.
A Martian day (known as a Sol) is slightly longer than a day on Earth—24 hours, 39 minutes, and 35 seconds, to be precise. Mars’s axis is tilted 25.19° to its orbital plane, which is close to Earth’s axial tilt of 23.4°. However, because Mars is about fifty percent farther from the Sun, a Martian year lasts about twice as long as a year on Earth: 686.98 Earth days. Still, Mars has a defined four-season cycle that corresponds to its orbit around the Sun and the tilt of its axis. Like Earth, Mars even has polar ice caps that grow and recede with the seasons.
Lastly, volumes of evidence indicate that Mars once had large oceans on its surface, and much of that water may still exist today beneath the surface. Given these similarities, it’s not hard to see why Mars is colloquially known as “Earth’s Twin.” But beyond these similarities, our two planets could not be more dissimilar.
For example, life on Earth is protected by a strong magnetic field that is the result of action in the planet’s interior. This consists of a molten outer core rotating around a solid inner core in the opposite direction of Earth’s rotation. Mars also had a magnetic field at one time. But about four billion years ago, the more-rapid cooling of the planet’s interior caused all geological activity to cease and the field disappeared.
This caused Mars’s atmosphere to be slowly stripped away by solar wind over the next few hundred million years. While it is replenished by outgassing, the resulting atmospheric pressure is less than one percent that of Earth’s. What little atmosphere it has is also unbreathable for humans and animals, composed predominantly of carbon dioxide, argon, and nitrogen, with traces of methane and water vapor. Then there are the big variations in the temperature.
The average surface temperature on Mars is -63°C (-82°F), but this ranges considerably based on location and the time of year. During a Martian summer, temperatures can reach as high as 35°C (95°F) around the equator but plummet to -135°C (°F) around the poles during winter. But even when temperatures are at their warmest, the very thin atmosphere ensures that very little of the heat is retained.
So from a terraforming standpoint, transforming Mars comes down to three goals:
1. Thicken the atmosphere
2. Warm up the planet
3. Convert the atmosphere to make it breathable
There are other necessities, such as the need to import water to create a water cycle large enough to maintain oceans, lakes, and streams, balancing the salinity of its soil, and pairing it with a satellite to help maintain its axial tilt. However, getting the ball rolling comes down to these three major steps.
Luckily, they are complementary. By thickening the atmosphere, less radiation will reach the surface. A thicker atmosphere will also absorb more solar radiation, increasing the temperature. Warmer temperatures will release the water and dry ice locked away in the poles, creating a greenhouse effect that will further warm the planet.
Step one would be to trigger a greenhouse effect on Mars, which could be done by introducing ammonia, methane, or chlorofluorocarbons (CFCs) into the Martian atmosphere. Aside from being super-greenhouse gases, their introduction would thicken the atmosphere and raise the planet’s temperature.
Once Mars has a warmer, thicker atmosphere, liquid water would be able to flow across the surface again. This would also lead to clouds, precipitation, and the creation of a water cycle. It might be necessary to import hydrogen and/or water at this point to ensure there’s an adequate amount. To complete the process, photosynthetic organisms, plants, and vegetation would need to be introduced to the surface.
These would stabilize the sands of Mars, allow for water to be absorbed into the ground, and feed the lichens, grass, and plants. These plants would slowly convert the CO2 in the atmosphere into oxygen gas and allow for the introduction of organic nutrients (largely in the form of carbon compounds from decayed plant matter). Once oxygen levels were high enough, insects and animals could also be introduced to create a self-sustaining life cycle.
An artificial magnetosphere would also ensure long-term habitability and reduce the amount of radiation reaching the surface. According to a NASA proposal, this shield would be positioned at the Sun-Mars L1 point, where the gravitational pull of the Sun and Mars allows it to remain in a stable position directly between them.
The magnetic shield would intercept radiation from the Sun and create a bow shock that Mars would fit comfortably inside. Because of this, a thickened atmosphere on Mars would remain dense, warm, and breathable over time, and not be stripped away by solar wind.
