Centauri Dreams

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).

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).

When it comes to planetary habitability, it is all too easy to let our assumptions slide past without review. It’s a danger to be avoided if we want to understand what may distinguish various types of habitable worlds. That’s the implication of a presentation at the recent Europlanet Science Congress (EPSC), which finished its work on September 23 at the Palacio de Congresos de Granada (Spain). Tilman Spohn (International Space Science Institute) and Dennis Höning (Potsdam Institute for Climate Impact Research) have been investigating the ratio of land to ocean and the evolution of biospheres.

The assumptions the duo are examining revolve around the kind of habitable world our Earth represents. Our planet draws on solar energy through continents balanced against large oceans that produce abundant rainfall. Would a given exoplanet have similar geological properties? According to the scientists, it is a balance between the emergence of continents and the volcanism and continental erosion of subduction that maintains Earth’s particular ratio of ocean to land. If we assume a similar internal state on an exoplanet, we could wind up with a similar equilibrium between the production of continents and their erosion, producing a continental land fraction like Earth’s.

But this is conjecture, not observation, and Spohn and Höning believe that several different outcomes may be produced depending on the coupling of continental crust cycle and water in the mantle. We might well wind up with a ‘land planet,’ one having 80 percent of its surface in the form of continental crust (that makes for about 70 percent land surface when continental shelves covered with water are accounted for).

At the other extreme is a planet only 20 percent or so covered in continents; here the land fraction is about 10 percent. Both worlds maintain equilibrium, and it’s rather startling that 80 percent of the team’s randomly chosen sets of initial conditions end up with the land planet outcome.

Image: Planet Earth is visible as a bright speck within the sunbeam just right of center and appears softly blue in the famous ‘pale blue dot’ image originally published in 1990. This updated version uses modern image-processing software and techniques to revisit the well-known Voyager view while attempting to respect the original data and intent of those who planned the images. Earth is the famous ‘pale blue dot,’ but would other habitable planets present the same aspect? Credit: NASA/JPL-Caltech.

Earth-like planets (with continental coverage in the range of about 40 percent, or a land fraction of 30 percent) result in only one percent of these evolutionary models, suggesting that the kind of equilibrium we see on our planet is unstable. Here’s Spohn on the matter:

“In the engine of Earth’s plate tectonics, internal heat drives geologic activity, such as earthquakes, volcanoes and mountain building, and results in the growth of continents. The land’s erosion is part of a series of cycles that exchange water between the atmosphere and the interior. Our numerical models of how these cycles interact show that present-day Earth may be an exceptional planet, and that the equilibrium of landmass may be unstable over billions of years. While all the planets modeled could be considered habitable, their fauna and flora may be quite different.”

Image: Terrestrial planets can evolve in three scenarios of land/ocean distribution: covered by lands, oceans or an equal mix of both. The land-covered planet is the most probable scenario ( around 80%), while our “equal mix” Earth (<1% chance) is even more unique than previously thought. Credit: Europlanet 2024 RI/T Roger.

When the authors included CO₂ outgassing in their model as well as the long-term carbonate-silicate cycle, they found only a 5 K average surface temperature difference between the land planet outcome and the ocean planet. But the scientists point out that a land planet’s climate would be considerably different from our own, accounting for the difference in flora and fauna that Spohn alludes to above: “…we would expect that the land planet has a substantially dryer, colder and harsher climate,” they write in their presentation abstract, “possibly with extended cold deserts in comparison with the ocean planet and with the present-day Earth.”

We need look no further than Earth’s geological history to see analogues. A land planet scenario produces the kind of climate that Earth would have had in the Pleistocene, while the ocean planet conditions are similar to the climate in Earth’s Paleocene. We’ll see how these numbers stack up when this work evolves out of the conference presentation stage and into a formal paper, but if they hold, the implications for habitable planet detection seem clear. We’re far more likely to find land planets and water worlds than the expected ‘pale blue dot’ signature characteristic of Earth.

The presentation is Spohn, T. and Hoening, D., “Land/Ocean Surface Diversity on Earth-like (Exo)planets: Implications for Habitability,” Europlanet Science Congress 2022, Granada, Spain, 18-23 Sep 2022, EPSC2022-506, 2022. Abstract. See also this useful overview on habitability and its geological constraints: Dehant et al., “Geoscience for understanding habitability in the solar system and beyond,” Space Science Reviews 215, 42 (20 August 2019).Abstract.