Astronomers have discovered thousands of planets outside our solar system, known as exoplanets. Many of these planets orbit small stars called red dwarfs. Because red dwarfs are the most common stars in the Milky Way galaxy, scientists once believed they might host a large number of habitable worlds.
However, new research suggests the situation may not be so simple. A recent study by scientists Giovanni Covone and Amedeo Balbi indicates that planets orbiting red dwarf stars may struggle to support complex life. The reason is not just how much light these stars produce, but the quality of that light.
According to their study, the type of light emitted by red dwarfs might not provide enough usable energy for plants or similar organisms to perform photosynthesis—the process that produces oxygen. Without oxygen, the evolution of complex life becomes far less likely.
Why Red Dwarfs Are So Important in Astrobiology
Red dwarf stars are small, cool stars that make up nearly 75 percent of the stars in our galaxy. Because they are so common, astronomers often focus on them when searching for potentially habitable planets.
These stars also have long lifespans—sometimes trillions of years—which gives life plenty of time to develop if the conditions are right. Additionally, many rocky planets have been discovered orbiting within the so-called habitable zone of red dwarfs. The habitable zone is the region around a star where temperatures could allow liquid water to exist on a planet’s surface.
Since water is essential for life as we know it, these discoveries initially made red dwarfs look like excellent targets in the search for extraterrestrial life.
But recent research suggests that there may be a major limitation: the energy carried by their light.
Quantity vs. Quality of Light
When scientists determine whether a planet is habitable, they usually measure how much energy it receives from its star. They focus on the number of photons—particles of light—reaching the planet, especially in the visible light range between 400 and 700 nanometers.
However, Covone and Balbi argue that not all light is equally useful for life.
Their study focuses on a concept called exergy. Exergy measures the maximum amount of useful work that can be extracted from energy. In simple terms, it describes how effective a source of energy is for driving important processes.
For living organisms, especially plants or photosynthetic microbes, light must provide enough energy to drive chemical reactions. If the light lacks sufficient quality, biological processes like photosynthesis may not work efficiently—even if plenty of light is present.
The Role of Photosynthesis in Producing Oxygen
Photosynthesis is one of the most important biological processes on Earth. Plants, algae, and certain bacteria use sunlight to convert carbon dioxide and water into energy-rich molecules. A by-product of this process is oxygen.
A crucial step in photosynthesis is water oxidation—splitting water molecules into hydrogen and oxygen. This reaction requires a significant amount of energy and acts as a bottleneck in the overall process.
Without enough energy to split water, oxygen cannot accumulate in a planet’s atmosphere. And without oxygen, the development of complex organisms like animals becomes extremely difficult.
Therefore, the ability of starlight to drive water oxidation is essential for creating oxygen-rich environments.
The Problem with Red Dwarf Light
Red dwarf stars are much cooler than stars like our Sun. Because of their lower temperatures, the light they emit is shifted toward longer wavelengths, mainly in the infrared region.
This shift creates two major problems for oxygen-producing life.
1. Lower-Energy Photons
Infrared photons carry less energy than visible light photons. Many of them simply do not contain enough energy to drive the chemical reactions needed for water splitting.
2. Reduced Thermodynamic Efficiency
Even the photons that do have enough energy are less efficient at producing useful chemical work. According to the researchers, the exergy available for water oxidation around Sun-like stars is roughly five times higher than around red dwarfs.
This combination significantly reduces the ability of photosynthesis to produce oxygen on planets orbiting red dwarf stars.
Can Life Adapt to Infrared Light?
Astrobiologists often consider whether alien life might evolve solutions to environmental challenges. If red dwarfs produce mostly infrared light, could life evolve to use that light for photosynthesis?
At first glance, this idea seems possible. Some organisms on Earth can use longer wavelengths of light than typical plants.
However, the researchers highlight a physical limitation called the “red limit.”
The red limit represents the longest wavelength of light that can still drive photosynthesis. Beyond this point, the photons simply do not carry enough energy to power the necessary chemical reactions.
Covone and Balbi argue that this limit is not fixed but depends on several factors:
The spectrum of the star
The planet’s atmosphere
The chemical reactions required for photosynthesis
For planets around red dwarfs, the estimated red limit is around 0.95 micrometers. For planets around Sun-like stars, the limit is closer to 1.0 micrometers.
In practice, this means life cannot simply shift its photosynthesis deeper into the infrared to compensate for the weaker light.
Competition from Infrared-Using Microbes
Another challenge involves the possible evolution of early microbial life.
Some bacteria on Earth perform anoxygenic photosynthesis, which does not produce oxygen. These organisms can use infrared light more efficiently than oxygen-producing photosynthetic organisms.
On a red dwarf planet, these microbes might dominate the ecosystem. If they spread widely across the planet, they could prevent oxygen-producing organisms from becoming dominant.
This could stop the planet from experiencing a major atmospheric transformation similar to Earth’s Great Oxidation Event, which occurred about 2.4 billion years ago. During that time, oxygen levels in Earth’s atmosphere increased dramatically, paving the way for complex life.
Without such an event, planets around red dwarfs might remain dominated by simple microbial ecosystems.
Life Could Still Exist—But It May Be Rare
Despite these challenges, the researchers do not completely rule out life around red dwarfs.
Life on Earth is actually very inefficient at converting energy into biological work. The planet’s biosphere uses energy levels far below the theoretical thermodynamic maximum.
This means that even weak energy sources might still support some form of life.
However, the conditions required for a thriving oxygen-producing biosphere around red dwarfs may be extremely rare. Special atmospheric compositions, unusual ecosystems, or unique biological adaptations might be necessary.
Implications for the Search for Alien Life
The findings have important implications for astrobiology and future space missions.
Because red dwarfs are common and easier to study, many exoplanet searches focus on them. Telescopes such as the James Webb Space Telescope often analyze the atmospheres of planets around these stars.
But if oxygen-producing life is unlikely around red dwarfs, scientists may need to shift some of their attention toward Sun-like stars.
Stars similar to our Sun produce light with a higher thermodynamic quality, making them better candidates for supporting photosynthesis and oxygen-rich atmospheres.
In other words, finding an alien forest filled with oxygen-producing plants may be far more likely around stars like our own.
Conclusion: Rethinking Habitable Worlds
The discovery of thousands of exoplanets has made the search for alien life more exciting than ever. Yet this new research reminds us that habitability is far more complex than simply finding a planet with water.
The quality of starlight plays a critical role in powering the chemistry of life.
Red dwarf stars may host many rocky planets, but their infrared-heavy light could make it difficult for oxygen-producing photosynthesis to evolve. Without oxygen, the development of complex ecosystems similar to those on Earth becomes much less likely.
While life may still exist on some of these worlds, flourishing oxygen-rich biospheres could be rare.
For scientists searching the galaxy for signs of life, this means one thing: the most promising targets might still be stars that resemble our own Sun.
Reference: Giovanni Covone et al, Photosynthetic exergy I. Thermodynamic limits for habitable-zone planets, arXiv (2026). DOI: 10.48550/arxiv.2602.20789
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