Terraforming Venus, meanwhile, requires a somewhat opposite approach. Whereas Mars is too cold and its atmosphere is too thin, Venus is too hot and its atmosphere could crush a person whole! Hence, for Earth’s “Sister Planet,” the necessary steps are to:
1. Thin the atmosphere
2. Lower the temperature
3. Convert the atmosphere to something breathable
4. Speed up the planet’s rotation
Once again, these steps are complementary. Since Venus’s heat is due to its incredibly dense (and predominantly CO2) atmosphere, thinning and converting it go hand in hand. As previously mentioned, one method entails introducing hydrogen into the atmosphere to trigger massive rains that would cover the surface in oceans. The remaining atmosphere would be an estimated three bars (three times that of Earth) and mainly composed of nitrogen—much like Earth’s.
Introducing calcium and magnesium into the atmosphere could also sequester carbon in the form of calcium and magnesium carbonates. This would have the effect of reducing atmospheric pressure and atmospheric temperatures considerably.
The concept of solar shades has also been explored, which could take the form of a large lens positioned at the Sun-Venus L1 point. This would reduce solar heating and protect the atmosphere against solar wind. If temperatures were reduced enough, the atmospheric CO2 would freeze to become dry ice that would fall to the surface.
Last, there’s the possibility that Venus’s rotational speed could be altered to ensure long-term changes in its climate. Presently, Venus rotates once every 243 days, the slowest rotational period of any major planet. This results in Venus having incredibly long days and nights, equivalent to 116.75 Earth days!
This is also the reason why Venus is consistently hot across the entire planet, day or night, and could account for the fact that Venus lacks a significant magnetic field. It has been argued that the planet’s rotational velocity could be sped up by either striking the surface with impactors at a precise angle or conducting close flybys using large asteroids.
Another idea is to use mass drivers and compression struts (aka dynamic compression members) to transform kinetic energy into the rotational force necessary to increase Venus’s rotational period to twenty-four hours. Impactors and mass drivers would have the added benefit of blowing some of Venus’s atmosphere into space.
As you can see, there are many possible methods, and they are complementary to one another.
“Ocean Worlds” and Icy Moons
Beyond the inner solar system, many icy moons and bodies could be transformed with the right kind of engineering. These bodies present more challenges than their rocky counterparts since they are largely composed of water by volume. This is true of bodies like Ceres in the Main Asteroid Belt, the largest moons of Jupiter, Saturn, Uranus, and Neptune, and Pluto and Charon.
In addition to thick icy crusts, most of these bodies may have oceans of salty liquid water in their interior floating above the rocky core. Some might even harbor life, but life far different from anything we’ve ever encountered! For these bodies, conventional terraforming is impractical because of the abundance of water and ice.
If these moons were enclosed with a shell and warmed up, the result would be oceans hundreds of kilometers deep. Removing this water/ice to get to the rocky core would be a massive undertaking and would leave only a tiny ball of rock and metal. Such worlds are probably better off with numerous surface and space settlements, rather than full terraforming.
First up, Ceres is the largest body in the Main Asteroid Belt and the only one to have achieved hydrostatic equilibrium (i.e., become spherical). The mean radius of this planetoid is 473 km (294 mi) and its mass is estimated at 9.39 × 1020 kg (103.5 quadrillion tons). This is roughly a third of the Asteroid Belt’s mass—between 2.8 × 1021 and 3.2 × 1021 kg (3,086.5 to 3,527.4 quadrillion tons)—which is equal to four percent of the mass of the Moon.
Based on its low density (2.16 g/cm3), Ceres is believed to be differentiated between a rocky core, a possible interior ocean, and an icy mantle. The mantle is estimated to be 100 km (62 mi) thick and contains up to 200 million km3 (48 million mi3) of water, equivalent to about ten percent of what is in Earth’s oceans. The surface may also contain iron-rich clay minerals, carbonate minerals, and various asteroids it has collected over the past several billion years.
