I hadn’t intended to return to habitability around red dwarf stars quite this soon, but on Saturday I read a new paper from Anna Childs (Northwestern University) and Mario Livio (STScI), the gist of which is that a potential challenge to life on such worlds is the lack of stable asteroid belts. This would affect the ability to deliver asteroids to a planetary surface in the late stages of planet formation. I’m interested in this because it points to different planetary system architectures around M-dwarfs than we’re likely to find around other classes of star. What do observations show so far?
You’ll recall that last week we looked at M-dwarf planet habitability in the context of water delivery, again involving the question of early impacts. In that paper, Tadahiro Kimura and Masahiro Ikoma found a separate mechanism to produce the needed water enrichment, while Childs and Livio, working with Rebecca Martin (UNLV) ponder a different question. Their concern is that red dwarf planets would lack the kind of late impacts that produced a reducing atmosphere on Earth. On our planet, via the reaction of the iron core of impactors with water in the oceans, hydrogen would have been released as the iron oxidized, making an atmosphere in which simple organic molecules could emerge.
If we do need this kind of impact to affect the atmosphere to produce life (and this is a big ‘if’), we have a problem with M-dwarfs, for delivering asteroids seems to require a giant planet outside the radius of the snowline to produce a stable asteroid belt.
Depending on the size of the M-dwarf, the snowline radius is found from roughly 0.2 to 1.2 AU, close enough that radial velocity surveys are likely to detect giant planets near but outside this distance. The transit method around such small stars is likewise productive, but we find no such giant planets in those M-dwarf systems where we currently have discovered probable habitable zone planets:
The Kepler detection limit is at orbital periods near 200 days due to the criterion that three transits need to be observed in order for a planet to be confirmed (Bryson et al. 2020). However, in the case of low signal-to-noise observations, two observed transits may suffice, which allows longer-period orbits to be detected. This was the case for Kepler-421 b, which has an orbital period of 704 days (Kipping et al. 2014). Furthermore, any undetected exterior giant planets would likely raise a detectable transit timing variation (TTV) signal on the inner planets (Agol et al. 2004). For these reasons, while the observations could be missing long-period giant planets, the lack of giant planets around low-mass stars that are not too far from the snow line is likely real.
Image: A gas giant in orbit around a red dwarf star. How common is this scenario? We know that such planets can exist, but so far have never detected a gas giant outside the snowline around a system with a planet in the habitable zone. Credit: NASA, ESA and G. Bacon (STScI).
In the search for stable asteroid belts, what we are looking for is a giant planet beyond the snowline, with the asteroid belt inside its orbit, as well as an inner terrestrial system of planets. None of the currently observed planets in the habitable zone around M-dwarfs shows a giant planet in the right position to produce an asteroid belt. Which is not to say that such planets do not exist around M-dwarfs, but that we do not yet find any in systems where habitable zone planets occur. Let me quote the paper again:
By analyzing data from the Exoplanet Archive, we found that there are observed giant planets outside of the snow line radius around M dwarfs, and in fact the distribution peaks there. This, combined with observations of warm dust belts, suggests that asteroid belt formation may still be possible around M dwarfs. However, we found that in addition to a lower occurrence rate of giant planets around M dwarf stars, multiplanet systems that contain a giant planet are also less common around M dwarfs than around G-type stars. Lastly, we found a lack of hot and warm Jupiters around M dwarfs, relative to the K-, G-, and F-type stars, potentially indicating that giant planet formation and/or evolution does take separate pathways around M dwarfs.
Image: This is Figure 2 from the paper. Caption: Locations of the giant planets, r, normalized by the snow-line radius in the system, vs. the stellar mass, M?. The point sizes in the top plot are proportional to m?. Red dots indicate planets around M dwarf stars and blue dots indicate planets around FGK-type stars. The point sizes in the legend correspond to Jupiter-mass planets. The bottom plot shows normalized histograms of the giant planet locations for both single planet and multiplanet systems. The location of the snow line is marked by a black dashed vertical line. Credit: Childs et al.
The issues raised in this paper all point to how little we can say with confidence at this point. Are asteroid impacts really necessary for life to emerge? The question would quickly be resolved by finding biosignatures on an M-dwarf planet without a gas giant in the system, presuming no asteroid belt had formed by other methods. As one with a deep curiosity about M-dwarf planetary possibilities, I find this work intriguing because it points to different architectures around red dwarfs than other stars. It’s a difference we’ll explore as we begin to fill in the blanks by evaluating M-dwarf planets for early biosignature searches.
The paper is Childs et al., “Life on Exoplanets in the Habitable Zone of M Dwarfs?,” Astrophysical Journal Letters Vol. 937, No. 2 (4 October 2022), L42 (full text).
They may not need an asteroid belt because the Kuiper belt and Oort cloud may be much smaller and have a greater disturbance by nearby passing stars. The passage through the spiral arm may also cause large impacts from debris in the arm. What would also have a much higher probability in these tightly packed systems is that the short orbital period of the planets would increase exponentially the chance of impact from any comet that passed close to the red dwarf. Because of this factor I would say that M Dwarf planets should have a very organic environment.
The two problems that need advance study are what do these minuscule systems like Trappist 1 have in the way Kuiper belt and Oort clouds. The Kuiper belt may have been depleted quickly because of the comets short period of 5 to 10 years and being in the plane of the ecliptic. Their impacts higher rate with the planets may have been similar to the the Late Heavy Bombardment in our system. The Oort cloud may have an average period of the comets of 30,000 years compared to ours of a million or more. The question is how much interaction would take place with these comets from the very close planets such as Trappist 1 b (1.5 day orbit) and c (2.4 day orbit). Hopefully we will know soon as to how much volatile elements exist on the Trappist 1 planets and other nearby M Dwarf systems. We may be able to see a clearer picture of how they evolved and were affected by comets.
