Red dwarfs make up the vast majority of stars in the galaxy. Such ubiquity means they host the majority of rocky exoplanets we’ve found so far - which in turn makes them interesting for astrobiological surveys. However, there’s a catch - astrobiologists aren’t sure the light from these stars can actually support oxygen-producing life. A new paper, available in pre-print on arXiv, by Giovanni Covone and Amedeo Balbi, suggests that they might not - when it comes to stellar light, quality is just as important as quantity. And according to their calculations, Earth-like biospheres are incredibly difficult to sustain around red dwarfs.
Their argument is based on the concept of exergy - a measurement of the maximum amount of useful work that can be extracted from a radiation field. In other words, it measures the thermodynamic quality of the light, not just the raw energy contained in it. Typically, when measuring the “habitable zone” of stars, astrobiologists look at the total number of photons, specifically in the visible light range between 400 and 700 nanometers of wavelength.
So what “useful work” does light do on exoplanets? Perhaps the most important is breaking apart water. This process, known as “water oxidation” is a kinetic bottleneck in the process of photosynthesis, and creates the oxygen expected to be seen in biosignatures. However, to do this, biological systems require a significant amount of kinetic energy to perform this chemical reaction. And red dwarfs have two strikes against them when it comes to providing that energy.
Fraser talks about habitable planets around Red DwarfsRed dwarfs are cool, and their light is heavily red-shifted into the infrared. Not enough of their photons pack enough energy to reach the threshold needed to split water. But even the ones that do have a smaller percentage of their energy that can actually be converted into useful chemical work. This one-two combination puts a huge dent in the potential of oxygenic life forming around red dwarfs. By comparison, the exergy available to drive water oxidation around Sun-like stars is around five times higher.
Astrobiologists are an optimistic bunch, though, so their immediate response to this concern would be - maybe life evolved around those stars to adapt to these higher infrared environments. Could they use longer, lower-energy infrared wavelengths under the skies of a red dwarf? The short answer is no, due to something called the red limit. This is the longest wavelength of light capable of supporting photosynthesis. The authors argue that this isn’t a set value, but an emergent property determined by a star’s spectrum, the planet’s atmosphere, and a targeted chemical reaction - in this case the water oxidation.
They estimate that for red dwarfs the red limit is 0.95 um, whereas for Sun-like stars its closer to 1.0 um. In practice, that means life cannot simply shift their primary absorption bands deeper into the near-infrared to adapt to their less powerful star. Another concern has to do with the evolution of life on one of these planets. Anoxygenic bacteria can effectively harvest infrared light. If allowed to proliferate, they could out-compete oxygenic bacteria, and the world would never experience a “Great Oxidation Event” equivalent to what happened on Earth. Without copious amounts of oxygen in the atmosphere, multicellular life would be severely hindered, if not outright eliminated altogether.
Fraser has a few videos on this topic, showing that there’s been an ongoing debate.Taking all of this into account paints a bleak picture for the possibility of life around red dwarfs. But let’s not rule it out entirely. Currently, the Earth’s biosphere only uses about three orders of magnitude below the maximum thermodynamic - proof that life itself is wildly inefficient. But even so, the conditions surrounding red dwarfs that would be favorable for life are likely extremely rare. This paper proves that our time searching for an oxygen-rich alien forest might be better spent around stars like our Sun, rather than chasing the statistical rarity of a flourishing biosphere surrounding a red dwarf.
Learn More:
G. Covone & A. Blabi - Photosynthetic exergy I. Thermodynamic limits for habitable-zone planets
UT - Red Dwarfs Are Too Dim To Generate Complex Life
UT - Habitable Zone Planets Around Red Dwarfs Aren't Likely To Host Exomoons
UT - New Research Suggests Red Dwarf Systems are Unlikely to Have Advanced Civilizations
