I’m going to start in the Kuiper Belt this morning before going further out, because the news that the Belt may extend much further than expected reminds us of the nature of exploration. The New Horizons spacecraft, well beyond Pluto’s orbit and approaching 60 AU from the Sun, is finding more dust than expected. Our theoretical models didn’t see that coming. In fact, the dust produced by collisions between Kuiper belt objects was thought to decline as we approached the Belt’s outer edge.

So just where is that outer edge? It had been pegged around 50 AU but now looks more like 80 AU, if not further out, a finding corroborated by the fact that New Horizons scientists have used Earth-based resources like the Subaru Telescope in Hawaii to find numerous KBOs beyond the assumed boundary. Is this a new population of Solar System objects, or are we actually seeing something more mundane, such as radiation pressure pushing inner belt dust further out than would be expected? It takes patient observation to decide, and to re-shape our notions according to hard data.

Which gets me into exoplanet territory, and specifically our understanding of red dwarf stars. Theory is always malleable and yields to observation which, in turn, re-energizes theory. Michaël Gillon (University of Liége), who is among other things the discoverer of the TRAPPIST-1 system, made this point in a recent email exchange. He was responding to my article What We Know Now about TRAPPIST-1 (and what we don’t) with a much needed note of caution. The question of whether rocky planets orbiting M-dwarf stars can retain atmospheres is one of the hottest controversies going. Observations, says Gillon, will tell the tale, not theory, no matter how elegant the latter. After all, we now have JWST, and a new generation of telescopes already under construction to help.

Image: An artist’s concept of Kepler-438b, shown here in front of its violent parent star, a red dwarf. It is regularly irradiated by huge flares of radiation, which could render the planet uninhabitable and possibly strip it of its atmosphere entirely. A variety of mechanisms for depleting the atmosphere of such a planet are now under discussion, using a wide range of models. Image credit: Mark A Garlick / University of Warwick.

On that score, it’s useful to look at a paper that Gillon suggests in his email, a study by Ignasi Ribas and collaborators that appeared in Astronomy & Astrophysics in 2016. The paper does a deep dive into the question of the habitability of Proxima Centauri b, that tantalizingly close Earth-mass planet in the star’s habitable zone. A key issue is whether the extreme dosing of X-ray and ultraviolet radiation such a planet would suffer would compromise a young atmosphere or prevent its existence at all.

There is a lot going on in the Ribas paper, but Gillon pointed me to its discussion of what we might learn from the lesson of our own Earth. Here the essential fuzziness of theory manifests itself, as Ribas and team point out how many uncertainties exist in our estimates of volatile loss, including the telling line “…they rely on complex models that were never confronted to actual observations of massive escape.” Even more telling is the lack of sufficient information about what might prevent such escape:

None of the available models include all the mechanisms controlling the loss rate, for example, the photochemistry of the upper atmosphere and its detailed interaction with the wavelength-dependent stellar emission, non-LTE [Non-Local Thermodynamic Equilibrium] cooling processes, and an accurate description of the outflow beyond the exobase where hydrodynamics no longer apply. Some key data are not known, like the intrinsic planetary magnetic moment, now and in the past, the detailed evolution of the atmospheric composition, of the high-energy spectrum and of stellar wind properties.

Nor do we have hard data on these things even now. Note the term ‘non-LTE cooling’ above. An LTE system is one in thermal equilibrium, maintaining a single temperature. Poking around in the literature, I learn that the lack of thermal equilibrium involves processes that have to be carefully measured to build an accurate profile of an atmosphere, with ramifications for any discussion of its long-term survival. In the absence of such data, it’s telling that other factors remain unknown, including local magnetic conditions and the actual properties of the stellar wind affecting the planet. And we have no good modeling for how volatiles may be distributed between the atmosphere of an M-dwarf planet and the internal processes that can replenish it.

Thus the Ribas paper, although eight years old, remains pertinent to this ongoing discussion, as scientists attack volatile retention in such systems. The authors point out that the protoplanets that built up the early Earth were exposed to a young Sun that was blasting our planet with X-rays, ultraviolet and stellar wind conditions that may equal, and perhaps surpass, what occurred at Proxima b. Note this (italics mine):

The XUV irradiation and stellar wind on the proto-Earth was therefore comparable, and possibly higher, than that of Proxima b. Proxima b spent 100–200 Myr in runaway before entering the HZ, which is longer than the runaway phases experienced by the proto-Earth by a small factor only (<10). Models predict that early Earth suffered massive volatile losses: hydrodynamic escape of hydrogen dragging away heavier species and non thermal losses under strong stellar wind exposure and CMEs (Lammer et al. 2012). Nonetheless, no clear imprint of these losses is found in the present volatile inventory.