“Because of this factor I would say that M Dwarf planets should have a very organic environment.”
More comets, than we get?
gbaikie, Your not seeing the difference to our system, the third planet from Trappist 1 is orbiting the M dwarf in 4 days. that’s basically 100 times faster then our third planet from the sun, earth. The system would evolve much faster then ours and live a 100 times longer. Just by the odds they would be the most likely to have life and intelligent life because for every G class stars there are 11 M dwarfs. They should have a larger amounts of cometary matter which are also high in organic material.
“The ESA’s Rosetta mission, which ended in September 2016, found that organic matter made up 40% (by mass) of the nucleus of comet 67P Churyumov-Gerasimenko, a.k.a. Chury. Organic compounds, combining carbon, hydrogen, nitrogen, and oxygen, are building blocks of life on Earth.”
https://www.cnrs.fr/en/does-organic-material-comets-predate-our-solar-system
Faster racetrack, faster to life and faster to intelligence – We only lived as mammals for 65 million years…
Why would an M_Dwarf system evolve 100x faster based on orbital period?
Elsewhere we have had posts on the possibility of “rocky” worlds being composed of a far higher C/Si ratio than ours. What we don’t know is what impact that would have on life. Consider that most of the carbon on Earth is locked up in carbonate rocks. If we released it all, would biomass be that much greater, or biodiversity be higher (or some other metric concerning proliferation? Is it not possible that more carbon might be a detriment as it impacts surface conditions and geology? [The carbon argument smacks a little of the argument that more CO2 emissions are positive as plants require it for growth.]
Let’s wait and see what JWST finds around M dwarf planets. I think the diversity is going to be tremendous.
Diversity of what exactly? How will JWST measure whatever it is you have in mind?
Well, we have 3 rocky planets with atmospheres, two of them dead from CO2 suffocation. So our knowledge level is very restrictive as to what will be found around M Dwarfs. Mini earths, Super Earths and Sub-Neptunes do not exist in our understanding of planets. Plus we have no one even talking about the high orbital speeds around M Dwarfs and how comet or asteroids could cause major changes in their evolution. We only have earth that has basically evolved extremely slowly and mostly by impactors. Now we find huge reservoirs of water in the earth’s mantle 660 kilometres down where large deep earthquakes take place. Its kind of like when they thought the deep ocean was dead. We really do not have a clue what exists. Trappist 1 is on the list for observation by JWST and should be able to detect whether they have water in their atmospheres. Is the earth at the center of the universe, no and planets can be just like life with a billion different forms.
OK, so you are talking about the diversity of planet conditions, not of life. Thank you for the clarification.
Well, one possible detection problem comes to mind:
If a jovian planet transited a red dwarf, it might blink it out entirely.
Much of this data collection is automated, of course. But I am trying to
think of a planetary transit of a luminous object which has a smaller
diameter than the planet has. E.g. a white dwarf? There are brown dwarf and red dwarf binaries that are known, but their identifications
rest on spectrographic or visual binary identifications.
It would seem that jovian planets would be detectable, but it would not be the first time survey results were skewed because of an observation problem.
Yes
One of my favorite subjects here are a few links for readers to follow up on
Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs
https://arxiv.org/abs/2209.14563
New Theory Concludes That the Origin of Life on Earth-Like Planets Is Likely
https://news.uark.edu/articles/62043/new-theory-concludes-that-the-origin-of-life-on-earth-like-planets-is-likely
Thanks for the interesting read Paul
Cheers Edwin
I read the paper Abiogenesis: the Carter argument reconsidered but I do not find it convincing. One can come to the opposite conclusion if the probability of biogenesis elsewhere is harder, not easier, than on Earth.
What we need is more evidence, not playing with maths. If we find that biosignatures are common (at least in suitable worlds), then we can argue that abiogenesis is easy (although we cannot rule out panspermia by some means). OTOH, if we do not find any biosignatures, that would imply that abiogenesis is hard and maybe we were just very lucky for some reason. It might also say something about the likelihood of ETI and the Fermi Paradox. [In another topic, consider the use of such Bayesian analysis to determine the existence of a G_d that answers pleas for help. Experiments on the results of praying should keep updating the probability that such a G_d exists.]
In summary, I don’t see much scientific value in speculation compared to gaining evidence. The discovery of the first exoplanet biosignature would have a significant impact on the topic. I would suggest that it would increase the activity of finding more, as Kepler did for finding exoplanets, and revitalize SETI and related activities for finding technosignatures.
Fortunately astronomers will be spoiled for data eventually about M dwarf planetary systems. No amount of analysis ever seems to resolve on the kind of jaw-dropping surprises that emerge immediately from improvements in observation.
Interesting article. I still have to stick with the latest idea that the terrestrial planets got their water from the early and late bombardment periods which indicates that we already had oceans four billion years ago. This would also remove the problem of Red Dwarfs stars not having any Jupiter sized gas giants needed to have water which came from the protoplanetary disk of the red dwarf star. I still expect to see water, but no oxygen in their exoplanet spectra.
I can’t rule out the idea that exoplanets around red dwarf stars without Jupiter sized gas giants might have less water any more than solar wind stripping or a smaller sized protoplanetary sized cloud of the smaller size red dwarf star than a G class star results in less water on the terrestrial exoplanets around a red dwarf star. These might result in less water and an Earth sized exoplanet with less water and oceans and closer to a desert planet. The water vapor still could be detected with a spectrometer. There is till interior water which can be replenished from volcanism.
This is why we either need to get probes to these worlds or at least build some huge space telescopes to resolve this issue:
https://cosmosmagazine.com/space/red-dwarf-alien-inhospitable/