Centauri Dreams

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.

We’re going to eventually get to know the seven planets of the TRAPPIST-1 system well. That’s because they present the ideal targets for upcoming attempts to probe the atmospheres of Earth-sized rocky planets. Orbiting an M-dwarf some 39 light years out, all seven of the planets here transit. Because of the faintness of their star and the size of the planets themselves, they present excellent candidates for transit spectroscopy, in which we look at the light from the star through planetary atmospheres to deduce their constituents.

Using the Space Telescope Imaging Spectrograph on the Hubble instrument, an international team led by Vincent Bourrier (Observatoire de l’Université de Genève) has been studying the ultraviolet radiation received by each of the planets in the system. The more we know about the star’s output, the more we can plug values into the atmospheric escape processes that can occur in its planets, with huge consequences for surface conditions. The team in particular was looking for the signature of an extended atmosphere for TRAPPIST-1 c.

Image: This is an artist’s impression of the TRAPPIST-1 system, showcasing all seven planets in various phases. When a planet transits across the disk of the red dwarf host star, as two of the planets here are shown to do, it creates a dip in the star’s light that can be detected from Earth. During such transits astronomers are able to study the potential atmospheres of these planets. Credit: NASA.

How would such an atmosphere be found? Low-energy ultraviolet radiation can break down water molecules, producing oxygen and hydrogen — the process is known as photo-dissociation. By heating the upper atmosphere, stronger ultraviolet (XUV) radiation as well as X-rays can play a role in letting the hydrogen and oxygen escape into space.

We wind up with hydrogen that can theoretically be detected in the exosphere of the planet, which can be seen as a possible marker for water vapor in the atmosphere. We’ve had hints in earlier transits of planets b and c, also studied by Bourrier, that “could either indicate extended atmospheres of neutral hydrogen or intrinsic stellar variability.” The current work, however, was unable to detect a hydrogen exosphere during a transit of c. We may still be able to find such hydrogen in future observations with the James Webb Space Telescope.

In the absence of such a detection, this paper focuses on the irradiation of the star and the wide range of consequences for water on its planets. We learn that the amount of ultraviolet radiation TRAPPIST-1 is putting out could lead to significant water loss over the history of the system. Based on their measurements, the researchers arrive at an estimate of present XUV radiation and go on to calculate the possible water loss.

This gets interesting because it covers the entire planetary system and describes markedly different rates of water loss. From the paper:

With our current knowledge of the TRAPPIST-1 system, the major uncertainty on the water loss estimates comes from the uncertainty on the planet masses, rather than on the XUV luminosity. Setting the masses to their nominal estimates from Gillon et al. (2017) we found that planets g and closer-in could have lost more than 20 Earth oceans through hydrodynamic escape, if the system is as old as 8 Gyr. Planets b, c, and possibly d could still be in a runaway phase, but if water loss drops down significantly within and beyond the HZ, planets e, f, g, and h might have lost less than 3 Earth oceans.

Image: This artist’s impression shows the view from the surface of one of the planets in the TRAPPIST-1 system. At least seven planets orbit this ultracool dwarf star 40 light-years from Earth and they are all roughly the same size as the Earth. Several of the planets are at the right distances from their star for liquid water to exist on the surfaces. Credit: ESO/N. Bartmann / spaceengine.org.

We would expect planets b and c, the two innermost worlds, to have the highest water loss over the eight billion years this system has been in existence, but it’s notable that planets e, f and g — the three thought to be in the habitable zone — could have lost a good deal less. That would imply the possibility that these perhaps temperate worlds still have water on their surfaces.

The authors point out that their estimates of water loss should be considered as upper limits, because their assumptions probably maximize the rate of the XUV interaction with the atmosphere. All planets except TRAPPIST-1 b and c could, they argue, harbor significant amounts of water, perhaps aided by outgassing, depending on planet formation models that include water content. But this is a tough calculation, and it depends on mass:

The amount of water outgassed after a few hundred Myr (after which TRAPPIST-1 h to e, and possibly d, have entered the HZ), is greater for planets with more massive refractory parent bodies. Improving our constraints on the planet masses of the TRAPPIST-1 system will therefore significantly improve our understanding of the variable outgassing capabilities of the TRAPPIST-1 planets and hence the current state of their atmospheres.

Are there other physical mechanisms that could affect the loss of atmospheres and, possibly, their replenishing? Other gases might slow the outflow, while stellar flares could increase it. On flares, however, Bourrier and team note that they observed no flaring activity in their observations at X-ray and far ultraviolet wavelengths, while earlier observations in the optical and infrared have shown a low rate of flares, with weak flares every few days but stronger flares once every two to three months, a rate the authors consider low.

We also have to take into account the possible effects of the stellar wind from TRAPPIST-1, which can likewise erode atmospheres, especially in the earlier phases of the system’s evolution. Our lack of knowledge of possible magnetic fields, which can affect the rate of atmospheric loss, is due to our lack of information about the composition and structure of the planets here. No wonder, then, that Bourrier is careful about his assessment:

“While our results suggest that the outer planets are the best candidates to search for water with the upcoming James Webb Space Telescope, they also highlight the need for theoretical studies and complementary observations at all wavelengths to determine the nature of the TRAPPIST-1 planets and their potential habitability.”

So much depends on JWST, which may be able to nail down our estimates on any hydrogen exospheres in TRAPPIST-1 planets. The paper is Bourrier et al., “Temporal evolution of the high-energy irradiation and water content of TRAPPIST-1 exoplanets,” accepted at the Astronomical Journal (preprint).