Red dwarf exoplanets face uphill battle in supporting complex life – Part 1

M-type stars, also known as “red dwarfs,” are the smallest class of stars in the Universe. Compared to stars like our Sun, they are low mass, cooler, and dimmer. They are also the most common in the Universe, accounting for more than 75 percent of stars in the Milky Way galaxy alone.

For decades, scientists have debated whether or not these stars are capable of hosting habitable planets. In the past 20 years, the debate has intensified thanks to the explosion in the number of known extrasolar planets.

This is largely because virtually all Earth-like planets discovered within 50 light-years of Earth orbit red dwarf suns.

Moreover, many of these were found within their star’s circumsolar habitable zone (CHZ), or “Goldilocks Zone.” This refers to the orbit a planet would need to have with its star so that it would receive enough solar energy to maintain liquid water on its surface. Planets that orbit within this zone are potentially habitable, meaning there is a chance they could support life.

Unfortunately, scientists are still unsure if life could survive there for long. Some argue that red dwarf suns are too volatile and prone to flare activity for life to survive on planets that orbit them. Others maintain that rocky planets with sufficiently dense atmospheres, a magnetic field, and enough water could maintain the conditions for life to emerge and thrive.

The debate continues today and is infused with additional energy as new research emerges.

Exoplanets abound!

More than 5,700 exoplanets have been confirmed in 4,294 star systems, with more than 10,000 candidates awaiting confirmation. Of these, only 205 have been “terrestrial,” meaning they are rocky planets comparable in size to Earth. Another 1,733 have been identified as “super-Earths,” rocky planets several times the size and mass of Earth.

Within 50 light-years of Earth, 30 rocky planets have been confirmed in 30 star systems, 29 of which are red dwarf stars. Each of the planets identified orbit within the HZ of their parent star, making them “potentially habitable,” and twenty fall into the “Earth-like” category. Some notable examples include (in order of distance):

• Proxima b: Located 4.25 light years from Earth, Proxima b is the closest exoplanet to our Solar System. It is comparable in size and mass to Earth and has an equilibrium temperature of about -49 °F (-43 °C), which is close to Earth’s (-0.67 °F; -18.15 °C).
• Ross 128 b: This rocky planet, located 11 light-years from Earth, is about 1.8 times the size of Earth and 1.4 times as massive. Its equilibrium temperature is also close to Earth’s, at about 44 °F (7 °C).
• Luyten b: This rocky planet orbits Luyten’s Star (12.3 light-years away) and is 1.35 times the size of Earth and 2.89 times as massive. Its equilibrium temperature is a relatively balmy 5 °F (-15.5 °C).
• Teegarden’s Star b and c: These two planets orbit Teegarden’s Star, roughly 12.5 light-years away, and are comparable in size and mass to Earth. Their equilibrium temperatures are around 15.5 and -101.5 °F (-9 and -74 °C), respectively.
• Gliese 1002 b: Located close to 16 light-years from Earth, this rocky planet is also comparable in size and mass to Earth and has an equilibrium temperature of -44 °F (-42 °C).
• Wolf 1069 b: Located just over 31 light-years from Earth, this rocky planet is comparable in size to Earth and has an equilibrium temperature of -10 °F (-23 °C).
• TRAPPIST-1 d/e/f/g: As part of the seven rocky planets that make up the TRAPPIST-1 system, 41 light-years from Earth, all four of these planets reside in the star’s HZ. Whereas d is 0.78 times the size and 0.39 times as massive as Earth, e is 0.92 times the size and 0.69 times as massive. They have equilibrium temperatures of about 5 and -46 °F (-15 and -46 °C), respectively. Meanwhile, f and g are more massive, roughly the same size and mass as Earth, but have lower equilibrium temperatures of -100 and -132 °F (-73 and -91 °C), respectively.

As you can see from this modest sampling, many potentially habitable worlds exist in our cosmic backyard. Unfortunately, ongoing studies of these and other star systems have revealed some uncomfortable discoveries. The challenges to their habitability fall into three main categories: flare activity, tidal-locking, and not enough of the “right” kinds of photons.

Flares everywhere!

Red dwarf stars are known for being far more variable and volatile than their larger, massive peers. They can lose up to 40 percent of their luminosity for months due to starspots forming across their surfaces.

At other times, they can double their brightness in minutes by emitting giant flares (aka. “superflares”). Based on data obtained by the NASA Galaxy Evolution Explorer (GALEX) mission, even calmer and older red dwarf stars produce lots of flares.

While these flares are lower in intensity than other stars, they are much more frequent and particularly bright in the ultraviolet (UV) wavelength. 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 will likely reduce or even blow off the planet’s atmosphere entirely.