The surface of Ceres experiences a maximum temperature estimate of 235 K (-38°C; -36°F) when exposed to direct sunlight. Assuming there’s a sufficient amount of antifreeze (like ammonia), the water ice would become unstable at this temperature. Therefore, Ceres may have a tenuous atmosphere caused by outgassing on the surface. Possible mechanisms include sublimation from exposure to the Sun or cryovolcanic eruptions resulting from internal heat and pressurization.
However, to ensure that the surface doesn’t melt indefinitely, engineers would need to create a barrier between the ice and the dome’s atmosphere. Crushed regolith from the local asteroids could be used to do this, either as 3-D printed concrete and ceramics or as a deep layer of dirt. In addition to providing a barrier, this dirt could be made into soil by adding organic nutrients and water.
It would be possible to convert Ceres into a shell world, where the entire planet would be enclosed with a material shell, thus allowing the entire surface to be terraformed into a habitable, Earthlike environment. Within this shell Ceres’ temperature could be increased, UV lights could convert water vapor into oxygen gas, ammonia could be converted to nitrogen, and other elements could be added as needed. The resulting hydrogen could also be used as to manufacture propellant. A thick layer of regolith plus insulation would ensure that the crust doesn’t melt completely…
The same processes could be used on all the other “airless” bodies in the outer solar system that are mainly satellites that orbit gas giants. This includes three of Jupiter’s “Galilean” moons, Callisto, Ganymede, and Europa; Saturn’s moons Titan, Enceladus, Dione, and Mimas; Uranus’s moons Titania and Oberon; Neptune’s largest moon, Triton; and Pluto and Charon.
In all cases, a significant portion of these bodies is composed of water and other volatiles (like ammonia, methane, and nitrogen). These bodies are also part of systems that include many asteroids that could provide the necessary regolith, minerals, and carbon compounds to construct space settlements, and even conduct major terraforming efforts.
The only exception is Titan, Saturn’s largest moon and the only satellite in the solar system to have an atmosphere. It’s also the only celestial body beyond Earth to have a nitrogen-rich atmosphere and bodies of liquid on its surface (its famous methane lakes). These quirks could present opportunities for creating a livable environment on Titan, as does the rich prebiotic environment and organic chemistry on the surface.
Titan’s atmosphere is estimated to be 1.45 times as dense as Earth’s and is predominantly composed of nitrogen gas with small amounts of methane, hydrogen, and trace amounts of other hydrocarbons. There is also evidence of cryovolcanoes, a possible indication that Titan has a subsurface ocean composed of water and ammonia. Combined with the organic chemistry on the surface, some have suggested that this ocean might even support life.
All of this makes Titan virtually ideal for a shell world. Rather than melting a small portion of the surface ice, the shell would enclose the existing nitrogen atmosphere. The surface could then be covered in regolith, leaving small pockets of ice and the methane lakes to remain. The organic molecules on the surface would naturally combine with the regolith to create soil rich in prebiotic molecules.
Lighting units located on the underside of the shell would provide light identical to that on Earth and additional infrared light to heat the moon surface to any desired temperature. Over time, the methane could be broken down to create additional carbon compounds for the soil and hydrogen that could be used to manufacture propellant and fuel for power reactors, or just be released into space.
The one drawback of these icy bodies is their gravity, which ranges from a low of 0.012 g for Enceladus to a high of 0.138 g for Titan—between one and fourteen percent that of Earth. The long-term effects of these microgravity and low-gravity environments on humans specifically and life in general are unknown and will remain an ongoing issue for settlers.
Rocky Planets Under Crimson Skies
When it comes to exoplanet studies, a lot of attention is currently directed at M-type (red dwarf) star systems. As noted, red dwarfs are the most common type of star in the Universe and appear to be the most likely place to find rocky planets orbiting within their HZs. Within forty light-years of Earth, no less than seventeen candidates have been confirmed in twelve red dwarf star systems.
These include our closest stellar neighbor, Proxima Centauri. Located just 4.24 light-years away, this red dwarf star also hosts our closest exoplanet neighbors—Proxima b, c, and even a d (a possible Mars-sized planet). Proxima b is of particular interest to exoplanet scientists and astrobiologists since it is a rocky planet that is comparable in size and mass to Earth. It also orbits within the star’s HZ.
