Red dwarfs make up about 75 percent of the stars in our galaxy. These are small (with a diameter less than six-tenths that of the Sun) and cool stars, but they can host planetary systems, and planetary systems have already been discovered around many of them.
However, in terms of the existence of Earth-like worlds, these are not the most suitable candidates. Most planets located in the habitable zone of these stars are tidally locked. This means that, as they orbit their parent star, they always present the same side of their surface to that star. The Moon, for example, orbits the Earth in a similar manner.
For planets with atmospheres, this can cause constant, turbulent airflow as temperature differences are balanced between the perpetually sunlit, heated side of the planet and the far side, which remains cool.
Due to their tidally locked rotation, these planets may also fail to develop a sufficiently strong magnetic field to protect any potential life from the star’s harmful radiation.
From the perspective of human habitability, therefore, these planets—even if they orbit a star within the habitable zone and meet other conditions—may not be suitable candidates for potential colonization.
Advantages of Red Dwarf Planetary Systems
On the other hand, planetary systems around red dwarfs also have advantages that could be leveraged as humanity continues to expand into space.
If we look at some of the already known planetary systems around stars of this type, we can see that the planets’ orbits are very close together.
For example, three planets (e, b, and c) have now been confirmed around the star Gliese 581, at distances of approximately 4.2, 6.1, and 10.8 million kilometers from the star. The distances between the orbits of these confirmed planets are therefore minimal compared to the distances between the planets in our Solar System (58 million kilometers from the Sun for Mercury, 108 million kilometers for Venus, and 150 million kilometers for Earth).
As a result, these planets are more easily accessible to one another, and travel between them takes a minimum amount of time.
The smaller size of these systems may also mean that remnants from the period when the entire system formed—such as comet nuclei (analogous to the Kuiper Belt or the Oort Cloud in our Solar System)—are located closer by, making them suitable for potential utilization.
Dead Worlds
From the perspective of human expansion, planets whose characteristics differ from those of Earth may be of interest. More specifically, planets more similar to our Moon or Mercury—dead worlds. These planets offer many advantages for extended stops, restocking supplies, or establishing smaller colonies (and perhaps even permanent settlements)—or even building massive generation ships.
Because one side of these planets’ surfaces is constantly exposed to stellar radiation that is not filtered by an atmosphere or magnetic field, the sunlit side may contain enormous amounts of the helium isotope He-3, which is produced there in the same way as on our Moon (where it reaches the surface through billions of years of bombardment by charged particles from the solar wind). In the future, this isotope is expected to serve as fuel for energy production through thermonuclear fusion and will most likely also be used to power spacecraft.
Another advantage of the perpetually illuminated side is, for example, the use of the star’s own radiation to generate energy — thanks to the perpetual day on the sunlit side, there will be no fluctuations in electricity generation (such as those that can occur, for example, when using photovoltaics on the Moon’s surface—the Luna Ring project, for instance, plans to compensate for this problem by placing photovoltaic panels around the entire circumference of the Moon)—and the radiation is not attenuated by an atmosphere.
The far side, hidden in perpetual shadow, may in turn offer abundant water reserves in the form of ice. This ice could have reached this part of the planet either during the formation of the planetary system or during later comet impacts.
On planets like this, the absence of an atmosphere—which would otherwise transfer heat from the sunlit side and could cause the gradual “destruction” of accumulated ice reserves—is a huge advantage. Meteorite impacts could pose a similar problem (since there is no atmosphere to slow or dissipate them, essentially everything strikes the surface), during which the accumulated ice could be gradually removed. But how significant would these losses be in this case?
The Moon and Mercury, for example, have similar ice deposits of this kind hidden in perpetual shadow. However, these reserves are found only in a few locations near their poles. On planets with a locked rotation, these reserves can be incomparably larger due to the size of the surface area (multiplied, if we consider planets close in size to Earth) that is hidden in perpetual shadow.
The problem with the atmosphere will increase as the planets’ mass increases, since greater gravity allows lighter, “more volatile” molecules to be retained near the surface. Therefore, the most likely candidates for “dead worlds”—those with gravitational conditions suitable for long-term human habitation—will be located much closer to the star. The following principle should apply here: the closer a planet is to the star, the more certain it is that it will lack any form of atmosphere.
However, they must not be too close to the star—on the one hand, the star’s intense radiation could complicate movement around the planet, and on the other hand, in the most extreme cases, heat could be transferred to the far side, for example, via lava flows or through an atmosphere formed by vaporized material from the near side.
Colonization
From the perspective of establishing long-term colonies, we are interested in planets where the temperature on the day side does not rise to extreme levels that could interfere with, for example, the extraction of He-3 and other raw materials, or the generation of electricity. This narrows the range in which to search for such planets around red dwarfs, but on the other hand, these limits are not as restrictive as those imposed by the habitable zone.
The outer parts of the system would also be reachable more quickly from planets orbiting closer to the star. The following illustration probably probably best illustrates this point:
As we can see, the asteroids in the asteroid belt can be reached more quickly from Venus than from Earth, thanks to Venus’s proximity to the Sun (and thus the shorter distance required). The same, of course, applies to planets orbiting other stars that are located closer to their star. However, a shorter journey does not mean a cheaper one: a planet closer to its star lies deeper in its gravitational well, so sending a probe on a trajectory to the outer part of the system requires a greater change in velocity (delta-v), and thus more energy or fuel. This is especially true for planets in tight orbits around red dwarfs.
Furthermore, these planets have relatively simple, more or less familiar environments, where experience gained from, for example, the colonization of the Moon can be put to use. In contrast, planets more similar to Earth pose a risk of greater environmental variability and potential changes, creating more obstacles during their colonization. Another risk associated with these “more habitable” planets is the possibility of extraterrestrial life, which could be threatened by the presence of Earth-based organisms—or vice versa.
