M-Dwarfs: The Asteroid Problem

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).


M-Dwarf Habitable Planets: The Water Factor

Small M-dwarf stars, the most common type of star in the galaxy, are likely to be the primary target for our early investigations of habitable planets. The small size of these stars and the significant transit depth this allows when an Earth-mass planet crosses their surface as seen from Earth mean that atmospheric analysis by ground- and space-based telescopes should be feasible via transmission spectroscopy. Recent studies have shown that the James Webb Space Telescope has the precision to at least partially characterize the atmospheres of Earth-class planets around some M-dwarfs.

Soon-to-be commissioned ground-based extremely large telescopes will likewise play a role as we examine nearby transiting systems. But M-dwarfs make challenging homes for life, if indeed it can exist there. In addition to flare activity, we also have to reckon with the presence of water. Too much of it could suppress weathering in the geochemical carbon cycle, but too little does not allow for the development of a temperate climate. Thus new work on water content in such systems is welcome.

For purposes of reference, Earth’s seawater accounts for 0.023% of the planet’s total mass. According to Tadahiro Kimura, a doctoral student at the University of Tokyo, and Masahiro Ikoma (National Astronomical Observatory of Japan), a number of models suggest that terrestrial planets around M-dwarfs would have either too much water or no water at all. Are habitable planets around such stars, then, a celestial rarity?

In a new paper in Nature Astronomy, the authors argue that there is a mechanism beyond the infall of icy planetesimals that can produce water as a young planet accumulates its atmosphere. It involves interactions between the hydrogen-rich atmosphere, drawn from the protoplanetary disk, and the magma ocean that would be present from impacts during the early days of planet formation. Water is accumulated through the chemical reaction between atmospheric hydrogen and the oxides found in the surface magma – a magma ‘ocean’ – of the young planet. From the paper:

…water can be secondarily produced in a primordial atmosphere of nebular origin through reaction of atmospheric hydrogen with oxidising minerals from the magma ocean, which is formed because of the atmospheric blanketing effect[8], thereby enriching the primordial atmosphere with water. By assuming effective water production, we recently showed that nearly-Earthmass planets can acquire sufficient amounts of water for their atmospheric vapour to survive in harsh UV environments around pre-main-sequence M stars [9]. The results suggest that including this water production process significantly affects the predicted water amount distribution of exoplanets in the habitable zone around M dwarfs.

Image: Probability distribution of seawater mass fractions for planets of Earth-like mass (0.3-3 times Earth mass) located in the habitable zone around M-type stars (0.3 solar masses). Green is the result of calculations following the conventional model and considering only the acquisition of water-bearing rocks. Orange is the result when the model of the present study is used and the effect of water production in the primordial atmosphere is taken into account. The dotted line is the present-day seawater amount on the Earth. Credit: National Astronomical Observatory of Japan.

In this scenario, the amount of water present depends on how the planet forms. The authors have created a planetary population synthesis model that tracks the mass and orbital evolution of planets in formation, including among other things the structure of the protoplanetary disk, potential orbital migration, instabilities in multi-planet systems and the effects of water production in the primordial atmosphere. The model, which refines that presented in an earlier paper by the same researchers, allows the calculation of the amount of water that should be produced through the atmosphere/magma interaction.

The range of water outcomes is wide, but if we narrow it to planets with seawater mass fractions similar to Earth, most of this water is found to come through atmosphere/magma interaction rather than by incoming impacts by comets and other water-bearing objects. And it turns out that a few percent of planets with a radius between 0.7 and 1.3 times that of Earth produce the right amount of water to sustain temperate climates. Let me quote the paper on this – note that in the passage below, HZ-NEMP refers to nearly-Earth-mass planets in the habitable zone:

The HZ-NEMPs of 0.7–1.3 R?… have lost their hydrogen atmospheres completely, ending up with rocky planets covered with oceans. It turns out that those planets are diverse in water content and do include planets with Earth-like water content. Several climate studies argue the amounts of seawater appropriate for temperate climates, considering the effects of seafloor weathering, high-pressure ice, water cycling and heterogeneous surface water distribution… According to those studies, the appropriate seawater amount ranges from ?0.1 to 100 times that of the Earth.