The authors point out, then, that geochemical evidence alone shows us no signs of significant depletion of Earth’s inventory of volatiles, which can lead to the possibility that volatile loss was extremely limited under conditions that some of our models would suggest should deplete them radically. If this analysis is correct, then the idea that the planets of red dwarf stars will likely be barren rock stripped of atmospheres is questionable. I come back to Gillon’s point. We’re only going to know from observation, just as we can only know about the extent of the Kuiper Belt through hard data.

Now comes a new paper from Ofer Cohen (University of Massachusetts) and colleagues. Writing in The Astrophysical Journal, the authors again address the TRAPPIST-1 question, this time with a new twist. They’re looking at electric currents that would be produced in the ionosphere of TRAPPIST-1e, a planet that may be in the star’s habitable zone. The question is whether such currents would produce atmospheric heating that would contribute to dissipating the atmosphere entirely.

So this is another stripping mechanism to consider, one produced by the planet’s upper atmosphere encountering the star’s changing magnetic field as the planet proceeds along its orbit. The operative term is ‘Joule heating.’ I only received the paper this morning, so I won’t go too deeply into it. But my early reading suggests that the results from the models used in it point to serious atmospheric loss. This adds to earlier modeling involving the stellar wind and ionized upper atmosphere, some of this conducted by the same authors. The conclusion draws naturally from the modeling:

The JH [Joule heating] is the result of a dissipation of electric current, which is driven by the rapidly varying magnetic field along the planetary orbit. We estimate the JH energy flux on the exoplanet Trappist-1e as well as similar planets orbiting the Sun in close-in orbits. We find that the JH energy flux is larger than the anticipated EUV energy flux at the planet, and it may reach a few percent of the stellar constant energy flux. Such an intense heating could drive a strong atmospheric escape and could lead to a rapid loss of the atmosphere. Thus, the rapid orbital motion of short-orbit exoplanets may exhaust a significant portion of their atmospheres over time.

Again we find useful theories painting a landscape of possibilities. But it’s also true that we lack observational data on the properties of the stellar wind, its evolution over time, and the magnetic fields affecting the planet. The authors call attention to this fact:

VDJH [voltage-driven Joule heating] depends on the variations of the interplanetary magnetic field (IMF) strength along the planetary orbit. Such detailed IMF data are not available for exoplanets (some observations were made for the stellar wind interaction with the interstellar medium; e.g., Wood et al. 2021), nor it is available for short orbits around the Sun (limited data at specific locations are available from the Parker Solar Probe; Raouafi et al. 2023). Due to the lack of observational constraints, we must rely on models to estimate the relevant stellar wind conditions.

Thus energy output, stellar wind and magnetic field changes all factor into a model that suggests atmospheric escape and, like other models, is in need of confirmation with future instrumentation. We can only turn to such observation to begin to understand how diverse theories mesh. I think all the scientists involved in the study of planetary atmospheres around M-dwarfs would agree with this. And headlines in the popular media announcing barren rocks at TRAPPIST-1 are making ongoing investigations into settled science.

Getting too comfortable with theory can mislead us. Recently we saw that the astronomer Otto Struve proposed detecting Jupiter-class worlds in tight orbits around their host stars, only to have the suggestion ignored for decades because ‘hot Jupiters’ simply didn’t fit into then current thinking.

For that matter, nobody thought ‘super-Earths’ were likely, especially in the kind of numbers we’ve found them, nor ‘mini-Neptunes,’ and I doubt many were expecting tiny compact systems of numerous planets, like those Gillon identified at TRAPPIST-1. All in all, I appreciate Gillon’s reminder that patience and data gathering are needed as we explore the question of life around small red stars, an issue that is under deep study but has been by no means resolved. Perhaps the Habitable Worlds Observatory (Habex) will allow a definitive answer for TRAPPIST-1 in the not so distant future.

The Ribas paper is “The habitability of Proxima Centauri b I. Irradiation, rotation and volatile inventory from formation to the present,” Astronomy & Astrophysics 506 (2016), A111 (full text). The Cohen paper is “Heating of the Atmospheres of Short-orbit Exoplanets by Their Rapid Orbital Motion through an Extreme Space Environment,” The Astrophysical Journal 962 (16 February 2020), 157 (full text).