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Red Dwarf UV: Too Little to Spark Life?

We’re going to need a lot more information about the effects of ultraviolet light as we begin assessing the possibility of life on the planets of red dwarf stars. We already know that young red dwarfs in particular can throw flares at UV wavelengths that can damage planetary atmospheres. They can also complicate our search for biosignatures through processes like the photodissociation of water vapor into hydrogen and oxygen, a non-biological source of oxygen of the kind we have to rule out before we can draw even tentative conclusions about life.

Could flares have astrobiological benefits as well? That’s a question that emerges from a new paper from Sukrit Ranjan (Harvard-Smithsonian Center for Astrophysics) and colleagues. What concerns Ranjan’s team is that red dwarf stars may not emit enough ultraviolet to benefit early forms of life. On the primitive Earth, UV may have played a key role in the formation of ribonucleic acid. If this is the case, then UV flare activity could actually help red dwarf planets by compensating for the lower levels of UV red dwarfs produce when they are quiescent.

“We still have a lot of work to do in the laboratory and elsewhere to determine how factors, including UV, play into the question of life,” said co-author Dimitar Sasselov, also of the CfA. “Also, we need to determine whether life can form at much lower UV levels than we experience here on Earth.”

Image: This artist’s impression shows how the surface of a planet orbiting a red dwarf star may appear. The planet is in the habitable zone so liquid water exists. However, low levels of ultraviolet radiation from the star have prevented or severely impeded chemical processes thought to be required for life to emerge. This causes the planet to be devoid of life. M. Weiss/CfA

But back for a moment to the question of UV and the early Earth. As the authors show in their paper, UV can power up prebiotic photochemistry. The literature on this is diverse and includes discussions of UV in relation to the origin of chirality, the synthesis of amino acid precursors and the polymerization of RNA. We can draw some inferences from RNA and DNA themselves, as the paper notes:

Measurements of nucleobase photostability suggest that the biogenic nucleobases (the informational components of the RNA and DNA monomers) are exceptionally stable to UV irradiation compared to structurally similar molecules with comparable thermal properties, suggesting they evolved in a UV-rich environment (Rios & Tor 2013; Beckstead et al. 2016; Pollum et al. 2016). This scenario is consistent with our understanding of conditions on prebiotic Earth: UV light is thought to have been abundant on young Earth due to the absence of a biogenic ozone layer (Cockell 2000a,b; Ranjan & Sasselov 2016).

Recent work by Ranjan and Sasselov has explored the interaction between UV radiation and prebiotic chemistry to the point that the authors now argue such interactions will be an important consideration as we try to understand the surface environment of planets orbiting red dwarfs. If correct, the hypothesis could give us a new criterion for assessing planetary habitability.

These matters are significant because the first atmospheres of planets in their stars’ habitable zones that we will be able to analyze will be found around stars like TRAPPIST-1, LHS 1140 and, of course, Proxima Centauri, the closest red dwarf of all. The figures for UV radiation are striking, with the authors estimating that prebiotic Earth-analog planets would experience between 100 and 1000 times less UV than would have been available on the early Earth.

“It may be a matter of finding the sweet spot,” said co-author Robin Wordsworth of the Harvard School of Engineering and Applied Science. “There needs to be enough ultraviolet light to trigger the formation of life, but not so much that it erodes and removes the planet’s atmosphere.”

If the key question for future laboratory work is whether UV levels this low can support life’s formation, we’re still forced to cope with the fact that we have only one known instance of abiogenesis. And even here on Earth, the question of exactly how life emerged remains the subject of debate. It is not inconceivable that we might find signs of life on a red dwarf planet that emerged along entirely different lines than the life that appeared on our own.

This is interesting stuff, because in most papers treating the question of UV on red dwarf planets, the radiation is considered a negative for habitability. A large amount of work has gone into analyzing whether various mechanisms exist that could protect surface life from UV flares, everything from biofluorescence to ozone layers and oceans. But if we exclude very active young stars, the authors argue, non-flare UV experienced at the surface of habitable zone planets could be clement for life. The question becomes, is it strong enough for life to begin?

The authors’ concern:

…uncertainty over whether the UV-dependent prebiotic pathways that may have led to the origin of life on Earth could function on planets orbiting M-dwarfs, such as the recently-discovered habitable zone planets orbiting Proxima Centauri, LHS 1140, and TRAPPIST-1. Even if the pathways proceed, their reaction rates will likely be orders of magnitude lower than for planets around Sunlike stars, potentially slowing abiogenesis.

Thus the paper calls for laboratory work measuring the reaction rate of UV-dependent prebiotic pathways, and analyzing their susceptibility to changes in radiation level. If we learn that red dwarf planets do not receive sufficient radiation at these wavelengths, then we may want to turn our attention to the more active red dwarfs, whose frequent flares can power photochemistry.

The paper is Ranjan, Wordsworth and Sasselov, “The Surface UV Environment on Planets Orbiting M-Dwarfs: Implications for Prebiotic Chemistry & Need for Experimental Follow-Up,” Astrophysical Journal Vol. 843, No. 11 (10 July 2017). Abstract / preprint.


