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).
The paper by Ribas et al was very interesting to read also for me who is not any expert in their field of study. But the general conclusion they make is that the impact of the stellar wind has been even more devastating in the past.
And also the fact that Proxima B was not even in the habitable zone when it formed, which might have prevented it from gathering much volatiles in the first place. – This is also pointed out in their study.
Now in the Discussion they do however bring some facts where I do have a bit of expertise, and this on condition that Proxima b against all odds had gathered quite a bit of water against all odds.
Quote:
“During the runaway phase the erosion of a significant water reservoir would have left large amounts of residual oxygen, possibly in the form of atmospheric O2, of up to 100 bar.”
For a non biologist this might sound as a potentially good thing for the possible emergence of life, but it is in fact a situation that would be disastrous for the emergence and evolution of life on Proxima b. And this being a magnitude worse than the oxygen catastrophe on Earth during the Rhyacian era. Low levels of oxygen is something that the life on Earth could cope with, but when that level went up trough the appearance of cyanobacteria. It resulted in what might be labelled a mass extinction – I say might, as that term is a bit controversial, not the fact that it happened. Since there was a serious collapse – since life struggled badly to adapt to the new conditions. And this is one of the turning points where evolution on other worlds might fail to develop any further than the unicellular stage. Which is one of several reasons I expect we will find many worlds with only microbial life that ended up in a cul de sac situation.
“…radiation pressure pushing inner belt dust further out…”
It may not be quite that simple. The Poynting-Robertson Effect suggests radiation pressure may have just the opposite result!
Radiation pressure striking dust particles in a direction radial to orbital motion will appear to be coming from a direction somewhat FORWARD of orthogonal of a vector from the source to intersecting with the orbital path. The result is that the pressure will have a component AGAINST the velocity vector of orbital motion component, at least from the POV of the affected particle. We are all familiar with this effect due to its effect on the Aberration of Light phenomenon*, or more commonly, a vertically falling raindrop appears to be rushing TOWARD a speeding car’s windshield. The net result is a slowing of orbital motion, causing the particle to spiral inward toward the star, not outward.
*For example, due to the earth’s orbital velocity of approximately 30 km/sec, light from stars in the direction of motion appears to be shifted to an apparent distance forward of its true origin (by as much as 20″ of arc!). This effect is easily predictable, and must be routinely corrected for in astrometric work.
Wikipedia has some good articles on these phenomena.
Reminiscent of the description of what happens when a moving object approaches the speed of light: only light ahead of it from the sides appears to reach it while light approaching from the sides misses and passes behind it: at the speed of light all the light reaching it appears to be from a spot ahead of it with an intensity approaching that of the Big Bang.
Many musings from many a source, but the wise assert that the proof of the pudding is in the eating.
Or perhaps the proof of an atmosphere may well be in the breathing..
Do androids snore?
If their soft palate can contact the nasopharynx.
As with particle physics, theories abound, but experimental data is paramount. It took the LHC to discover the Higgs particle that was long theorized, and only then was Peter Higgs awarded the Nobel. Supersymmetry has long been theorized, but no confirmation has arrived, suggesting that it is an incorrect theory. Math is a powerful tool, but it must not supersede observation. Relying on models with experimental support is not unlike philosophy which can build all sorts of logical edifices that may be completely false. The “mathiness” of economics is similarly problematic, resulting in schools of economic thought very different from each other.
New telescopes, like new “atom smashers” help guide the theories, both weeding out incorrect ones built on wrong models and generating new observations to be explained.
Observation of phenomena without theory and models is stamp collecting. Theories and models without relevant observations are mere intellectual play.
Factoid:
The lifetimes of main sequence stars therefore range from a million years for a 40 solar mass O-type star, to 560 billion years for a 0.2 solar mass M-type star.
———
For the last 100 years or so, the Hertzsprung-Russell diagram has been a very useful tool for tying together what appeared otherwise as loose ends in astronomy. The H-R diagram does not explicitly show the stellar mass-main sequence lifetime relation since the track of a star on the MS line is small.