Clearly, target selection for exoplanet habitability would benefit from being able to exclude planets that are unlikely to be habitable, which according to this paper would include habitable zone worlds with radii > 1.3R? that have deep oceans with high-pressure ice, and planets with ocean mass fractions greater than 100 times that of Earth. The authors believe that we should be able to identify such worlds if planetary mass and radius can be measured within ? 20% and 5% accuracy respectively. Having eliminated these, we turn to planets in the 0.7 to 1.3R? range. The authors refer to them as ‘water-poor,’ in comparison to their larger cousins, but they still can have seawater fractions similar to that of Earth:

…the HZ-NEMPs with appropriate amounts of seawater for habitability are estimated to account for ?5% of the “water-poor rocky planets” orbiting 0.3M M dwarfs. This frequency becomes higher for larger stellar mass, and around 0.5M stars, for example, more than 10% of the water-poor rocky planets are expected to have the appropriate amounts of seawater.

So 5% to 10% of the M-dwarf exoplanets in the appropriate size range (< 1.3R?) have the fraction of water needed for habitability. The paper makes this prediction: Survey missions like TESS and the upcoming PLATO should detect approximately 100 Earth-sized planets in the habitable zone around M-dwarfs. 5 to 10 of these, according to this model, are likely to be planets with oceans and temperate climates, a sharp contrast to earlier studies indicating such worlds should not exist.

The paper is Kimura & Ikoma, “Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs,” Nature Astronomy 29 September 2022 (abstract / preprint). The authors’ earlier paper on water enrichment is Kimura & Ikoma, “Formation of aqua planets with water of nebular origin: effects of water enrichment on the structure and mass of captured atmospheres of terrestrial planets,” Monthly Notices of the Royal Astronomical Society 496, 3755 (2020) (abstract).


Great Winds from the Sky

Do we need to justify pushing our limits? Doing so is at the very heart of the urge to explore, which is embedded in our species. Recently, while doing some research on Amelia Earhart, I ran across a post on Maria Popova’s extraordinary site The Marginalian, one that examines the realm of action within the context of the human spirit. Back in 2016, Popova was looking at Walter Lippmann (1889-1974), the famed journalist and commentator, who not long after Earhart’s fatal flight into the Pacific discussed the extent of her achievement and the reasons she had flown.

Here’s a passage from Lippmann’s New York Herald Tribune column, written on July 8, 1937, just six days after the aviator and her navigator, Fred Noonan, disappeared somewhere near Howland Island between Hawaii and Australia. Lippmann asks whether such ventures must be justified by a utilitarian purpose and concludes that what is at stake here transcends simple utility and speaks to the deepest motivations of our explorations. It is a belief in a goal and the willingness to risk all. Practicality carries little weight among those who actually do the deed:

“The best things of mankind are as useless as Amelia Earhart’s adventure. They are the things that are undertaken not for some definite, measurable result, but because someone, not counting the costs or calculating the consequences, is moved by curiosity, the love of excellence, a point of honor, the compulsion to invent or to make or to understand. In such persons mankind overcomes the inertia which would keep it earthbound forever in its habitual ways. They have in them the free and useless energy with which alone men surpass themselves.

Such energy cannot be planned and managed and made purposeful, or weighted by the standards of utility or judged by its social consequences. It is wild and it is free. But all the heroes, the saints, the seers, the explorers and the creators partake of it. They do not know what they discover. They do not know where their impulse is taking them. They can give no account in advance of where they are going or explain completely where they have been. They have been possessed for a time with an extraordinary passion which is unintelligible in ordinary terms.

No preconceived theory fits them. No material purpose actuates them. They do the useless, brave, noble, the divinely foolish and the very wisest things that are done by man. And what they prove to themselves and to others is that man is no mere creature of his habits, no mere automaton in his routine, no mere cog in the collective machine, but that in the dust of which he is made there is also fire, lighted now and then by great winds from the sky.”

Image: Amelia Earhart’s Lockheed Electra 10E. During its modification, the aircraft had most of the cabin windows blanked out and had specially fitted fuselage fuel tanks. The round RDF loop antenna can be seen above the cockpit. This image was taken at Luke Field in Hawaii on March 20, 1937. Earhart’s final flight in this aircraft took place on July 2, 1937, taking off from Lae, New Guinea. Credit: Wikimedia Commons. Scanned from Lockheed Aircraft since 1913, by René Francillon. Photo credit USAF.

Lippmann’s tribute is a gorgeous piece of writing, available in The Essential Lippmann (Random House, 1963). Naturally, it makes me think of other flyers who rode those same winds, people like Antoine de Saint-Exupéry and Beryl Markham, who in 1936 was the first to dare a solo non-stop flight across the Atlantic from east to west. As I’ve recently re-read Markham’s elegant West With the Night (1942), she as well as Earhart has been on my mind. What a shame that Earhart didn’t live to pen a memoir as powerful, but perhaps Lippmann in some small way did it for her.