There’s also Bernard’s Star b, a super-Earth orbiting a red dwarf star roughly 6 light-years away. While extremely frigid, with surface temperatures at about -170°C (-274°F), recent research suggests that life could exist beneath the surface, where heat generated by geothermal processes could sustain pockets of liquid water.
This brings us to Ross 248d, a fictional Earth-sized exoplanet located within the inner habitable zone of its parent star—a red dwarf located 10.3 light-years from our solar system that appears in this anthology. This planet is a water world and considered a good candidate for potential habitability based on its orbit and official estimates of its surface temperatures.
Arguably, the most intriguing red dwarf star system is Trappist-1, an ultracool red dwarf located about 39 light-years from Earth. Between 2016 and 2017, seven rocky exoplanets were confirmed in this one system—three or four of which orbit within the star’s HZ. These include Trappist-1d, e, f, g, and maybe h. Based on various climate models, these planets are believed to have retained their atmospheres and water on their surfaces.
However, there are multiple indications that life would have a hard time arising on these planets, owing to the nature of their suns.
Flare-ups and Tidal Locking
For starters, M-type stars are known for being variable and prone to flare-ups. This is based on data obtained by the NASA Galaxy Evolution Explorer (GALEX) mission, which monitored stars for signs of solar flare activity between 2003 and 2013. This data, now part of the GALEX Photon Data Archive (gPhoton), indicates that even calmer and older red dwarf stars are subject to flare-ups.
While these flares are lower in intensity than other stars, they are a lot more frequent. They are also particularly bright in the ultraviolet (UV) wavelength. Unless the planet(s) orbiting a red dwarf star have a protective ozone layer, magnetic field, and sufficiently dense atmosphere, this radiation could be very hazardous to life-forms on the surface.
This is similar to what astronomers have observed with Mars, which had a thicker atmosphere and flowing water on its surface billions of years ago. Based on data obtained by missions like the Mars Express, Mars Global Surveyor, and Mars Atmosphere and Volatile EvolutioN (MAVEN), this changed about four billion years ago when Mars lost its global magnetic field.
As a result, Mars began to lose its atmosphere to space over the next five hundred million years as solar wind slowly stripped it away. This caused Mars to undergo a transition where most of its surface water was lost, temperatures dropped considerably, and the surface became the freezing and desiccated environment we see there today.
What’s more, red dwarf stars are 2000 K cooler than Sun-like stars on average, in addition to being far less massive. As a result, their habitable zones (HZs) are narrower and much closer. This means that rocky planets that orbit within their HZs are likely to be tidally locked or have an orbital resonance of 3:2 (like Mercury), 5:3, 7:4, and so on.
Therefore, rocky planets that are tidally locked with red dwarfs will be exposed to bursts of radiation that will consistently hit the planets’ atmospheres on the same side. Over time, unless the planets have particularly dense atmospheres and a planetwide magnetic field, this is likely to reduce or even blow off the planet’s atmosphere entirely.
In fact, multiple superflare events have been observed in recent years that were powerful enough to obliterate the atmospheres of any planets that orbit them. These events typically consist of a flood of very short-wavelength radiation, including X-rays and even gamma rays, followed by a coronal mass ejection (CME) of slower charged particles.
The intensity of the flares also meant that any life on the surface would be heavily irradiated. In short, a single superflare would render the dayside of potentially habitable planets completely sterile.
Luckily, the Transiting Exoplanet Survey Satellite (TESS) recently observed many superflares that suggest red dwarfs release their largest events from the poles. While this does not mean that orbiting planets are spared from all flare activity, the possibility that superflares only emerge from above 60° latitude means they would be spared from the worst, but still subject to regular flare activity.
As noted, rocky planets need to orbit very closely to their red dwarf suns to get enough heat for liquid water to exist on their surfaces (i.e., to be considered “potentially habitable”). This inevitably results in tidal-locking or a 3:2 orbital resonance, which means that one side of the planet is exposed to constant sunlight while the other is not, or that each side of the planet experiences extremely long periods of day and night.