But trace lines such as the Hayashi indicating arrival path tagged with a time
line has a similar exponential relation. The more massive the protostar, the shorter it takes to get to the MS.
Now how does that relate to planet formation in the circumstellar disk surrounding the newly forming star? Consider that the musing part.
The brightness-color relation of main sequence stars is the basis of the many HR diagram’s graphic facets. Transformed to luminosity – temperature on Y and X axes, the hottest O stars are lodged up at the upper left – and the M red dwarfs are down at the lower right. Inherent in the diagram are details about their mass, luminosity, effective temperature and lifetime burning hydrogen into helium.
The O s burn brightly for a short lifetime of about a millions years, intermediate mass stars shine longer in ascending orders of magnitude ( tens of millions, hundreds of millions, a billion years) and our G spectral type sun hanging around for 10 billion or so. The Red Dwarfs can hang on for hundreds of billions or trillions.
Now given that, with most everyone familiar with such already, it is possible to place overlays atop of this diagram:
1. the path to main sequence as a proto-star emerges out of a nebula and becomes surrounded by a circumstellar disk. –
2,and the route off the Main Sequence to a a fate such as white dwarf or a nova or – for red dwarfs – an extended ember like state without distinct transformation.
Now given that background, I say it’s time for another overlay or two.
1. The temporal path of the surrounding circumstellar disk that results in planets
2. The time it takes for proto-planet(s) to emerge out of the circumstellar disk for a given stellar spectral type.
Years ago, when I would attend classes or (later) attend conferences, circumstellar disks surrounding stars similar to the sun were assumed to disperse after about ten million years. In class my instructors were more focused on stellar descriptions such as evolution, interior or atmosphere and did not give planets much address. They mentioned the Jeans Mass lower limit, however, which in the reckoning of the time, was right around the lower limit of nuclear fusion ignition for the MS ( 0.08 solar mass).
Coincidence? Well, they would get back to that later if they had access to really powerful scopes in Chile or else on orbit someday…
OK, later in the 90’s when there were IR and UV space observatories, the astronomical community could now explain where hot Jupiters were coming from because for some G stars ( e.g., 51 Pegasi) hot Jupiters or brown dwarfs were being detected with doppler techniques. No one was talking anymore about the lower Jeans’ mass limit. Nor were they yet focusing on planets in red dwarf systems either. But they were still saying that ten million years was a good time window for planet formation before the circumstellar disk dispersed.
Now, I have to wonder if the conversation then – and still – was a model for planet formation based on G stars.
Currently, the Hayashi and Henyey tracks for proto-sun paths are overlayed on the HR diagram. They have varied tracks in terms of their points of temperature and brightness, whether their tracks are largely vertical or horizontal. But as with their expected lifetime on Main Sequence, the protostar timelines migrating to the MS line vary by orders of magnitude. Shorter time to MS as mass increases.
Already, have taken a winding, tedious path on this subject myself. But according to models, a star with 5-6 solar masses takes about 100,000 years to get to MS ignition, a star of 3 solar masses, (e.g., Sirius) about a million years. The sun on its own track takes perhaps tens of million. And as near as I can judge from the Hayashi or Henyey diagrams on Wikipedia, it takes about 100 million years or more for 0.1 solar mass star to reach the ignition. And despite all all the flares associated with M stars, it might be a more subtle branch then the more massive stars.
The bottom line to all of this, is that although it is clear that there are very detailed models of planet models forming out of red dwarf circumstellar disks, I am not sure of when in the life of the primary star this process really begins – or else how it is connected on a timeline. If a fixed circumstellar disk dissipation timeline is applied for stars of all masses, it would be out of character with the other evolutionary processes driven by the mass of the primary star. Exponentially proportional to the stellar mass, of course.
This 2020 press release (which links the open-access Science paper) suggested the Solar system formed in just 200,000 years. https://phys.org/news/2020-11-solar-years.html At the moment I don’t really understand what is going on with the molybdenum isotopes or how this could be dated so very precisely, nor why this can be so much shorter than the period of time for stellar accretion.
