Red dwarfs make up the vast majority of stars in the galaxy. Because of their ubiquity, they host the most rocky exoplanets we have found so far – which in turn makes them interesting for astrobiological studies. However, there’s a catch: Astrobiologists aren’t sure whether the light from these stars can actually support oxygen-producing life. A new paper by Giovanni Covone and Amedeo Balbi, available as a preprint on arXiv, suggests that may not be the case – when it comes to starlight, quality is just as important as quantity. And according to their calculations, maintaining Earth-like biospheres around red dwarfs is incredibly difficult.
Their argument is based on the concept of exergy – a measure of the maximum amount of useful work that can be extracted from a radiation field. In other words, it measures the thermodynamic quality of light and not just the raw energy it contains. When measuring the “habitable zone” of stars, astrobiologists typically look at the total number of photons, particularly in the visible light range between 400 and 700 nanometers wavelength.
So what “useful work” does light do on exoplanets? Perhaps the most important is breaking up water. This process, known as “water oxidation,” represents a kinetic bottleneck in the process of photosynthesis and produces the oxygen expected in biosignatures. However, biological systems require a significant amount of kinetic energy to carry out this chemical reaction. And when it comes to providing that energy, red dwarfs have two advantages.
Fraser talks about habitable planets around red dwarfs
Red dwarfs are cool and their light is strongly redshifted into the infrared. Their photons do not concentrate enough energy to reach the threshold required for water splitting. But even those that do have a smaller percentage of their energy that can actually be converted into useful chemical work. This double combination enormously reduces the potential for the formation of oxygen-containing life around red dwarfs. In comparison, the exergy available to drive water oxidation around Sun-like stars is about five times higher.
However, astrobiologists are an optimistic bunch, so their immediate answer to this concern would be: perhaps life around these stars has evolved to adapt to these higher infrared environments. Could they use longer, lower-energy infrared wavelengths under a red dwarf sky? The short answer is “no” because it is the so-called red border. This is the longest wavelength of light that can support photosynthesis. The authors argue that this is not a fixed value, but rather an emergent property determined by a star’s spectrum, the planet’s atmosphere and a targeted chemical reaction – in this case water oxidation.
They estimate that the red limit for red dwarfs is 0.95 µm, while for sun-like stars it is closer to 1.0 µm. In practice, this means that life cannot simply shift its primary absorption bands deeper into the near-infrared to accommodate its less powerful star. Another concern concerns the development of life on one of these planets. Oxygen-deficient bacteria can use infrared light effectively. If they could multiply, they could outcompete oxygen-containing bacteria and the world would never experience a “major oxidation event” equivalent to what happened on Earth. Without abundant oxygen in the atmosphere, multicellular life would be severely limited, if not eliminated entirely.
Fraser has some videos on this topic that show there is an ongoing debate.
When all of this is taken into account, the possibility of life near red dwarfs paints a bleak picture. But let’s not rule it out completely. Currently, Earth’s biosphere is consuming only about three orders of magnitude below maximum thermodynamics – evidence that life itself is extremely inefficient. But even then, the conditions around red dwarfs that would be favorable for life are likely extremely rare. This article proves that our time searching for an oxygen-rich alien forest would be better spent near stars like our sun than chasing the statistical rarity of a thriving biosphere around a red dwarf.
Learn more:
G. Covone & A. Blabi – Photosynthetic exergy I. Thermodynamic limits for planets in the habitable zone
UT – Red dwarfs are too weak to generate complex life
UT – Planets in the habitable zone around red dwarfs are unlikely to host exomoons
UT – New research suggests advanced civilizations are unlikely to exist in red dwarf systems