So while the atmosphere would be warm on one side of the planet, they would be rather freezing on the other. But unlike Mercury and the Moon, rocky planets that orbit red dwarfs could redistribute this heat from one side of the planet to the other, provided they had a dense enough atmosphere and/or liquid oceans.
Too Much or Too Little?
Another issue with red dwarfs is the nature of the light they emit, otherwise known as their stellar flux. For starters, most of the light emitted by red dwarf stars is in the red and infrared range of the spectrum. This could be a problem for planets that orbit red dwarf suns and might prevent life from emerging on them.
On Earth, photosynthesis occurs when plants absorb light in the red and blue bands and reflect it in the green band, while also glowing in the infrared wavelength. For red dwarfs, much of the light they emit falls within the infrared and red parts of the spectrum. This essentially means that planets orbiting red dwarf suns may not receive enough of the right kind of light to sustain photosynthesis, which could have serious consequences for habitability.
On Earth, the emergence of photosynthetic organisms was a major step in the evolution of our atmosphere and life as we know it. Roughly 3.8 billion years ago, Earth’s atmosphere was composed primarily of nitrogen and CO2. This was the product of volcanic outgassing combined with volatiles deposited by comets during the Late Heavy Bombardment.
The first photosynthetic organisms are believed to have emerged around 3.5 billion years ago—and began converting the atmosphere by metabolizing the CO2. This led to the “Great Oxygenation Event” (ca 2.4–2.0 billion years ago), Earth’s current atmospheric composition, and the emergence of complex life-forms.
This means that planets orbiting within the HZ of a red dwarf are not getting the right kind of light for life to emerge and thrive. Conversely, there’s also research that suggests that red dwarf stars may not provide enough UV radiation for early forms of life.
According to this theory, UV radiation may have played a major role in the formation of ribonucleic acid (RNA) billions of years ago from Earth’s early prebiotic environment. Since rocky planets orbiting in red dwarf HZs receive one hundred to one thousand times less bioactive UV radiation than Earth, it is unclear if UV-sensitive prebiotic chemistry would occur.
Too Much Oxygen and Water?
Another issue is the presence of water on these planets, which astrobiologists consider to be a vital necessity for life. Based on Earth’s example, all indications show that a careful balance between ocean and continents is crucial to life, not to mention the regular exchange of energy and material between the ocean floor and surface.
But rather than there not being enough, there is also the possibility that rocky planets orbiting red dwarfs have too much water to support life. Several surveys and research studies indicate that “water worlds” may be particularly common around red dwarf stars. Once again, this could have significant implications for astrobiology studies and the habitability of red dwarf exoplanets.
A popular theory states that this is because of how red dwarf stars and their planets form. Compared to G-type stars like our Sun, red dwarf stars form slowly due to low gravity. As a result, they do not experience gravitational collapse (i.e., “ignite”) until after their planets have formed. Without light pressure to heat up and push away volatiles (such as H2O), they are simply incorporated into the planets. As the planet forms, heavy elements such as metals gravitate to the center, silicates on top of that, and lighter elements such as H2O form a layer on the surface. Hydrogen and methane and other gases form an atmosphere on top of this and the lighter elements such as hydrogen and helium are soon lost to space.
In 2016, the Pale Red Dot Campaign—a team of astronomers dedicated to finding rocky planets around red dwarfs—created a series of internal structure models that showed how super-Earths are likely to have many times the water of Earth. That same year, researchers from the University of Bern created a series of planet-formation models for red dwarf stars and concluded that in ninety percent of cases, water would account for more than ten percent of these planets’ mass—whereas Earth is just 0.05% water by mass.
In 2018, a study led by Arizona State University’s School of Earth and Space Exploration (SESE) calculated the mass distribution of the Trappist-1 planets. According to their mass-to-composition models, the two innermost planets of this system (b and c) are “drier” (fifteen percent water by mass), while the outermost (f and g) are more than fifty percent water by mass.