M.S.,
Duly noted. And checked it out. Stated as well in that paper is that it takes about 1.2 million years for a star to commence to MS or hydrogen ignition.
It’s worth commenting on.
For one thing, whatever the coalescence into star time period is, it is distinct from the coalescence into planets – and here cited as about 6 times as long.
On the other hand, my sources indicate that there is not one single time line from nebula to main sequence. Depending on mass of the finished product…
Well, see above. Orders of magnitude difference. But if the time is dependent on the mass of the star which is formed, then we could expect some proportional changes of the timeline for planets from one early star system to another. Consequently, we might be assuming certain events related to the star or the circumstellar disk occurring at the same time across the board – and that the planet might not even have started to form. Or else formed in a region where the radiative flux is a lot lower.
In the early days of exoplanets, all the samples were relatively hot Jupiters and deductive reasoning suggested that terrestrial planets like Earth would be exceptions to this general rule – unless higher resolution sensor results came to the database rescue. And the flow of data about exoplanets did so. But even in the 1990s, Jupiter like bodies such as 51 Pegasi seemed to have migrated to such close in orbits as a result of “momentum exchange” with a large remaining circumstellar disk. The majority of a gazillion particle flybys braked these planets to narrow width orbits with periods of a few days generating high doppler velocities.
Now does anyone want to build a case these days for these “hot jupiters” forming where they were found?
As some have cited, volatiles can often reside be the interior of terrestrial planets – and could be released as atmospheric material over a long period. And red dwarf planets might also experience inward migration in a process that transpires more slowly than with a G type star. Some of the implications could favor habitability. As the EPA used to say, “Your mileage may vary from ( our) laboratory model estimates.” Room for hope.
BTW, what’s the metric equivalent for “mileage”?
Interesting thoughts, and related topics.
And musings such as Is There a Black Hole in the Center of the Sun? With Earl Bellinger (Event Horizon podcast).
One could suppose that, within the high-dimensional phase space of habitability of planets orbiting a flare star, the effect of tidal locking *could* improve the chances of habitability.
Here’s another factor to consider: Tidal heating.
Trappist b & c generate from their primary through tidal flexing from non-circular orbits a heat flow of 1-2 w/m2, which is similar to Io’s (10x Earth’s).
The resulting volcanism could regenerate their atmospheres from mantle volatiles.
If I read Table I of the following correctly, there could be up to 100 times higher concentration of hydrogen in the core of a planet than in its mantle: https://arxiv.org/ftp/arxiv/papers/2311/2311.18262.pdf This depends on the size of the planet, so any effect of tidal flexing there might be something not seen on Io. (but I have no idea if tidal flexing of a planetary core is even a thing) Maybe a source of hope for old planets?
If we compare our solar system with Trappest-1 we should look first at the differences. Trappist-1 at 7.6 billion years is more than 3 billion years older than our system. There is a lot more time for the solar wind to strip away their atmospheres. A red dwarf does not have a red giant phase, so there will be no increased stellar radiation after billions of years. I image Venus would still have a thick atmosphere after 3 billion years due to jeans escape which ‘Joule heating,” is still dependent, the amount of gravity, the temperature and escape velocity is dependent on the size and mass of the exoplanet and the amount of radiation received from the star. The Trappist 1 exoplanets have had the same radiation for 7,6 billion which not including their youths. Volcanism replenishment of the atmosphere has already been mentioned. Carbon dioxide is a heavy gas. I still think it is the major constituent of all the planets there. I can imagine an atmosphere rich in carbon dioxide, but bone dry in water do to the photo dissociation of water molecules by high energy EMR.
We have not heard much of the Trappist 1 system from the JWST. Hopping there will be a more thorough observations of it in the future and other Earth sided exoplanets.
I meant a Venus sized expoplanet around a Red dwarf star still might have an a substantial atmosphere after seven billion years since it does not have a red giant phase.
This is an interesting paper
Heating of the Atmospheres of Short-orbit Exoplanets by Their Rapid Orbital Motion through an Extreme Space Environment
https://iopscience.iop.org/article/10.3847/1538-4357/ad206a