{ 10 comments… add one }
  • Abelard Lindsey September 5, 2017, 13:10

    Bio-energetics considerations themselves argue for the deep ocean alkaline vents and not surface bodies of water being the origin of life. Not only did prokaryotic life evolve in these vents, but such vents were likely the locations of where the first Eukaryotes emerged as well according to William martin’s Hydrogen Hypothesis of Endosymbiosis. Hence, the role of UV radiation in the origin of life on a planet is minimized.

    • Jens Knudsen September 5, 2017, 18:10

      I see the point. But wouldn’t an atmosphere rich in organics supply the ‘right’ kind of chemicals to these parts of the ocean, too?

      • Abelard Lindsey September 6, 2017, 18:19

        Yes, it would help. Mostly an atmosphere is necessary to ensure that water remains liquid (e.g. oceans) on the planetary surface.

        • Joe H. September 7, 2017, 14:48

          And don’t you need an electromagnetic field to keep the atmosphere from evaporating? And don’t you need a metallic core to get an electromagnetic field? IMO the “Rare Earth” theory is looking better and better.

  • Alex Tolley September 5, 2017, 16:16

    This paper makes 2 assumptions.
    1. That our early earth was fairly ideal for abiogenesis from a UV standpoint.
    2. That UV is an important factor in abiogenesis.

    Both of these assumptions may be very wrong.
    While we know that Earth is the only habitat for life that we know of today, and that life started fairly early in our planet’s history, we really have no good idea about the important factors for abiogenesis, or even that we are certain that earth was the habitat for abiogenesis and that panspermia was not the source of early life.

    While lab experiments are pushing forward possible chemistry, we are very far from a theory of abiogeneis, whether the RNA or metabolism first models. In the 1950’s, the Miller-Urey experiment used sparks (lightning) to create amino acids from a rather different primordial atmosphere. We might speculate that papers written about RD planets at that time might have focused on the frequency of lightening strikes.

    I think we are far better focused on finding life, and like Kepler, determining the conditions from a statistically useful sample set. The result may not accord with our expectations.

    I’m reminded of the sort of logic that was once used to determine what Martians would look like. I would describe those efforts as sand castles of assumptions and poor reasoning. This paper may be making very similar errors. The focus on measurable UV fluxes may also be analogous to looking under a street light for car keys lost under a bush because the light is better there.

  • Jens Knudsen September 5, 2017, 18:19

    I see your point. I see the article as if it has its focus on one out of a host of different ‘parameters’ regarding abiogenesis(?). We don’t really have a good grasp on this one.

  • Michael Fidler September 6, 2017, 2:48

    What is interesting about this concept is that most anything that can throw a system far from equilibrium can cause the right conditions to form the predecessor to life. UV, lightning, shock waves, chemical reactions and even radioactive elements on the early Earth, to name just a few.

    Hailed as the “Next Darwin,” MIT’s Jeremy England Sheds Little Light on Life’s Origins:

    Building on these insights, England’s lab simulated a network of interconnected chemical reactions with added external forcing. The thermodynamic forcing altered the dynamics of specific reactions. A real-world example of such forcing would be constantly adding a chemical that could accelerate or slow a particular reaction, depending on whether it was a reactant or a product. Other examples would be supplying UV light or shock waves. England ran simulations where the effect of the forcing depended on the configuration of the matter (i.e., the nature of interactions and the relative abundance of molecules) and only a few configurations resulted in strong forcing effects. His experiments demonstrated that the system often moved into the states that maximized the forcing and thus the efficiency of dissipating energy. In other words, the matter at times self-organized (emerged) into configurations that were “fine-tuned” for extracting energy from the environment.



  • Geoffrey Hillend September 6, 2017, 17:05

    The assumption that here is that red dwarf stars like Trappist -1 might not have enough UV radiation. Any UV radiation such as UVB and UVC over time is too much UV if it is not blocked by ozone. Without ozone, life will not survive and increase in number, evolve and develop a biosphere. All red dwarfs, even the smallest ones, emit UV radiation. One does not need to have active young stars to emit UV. There is always some UV coming from red dwarfs even without high flare activity. Little flares are always there due to magnetic fields and they emit x rays and UV. A smaller amount of UV is still deadly.
    The question is did life evolve and make an ozone layer for protection. The photo dissociation of H2O or water and Co2 into hydrogen, carbon and oxygen UV will leave only trace amounts of oxygen which is not enough to produce much ozone. Early Earth’s atmosphere needed a lot of oxygen to produce enough ozone to make it safe for life on the surface of the ocean. We might be able to differentiate between trace UV produced oxygen and life produced oxygen since the spectral lines might be stronger? The presence of other bio-markers like nitrous oxide, methane, ethane etc would help. In early Earth’s atmosphere, UV penetrates 5 to 10 meters or 10 to 30 feet without ozone. P. 4, Barbato and Ayer, 1981, Atmospheres. “Lovelock and Margulis also suggest that if life were to cease, levels of oxygen and nitrogen would decline until they were trace gases in an atmosphere of water vapor, carbon dioxide, and the noble gases. Without life, Earth would have an atmosphere whose composition was something of a compromise between the atmospheres of Venus and Mars.” P. 5, Ibid.

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