Another 2018 study examined data from Kepler and the European Space Agency’s Gaia mission to determine how common “water worlds” really are. They found that super-Earths 2.5 times as large, and up to 10 times as massive as Earth, are likely to be up to fifty percent water by mass. This essentially means that about thirty-five percent of all known exoplanets have too much water to support life.
Depending on where the exoplanet orbits relative to its star, this could result in all manner of scenarios. For water worlds located closer to their star than the inner boundary of the HZ, the world is likely to be shrouded in an atmosphere of hot water vapor (aka a “steam planet”). For those located beyond the HZ, they will probably be an ice planet, especially on their “night side” (if they are tidally locked).
If the exoplanet were large enough, it would be differentiated between an icy surface, an interior ocean several kilometers deep, and a layer of high-pressure ice (hundreds of km in depth) surrounding a rocky core. This ice layer would prevent the exchange of energy through geothermal activity at the core-mantle boundary, which is believed to have been essential to the emergence of life on Earth (and possibly in “ocean worlds” like Europa, Enceladus, etc.)
Research has also shown that planets orbiting red dwarfs may be “toxic” with oxygen gas. As noted, red dwarfs have extended pre-main-sequence phases that could last up to one billion years. During this period, rocky planets that orbit within what will eventually become the star’s HZ would be exposed to significant radiation.
For those that have abundant surface water, this will result in a runaway greenhouse effect that would last for several hundred million years. During this time, the planet would become a “steam world” and lose much of its water to photolysis, leaving behind an atmosphere rich in abiotic oxygen.
This is consistent with observations made of red dwarf planets that showed hydrogen escaping into space. While oxygen gas is considered an important biomarker, that only applies when and where photosynthesis is involved. On Earth, the appearance of an oxygen atmosphere was the result of photosynthetic organisms that evolved in the ocean and metabolized CO2 to create oxygen gas as a waste product.
During the Great Oxygenation Event, the emergence of an oxygen-rich biosphere likely triggered a mass extinction among anaerobic species on Earth. As a result, a planet that has plenty of atmospheric oxygen early in its history would not support the emergence of early single-celled microbes. This raises questions about evolutionary pathways and whether or not life as we know it would be possible on red dwarf orbiting exoplanets.
Terraforming to the Rescue?
There is some good news in all of this. For starters, the challenges of knowing how these systems are lacking mean that we can tailor our terraforming strategies to make them more liveable. For example, red dwarf planets with oxygen-rich atmospheres and lots of water on their surfaces may be “transiently habitable”—uninhabited but not uninhabitable.
In fact, these exoplanets could accommodate terrestrial organisms that were imported (i.e., the ecopoiesis process). One proposal, known as the “Genesis Project,” is to introduce basic organisms that could help build a life cycle on the planet and shave a few billion years off the evolutionary process.
For the sake of terraforming, this concept could be taken several steps further by introducing terrestrial flora and fauna. Alongside microorganisms, planets, shrubs, trees, and animals would be imported to fill every niche. These would help stabilize the planet’s atmosphere and climate and transform them into lush and life-sustaining worlds.
The Right Photons
Another idea for making rocky planets around red dwarfs more habitable is to alter the light they receive from their parent star. In much the same way as a magnetic shield could protect Mars, planets in the HZ of red dwarf stars could benefit from a large solar shield stationed at the star-planet L1 point. Autonomous robots could assemble this massive installation using locally sourced silica and minerals from asteroids.
During normal periods, the filter would absorb the light of the red dwarf sun and shorten its wavelength to produce light that corresponds to photosynthesis. During flare-ups, the shield’s large magnetic field could protect the orbiting planets from CMEs, which have the potential to strip a planet’s atmosphere away.
Another option is to alter the planets’ rotation to create a daily cycle similar to Earth’s, as with the terraforming of Venus. A possible method would be to capture comets and asteroids and redirect them to impact the surface. Another would be to station mass drivers on the surface to speed up rotation through kinetic energy.