These dead planets should also orbit red dwarfs that are older than approximately one billion years, since the first billion years of a red dwarf’s life tend to be quite turbulent. Frequent flares occurring on these stars during this period could cause major problems. However, there is no clear-cut boundary: flares occur on red dwarfs even billions of years after their formation, and the appropriate age therefore depends on the specific star.
As far as age is concerned, another major advantage of these stars is their estimated lifespan, which ranges from several tens of billions to trillions of years. This longevity—incomparably longer than the lifespan of stars similar to our Sun—could be of interest from the long-term perspective of humanity’s survival.
Suitable dead worlds could have a mass slightly smaller than Earth’s, and thus lower gravity, making it easier to work on the surface or to leave it (a slightly lower escape velocity; moreover, there is no need to deal with the aerodynamics of spacecraft due to the absence of an atmosphere—which also means no waste of fuel that would otherwise be needed to pass through the atmosphere). Of course, not by much, so as not to cause health problems during a longer stay.
Colonies
Colonies on these planets could be located on the far side, where the temperature—though significantly lower—is constant (there are not such large temperature fluctuations there as, for example, on Mercury), and where these colonies would be at least partially protected by the planet’s mass from any stellar eruptions.
There won’t be much of interest on the surfaces of these planets, and the colonists will also need to be protected from cosmic radiation, which is why most parts of the colonies will be hidden underground. However, the perpetual sunlight on the sunlit side could be used to power a substitute for Earth’s magnetic field. This could also be used to help colonists navigate around the bases, which would be protected in this way.
Thanks to their barren, moon-like surfaces—covered only by craters—no form of environmental protection will hinder the maximum utilization of local resources. In this regard, the surface of these planets can be considered a clean, blank canvas that can be used to build truly large colonies (which could accommodate even billions of people) and to modify the surface for any purpose.
Space Elevators
Space elevators could be of immense benefit to the colonies; their construction would not be hindered by any atmosphere, and the slightly lower gravity mentioned earlier could also be helpful here. These elevators would significantly simplify access to outer space and facilitate the further expansion of human colonies deeper into space.
In this regard as well, a tidally-locked rotation is advantageous, as it would ensure that these elevators would “protrude” from the surface—as seen from the star’s perspective—always from the same location.
The counterweight for a space elevator, located farther from the surface, could consist, for example, of mirrors that direct light onto colonial structures hidden in perpetual shadow, thereby simulating the cycle of day and night.
Photovoltaic panels (or another system capable of converting starlight into electrical energy) could also be installed here, supplying at least some of the energy to devices on the surface.
This solution could be advantageous for planets orbiting a star even closer, where photovoltaic systems would not have to be built on the scorching, sun-facing side (though they would still be exposed to intense stellar radiation in the space environment). Energy would not need to be transmitted to the surface via lossy methods such as lasers or microwaves (as with space-based solar power plants proposed for Earth’s vicinity), but could instead be transmitted using superconductors, for example.
Power could even be supplied here by a network of “elevators,” whose sole purpose would be to carry the appropriate number of photovoltaic panels and transport energy to the surface. As a result, cables would sprout from the planet’s surface like flowers in suitable locations, scattered around the planet’s terminator and carrying outstretched photovoltaic panels pointed toward the star.
Suitable, Already Discovered Planets
Described below are two confirmed planetary systems that could be suitable candidates for colonizable dead worlds. Both were discovered as part of the Kepler mission.
- Constellation: Lyra
- Distance: 215 light-years
- Age: 6 billion years
It is not a red dwarf, but a star more similar to the Sun, with a mass of eight-tenths that of the Sun, 0.77 times its diameter, and a surface temperature of about 5,417 Kelvin (by comparison, the Sun’s surface temperature is 5,780 Kelvin).
Planet b, located closest to the star, is the smallest planet in the system, with a diameter close to that of our Moon and an equilibrium temperature estimated to be around 430 degrees Celsius. For us, this planet may, on the other hand, be too hot and too small.
An interesting potential dead world here is the planet located in the middle—planet c. This planet is reasonably large (with a diameter of approximately 9,500 km—or 0.742 times the diameter of Earth), and its equilibrium temperature is expected to be around 280 degrees Celsius. It is located about 20.5 million kilometers from the star, and one orbit takes approximately 21.3 days.
When viewed from the surface of this planet, the star would have an angular diameter approximately six times larger than that of the Sun in our sky. At its closest approach, Planet b would appear as a very bright object with an angular diameter of about one-twelfth that of the Moon.
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| A Comparison of the Sizes of the Planets in the Kepler-37 System with the Sizes of Objects in the Solar System (Credit: NASA/Ames/JPL-Caltech) |
- Constellation: Cygnus
- Distance: 126 light-years
Three planets are known to orbit this red dwarf (with a mass of 13 percent that of the Sun and a diameter of 17 percent of the Sun’s diameter), with diameters ranging between those of Mars and Venus. The planet located farthest from the star might be of particular interest to us. It has a mass of 0.9 times that of Earth, a diameter of about 7,600 kilometers, and its equilibrium temperature is estimated to be approximately 180 degrees Celsius. It orbits the star once in just under two days.
In the sky of this planet, the host star would appear nearly twelve times the angular diameter of our Sun. The planet closest to the star would be slightly smaller than the Moon in our sky at its closest approach (0.86 times its angular diameter), while the second planet from the star would be 2.3 times larger than our Moon (both planets would be visible mainly from this planet’s star-facing side).
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In this image, we can see a comparison of the size of the entire Kepler-42 planetary system with the size of the system of Jupiter's largest moons (Credit: NASA/JPL-Caltech).
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