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.