More importantly, a planet orbiting a red dwarf star must have a magnetic field to protect it. On Earth, the presence of a planetary magnetic field intercepts incoming charged particles (solar wind and cosmic rays) and deflects the majority of them.

Without this field, Earth’s atmosphere would have been slowly eroded by solar wind. This is precisely what happened to Mars, beginning ca. 4 billion years ago, eventually turning it into the extremely cold, desiccated, and irradiated planet we see today.

However, astronomers have observed superflare events that were 10,000 times more energetic than the average solar flare. So, while the average flare activity of a red dwarf would have a limited effect on a planet’s atmosphere, a superflare would be capable of stripping its atmosphere instantly!

A 2020 study by researchers from the University of North Carolina at Chapel Hill and the University of Barcelona assessed the impact of superflares on planetary habitability.

They found that planets orbiting within the HZs of red dwarfs may experience “life-prohibiting levels of UV radiation,” though some microorganisms might survive. This was followed by another study released in 2024 that revealed that red dwarf suns emit more radiation in the far-ultraviolet part of the spectrum than previously thought.

In 2017, researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) conducted two studies that attempted to model flare activity in the TRAPPIST-1 system.

In the first study , Professors Manasavi Lingham and Avi Loeb considered how the system’s planets would experience elevated levels of stellar wind pressure, which would strip their atmospheres and deplete their water. As Loeb told Interesting Engineering via email:

“Another challenge for life is the strength of the stellar wind at a close-in distance which can strip the atmosphere of the planet. If that happens, the water will evaporate and the planet will become a desert like Mars after losing its atmosphere. In addition, dwarf stars have strong flares in UV and X-rays that can further strip the planet’s atmosphere.

“The surface temperature of dwarf stars like Proxima Centauri is half that of the Sun, peaking in the infrared. This might suppress photosynthesis as we know it which requires optical-UV photons, except during UV flares.”

In the second study , the CfA team calculated that TRAPPIST-1 bombards its system of planets with stellar winds 1,000 to 100,000 times more powerful than solar wind. Furthermore, they found that TRAPPIST-1’s magnetic field is likely connected to its planets’ magnetic fields, allowing stellar wind to flow directly from the star onto the planets’ atmospheres.

Perpetual day and night

Another potential issue with planets orbiting red dwarfs is the nature of their orbit. Since red dwarf stars are far less massive and about 2000 K cooler than Sun-like stars, their habitable zones (HZs) are narrower and much closer.

This means that rocky planets that orbit within their HZs are tightly bound to their parent star, so the star’s gravity affects the planet’s rotation.

As a result, the planet’s rotation becomes synchronized with its orbit, causing one side to face toward the star constantly. This phenomenon is known as “tidal locking” (aka. synchronous rotation), where one side experiences constant daylight while the other is in perpetual darkness.

Alternately, the planet may achieve a 3:2 orbital resonance (like Mercury), which means that each side of the planet experiences extremely long periods of day and night.

Tidal locking applies to the Earth and its only natural satellite, the Moon. Looking up at the Moon, we see the same face every night (though in different phases).

Hence, we refer to the unseen side as “the dark side,” which is a bit of a misnomer, seeing as how it receives plenty of Sunlight. It is “dark” because it has remained a mystery to humans for most of our history.

As Loeb summarized, this orbital proximity has two major implications:

“First, the planet becomes tidally locked, having a permanent dayside and nightside. The dayside will be hot, and the nightside will be cold. When I mentioned this to my daughter, she said that if we ever visit Proxima b, she wants us to have a house on the strip that separates these two sides where we can see Proxima’s sunset forever. Life is most likely to be in that strip. But there may be strong winds between the two sides because of their different temperatures.”

Scientists speculate that a planet that receives all of its solar irradiance on one side is likely to become an “eyeball planet” – which may be “hot” or “cold.” Whereas hot eyeball planets are likely to be dry and irradiated on their sun-facing side, their dark side is likely to be covered in water. In contrast, cold eyeball planets will have liquid water on their sun-facing sides but will be covered in ice sheets on their dark side.

Unless there is adequate heat transfer between one side of the planet and the other through atmospheric and ocean circulation, the “eyeball planet” scenario is likely. This also requires a sufficiently dense atmosphere and a planetary magnetic field.

As you can see, there are doubts that “potentially habitable” rocky planets that orbit red dwarfs could support life. The list doesn’t end with flare activity or tidal locking; it includes other issues like the kinds of radiation these planets receive from their suns and their water content (they may have too much).

But there are also reasons to remain hopeful, and in the coming years, more information will be available so scientists can address the question further.

All of that will be covered in our second installment, coming soon!