This would allow plants and animals that are introduced to adapt more readily to the new environment and would have the added benefit of stabilizing temperatures planetwide. If altering a planet’s rotation is impossible, it could be feasible to create a solar mirror/solar shade in orbit that would circle the planet, shading the “dayside” to simulate night and reflecting light onto the “dark side” to simulate day.
Another possibility is to have an artificial “sun” with a twenty-four-hour orbital period that draws power from the magnetic shield positioned at the L1 point. If the shield is equipped with photovoltaic arrays, it will be able to convert ultraviolet radiation from the sun into abundant energy. This energy could then power a luminous satellite that would orbit the planet to provide artificial sunlight.
Living on Steam, Water, and Ice Worlds
Living on the surface of steam worlds will likely be similar to trying to live on the surface of Venus: dark and very hot. On these planets, the construction of habitats will likely need to be the same: floating airships that hover above the dense clouds where pressure and temperature conditions are far more hospitable. These airships could harvest valuable elements from the atmosphere (such as He-3) and would be deep enough into the atmosphere to be protected from solar flares. Such stations could be enlarged over time and could house fairly large populations.
On water worlds, floating cities would make an almost ideal habitat for humans, especially if the atmosphere could be made breathable. Once again, this is similar to floating cities in Venus’s atmosphere. And in the long run, these floating cities could be used as platforms for ecological engineering and the introduction of Earth-based marine life. Earth’s marine life.
Another option would be to remove water from these worlds. This would reduce the depths of the oceans and allow for interaction between the ocean and interior at the core-mantle boundary.
This could be done using solar mirrors in space to direct concentrated sunlight to evaporate the ocean’s upper layers. Another method would be to use a sunshade to freeze ice on the surface, which could then be lifted to space, possibly by a space elevator. If the oceans are so deep that thick layers of ice are present at the core-mantle boundary, removing water may need to be paired with operations to break the ice up—nuclear devices positioned in the ice sheet would do it!
As the ocean depth decreases and the pressure conditions alleviate, terraforming crews will have direct access to the planet’s mantle. At this point, ecological engineering could be undertaken to trigger hydrothermal activity, like geothermal tubes that would transfer heat and the necessary chemical elements to support life from the interior to the ocean depths.
However, each of these options would require tremendous amounts of time, energy, and resources. Compared to building floating cities that seed the oceans with marine life, this option could take several millennia, rather than a few centuries. On icy water worlds, the most viable solution would likely involve building settlements directly into the ice sheet, which would provide radiation protection.
This is similar to proposals for the colonization of Europa and other “icy moons” in the outer solar system. Because of the similarities involved, domed enclosures and even shell worlds could be built around icy water worlds. Inside them, ice could be harvested and exposed to light reflected by solar mirrors to create a warm and breathable atmosphere while regolith is used to create layers of soil and prevent the surface from melting.
All of this represents a huge challenge in terms of engineering, logistics, and resources. However, these steps are theoretically possible, though some will require significant technological advancements. But with the “what and how?” covered, the only remaining questions are “when and where?”
After all, we already know the answers to who and why: It will be future generations that will undertake this bold endeavor. The reason for it, beyond increasing the likelihood of human survival, will be the desire to meet a new challenge head-on. Someday, when humans have mastered the art of being multiplanetary, they will dream of taking the next great leap and becoming interstellar!
The year 2647 / 64 AA
Much has happened. Humanity has spread throughout the Ross 248 system. Small cloud facilities, modeled on the ones at Venus, float in the thick atmospheres of the two innermost planets that have been officially named Aeneas and Cupid—after the children of Venus. They mine He-3 from the atmosphere and export it throughout the Ross 248 system. Research ships sail the single ocean of the third planet, Poseidon’s World, researching its potential to be terraformed. A large solar array was constructed at the Ross/Poseidon’s World L1 Point to produce antimatter. On Liber, the moon of the seventh planet, the borough of Promise was completed and the Cerites now call it their home. The Primate Quarter, designed for normal humans and connected to the borough of Promise, comprise Toe Hold, the Cerite settlement. Well-off humans continue to live on the Copernicus in orbit above Liber while the less-well-off are consigned to the Primate Quarter.