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

Welcome back! In the previous installment, we examined the abundance of red dwarf stars in our galaxy and the surprising number of them (within 50 light-years of Earth) with rocky planets orbiting within their circumsolar habitable zone (HZs).

However, these planets have very short orbital periods, which means they are likely to be tidally locked to their stars—one side is constantly facing toward the star while the other is facing away from it.

These pose a challenge for life as we know it. But as we shall see today, it doesn’t end there. There are also unresolved questions about the kinds of radiation these stars produce and whether or not they have too much water for life to emerge. But as we shall see, there are still reasons to remain hopeful!

Not enough of the right photons

According to current research , another issue with red dwarfs is the wavelengths of the light they emit (aka stellar flux). Most of the photons emitted by red dwarf stars are in the red and infrared (heat) wavelengths, which could impede the emergence of life on rocky planets that orbit them.

Based on the fossil record, it is known that the emergence of photosynthetic organisms (ca. 3.5 billion years ago) altered Earth’s atmosphere through the introduction of oxygen gas, which led to the evolution of more complex organisms.

On Earth, photosynthesis consists of plants and bacteria absorbing light in the red and blue parts of the spectrum and reflecting it in the green. Since most of the light emitted by red dwarfs falls within the infrared and red parts of the spectrum, planets orbiting them may not receive enough light to sustain photosynthesis, which could have serious consequences for habitability.

Conversely, while red dwarf flares produce too much UV for life to thrive, research suggests that they may not provide enough UV radiation for early life to emerge. According to a 2017 study by astronomers at the CfA , UV radiation may have played a major role in forming ribonucleic acid (RNA) on Earth billions of years ago. However, the team noted that rocky planets orbiting red dwarfs receive (on average) an estimated one hundred to one thousand times less UV radiation than Earth.

Therefore, it is still being determined if UV-sensitive prebiotic chemistry could occur on these planets.

Too much water?

Another issue is the presence of water on these planets. Using Earth as an example, research has shown that a careful balance between ocean and continents is essential to the evolution of life.

In addition to ensuring a mix of terrestrial and aquatic environments and all the coastal niches and rivers essential to biodiversity, this balance also ensures the regular exchange of energy and material between the ocean floor and surface.

However, according to recent studies, rocky planets orbiting red dwarfs may have too much water to support life.

Additional research has shown that these “water worlds” may be particularly common around red dwarf stars. This includes a 2016 Pale Red Dot Campaign study, an international collaboration dedicated to finding rocky planets around red dwarfs. In their study, the team created a series of internal structure models that showed how super-Earths are likely to have many times the water of Earth.

“Life as we know it requires the interface between land and water. Puddles that dry up tend to concentrate chemical nutrients,” said Loeb.” A recent study also suggested that rain droplets might have given rise to the earliest cells, a phenomenon that requires landmass.”

In 2016, researchers from the University of Bern modeled the formation of planets around red dwarf stars and concluded that in nine out of ten cases, water would account for more than 10 percent of these planets’ mass .

While this might not sound like much, consider that Earth is just 0.05 percent water by mass. A study in 2018 led by Arizona State’s School of Earth and Space Exploration (SESE) made similar findings regarding the TRAPPIST-1 planets, concluding that they ranged from 15 to 50 percent water by mass.

Based on the current exoplanet census , super-Earths account for about 30% (1,733) of all the exoplanets confirmed to date (5,756). Using data from NASA’s Kepler Space Telescope and the ESA’s Gaia Observatory , a team of researchers released a study in 2018 that indicated super-Earths 2.5 times as large and up to 10 times as massive as Earth are likely to be up to 50% water by mass. Almost a third of all known exoplanets could have too much water to be habitable.

Research has also shown that planets orbiting red dwarfs could have too much oxygen gas (another biosignature) early in their history. As noted, red dwarf stars are extremely long-lived, which means they also have extended pre-main sequence phases.

During this period, which can last up to 1 billion years, rocky planets orbiting in the star main-sequence HZ would be exposed to higher-than-normal radiation. If these planets have water on their surface, it will eventually be chemically dissociated to create hydrogen and oxygen gas.

While the hydrogen gas will be lost to space, the oxygen gas will be retained, leading to an atmosphere rich in oxygen but not the result of biological processes (aka abiotic oxygen).

On Earth, life emerged in an environment characterized by volcanic outgassing and a predominantly carbon dioxide atmosphere. By ca. 2.4 billion years ago, the presence of biologically produced oxygen led to the “ Great Oxidation Event ,” which is believed to have triggered a mass extinction among anaerobic species on Earth.

Based on this, rocky planets with oxygen-rich atmospheres that orbit red dwarf suns may be sterile since oxygen gas is toxic to early life forms.

Some good news!

But before you think red dwarf stars are completely inhospitable to life, there’s also some encouraging data. According to a 2021 study, the Transiting Exoplanet Survey Satellite (TESS) data demonstrated that red dwarfs tend to release their largest megaflares above 60 ° latitude (around their poles). While orbiting planets are not spared from all flare activity, they would be spared from the worst events.

“Another advantage of dwarf stars is that they live much longer than the Sun, up to ten trillion years – a thousand times longer than the lifespan of the Sun,” Loeb added. “If life was common near dwarf stars, we would be most likely to live in the future.” These arguments were quantified by Loeb and colleagues from the CfA in a 2016 paper .

In addition, a study in 2018 led by Anthony D. Del Genio of NASA’s Goddard Institute for Space Studies (GISS) showed that planets like Proxima b could be habitable despite being tidally locked. According to the 3-D climate simulations, a dayside ocean would allow for sufficient heat transfer so that Proxima b would not become an “eyeball” planet.

In addition, a NASA-supported study released in 2023 showed how flare activity around red dwarfs could assist in forming amino acids and carboxylic acids. So, while planets orbiting red dwarfs may not receive enough UV radiation on average for life to emerge, adding flew flares could provide the needed push.

The debate surrounding red dwarf habitability is far from resolved. Much of this concerns our unanswered questions about the stars themselves and the types of planets they give rise to. And this, in turn, is due to the limits of our instruments and what we can detect. Because red dwarfs are particularly cool, they are difficult to observe using conventional methods.

Similarly, the faint nature of these stars means that planets orbiting them are extremely difficult to observe directly. In the coming years, astronomers hope to study these planets using next-generation telescopes like the James Webb Space Telescope (JWST), the Nancy Grace Roman Space Telescope (RST), and ground-based observatories like the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT).

Someday, we may have the opportunity to get a close look at planets orbiting Proxima Centauri, TRAPPIST-1, and other nearby red dwarf stars—possibly using nano crafts equipped with light sails.

Until then, scientists must infer what conditions could be like in these worlds based on what we can observe from a distance. Knowing that 75 percent of the stars in our Universe could host life would certainly be encouraging. If not, then we’ll just have to keep looking!