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Colors of a Habitable Exoplanet

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.

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{ 11 comments… add one }
  • henry cordova September 29, 2022, 15:40

    “Planet Earth is visible as a bright speck within the sunbeam…”

    I’ve often wondered about the “sunbeam” in that famous image. It was taken from a probe deep in space, in hard vacuum many times more tenuous than earth’s atmosphere. There should be no floating dust or aerosol available to create the “sunbeam” illusion. Could this be a lens flare or some other artifact of the imaging system? Or perhaps the spacecraft is surrounded by a thin cloud of paint flecks, droplets of lubricant or propellant, or some other similar material which would simulate dust. Can anybody answer this?

    • Ron S. September 30, 2022, 8:12
      • henry cordova October 1, 2022, 0:12

        I don’t think so.

        The ‘zodiacal light’ or ‘false dawn’ is bright, it is easily seen with the naked eye if you know exactly where and when to look, But it is brightest near the sun, and only visible a few degrees away from it (and the earth must obscure the sun), It can only be seen when the sun is close below the horizon but the sky is still totally dark. It is also oval in shape, and appears to be a narrow triangle arrayed along the ecliptic, with a fuzzy edge. Check out these photos:

        https://earthsky.org/astronomy-essentials/zodiacal-light-false-dusk-how-to-see-explanation/

        The’ sunbeam’ in the image looks like a beam, with sharp edges and
        is rectangular in outline. I think its more likely to be some kind of instrumental artifact. Then again, maybe its something really interesting…

        Incidentally, the Z.L. was the topic of Dr Brian May’s PhD thesis!

  • Geoffrey Hillend September 29, 2022, 17:46

    Quote from this paper: The authors “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. ” I don’t understand why the author’s here have come to this conclusion because it is a comparison with our Earth’s past with another Earth sized exoplanet. The Earth has a Moon, so it’s long term climate is restricted to exact Earth twins. The Paleocene was 56 to 66 million years ago. At that time, the carbon dioxide levels were over 1500ppm and there were no polar ice caps and the sea level was 210 higher. The rainfall was much greater because there was much more water vapor in our atmosphere. The climate at that was much wetter than today. Warm air always holds more water vapor than cold air in atmospheric physics, the adiabatic lapse which is proven as air rises up mountains. As the air cools due to the 5 and one half degree drop in temperature every 1000 feet of rise, the relative humidity increases as the warm air near the ground rises and cools off and cold air holds less water vapor and the water gets squeezed out of the air as it rises which is why we have rain and thunder in the mountains in the summer in California.

    Now an Earth sized exoplanet without a Moon, can’t have a strong magnetic field, so it might lose some of it’s water vapor, but probably not enough to lose all of its water if it is in the life belt at the same distance from Earth around a G class star. If exoplanets need a Moon to have plate tectonics, then there might be no carbon cycle and it would have a problem of carbon dioxide would not be recycled. The planet would be colder, and drier at some point. It might match the land planet model. The same thing would happen if the Earth sized exoplanet was outside the life belt on the snow side where Mars is. Cold water also holds less carbon dioxide, so the solubility of CO2 is less in cold water. It may be conjecture to assume that equal, Earth sized exoplanets can have a large variance in the amount of water they get from the protoplanetary accretion disk. This idea is not too far fetched, but I don’t think it is a general principle based on density. I’ll admit that I am conservative sticking to principles, but I don’t think it is wide open. Such a situation might be rare that there is a large variance of water in exoplanets. I think we must consider strictly sticking to how we define an Earth sized exoplanet’s type of system like a G class star, Moon or exact Earth twin, or Earth twin without a Moon since those contingencies might be important for the best possible chance of life evolving beyond simple single cells and reaching the planet and animal stage.

  • Robin Datta September 29, 2022, 18:57

    If by habitability one refers to a point in time when observations are made, the conditions for abiogenesis and/or seeding by panspermia could have come and passed: the current state of an inhabited planet may or may not permit ether or both.

    Camels and cacti come to mind: their ancestors would not arise in the environments they, thanks to evolution, now inhabit. And not just biological evolution, but technology also could be given consideration: a world inimical to life could be modified through what is manifestly or unmanifestly terraformng to be overtly or covertly habitable.

  • Alex Tolley September 29, 2022, 19:00

    So a quick google of land-to-sea ratios indicates that Earth was primarily a water world (2-3% ratio) 2.5 bya, and has steadily increased the ratio to today’s 28%. Ancient Earth may have been a “water world” with no dry land.

    If I understand the author’s argument, this is unusual, and either the ratio stays low, or becomes much higher over time.

    However, from the POV of life, it shouldn’t matter what the path of ratio change is, as a water world starting condition is likely the one conducive to abiogenesis. A water world with ocean depths and plate tectonics similar to Earth will end up with primarily marine life, much as our planet was before life invaded the dry land. However, small areas of land, whether as a single continent or many small islands will still have life emerging into the land. The main difference would likely be biodiversity. If the planet becomes dominantly a land planet, but perhaps with extreme climatic conditions on land, then the result may be similar, with biodiversity concentrated around the coasts and relatively absent in the interiors, much like the low diversity of hot/cold deserts.

    Either way, once life emerges, it should spread rapidly to occupy all possible niches. Once complex life emerges, perhaps aided by the evolution of oxygenic photosynthesis, then it should still be able to occupy viable niches. After all, life may show low diversity in low-productivity deserts, but animals so seem to colonize almost anywhere where there is some food availability.

    The Dehant paper suggests that life is important in the evolution the land-to-sea ratio:

    As the biosphere enhances weathering, it can be argued to play an important role in preserving the present-day Earth state with large emerged continents. An Earth-like planet without a biosphere could evolve into a water-world scenario with no or few emerging continents as suggested by Höning and Spohn (2016).

    From this, I don’t get the sense that there is a saddlepoint-like divergence of planetary evolution to the 2 extreme states, but rather that once life emerges, the evolution of the planet is regulated more by life, a regulation that goes beyond a homeostatic Gaian temperature regulation, but also contributes to the maintenance of the land-to-sea ratio which impacts climate and habitability. Shocks will occur, and their impact may be increased or lessened by the land-to-sea ratio and the biomass of life and its distribution, but life will drive the habitability to a more stable intermediate point of land-to-sea ratio as part of the climate regulation.

  • Michael Sloboda September 30, 2022, 8:34

    The colors on the illustrations are correct. If 80% land, mostly brown or yellow desert and semi-desert, with a bit of pale green. If only 5% or 10% land, all green because it’s not a great distance inland. Problem: before becoming hi-tech, ET first has to farm millions of square kilometers of land to support a large population. A few million hunter gatherers will not build a radio telescope.

    • Gary Wilson October 2, 2022, 15:47

      Or the inhabitants could possibly do a huge amount of aquaculture and fishing Michael. The ocean would hopefully be teaming with life.

  • Brett September 30, 2022, 12:31

    10% dry land with another 10% in submerged continental shelves during warm periods is still a pretty decent amount of land. That would be about 51 million square kilometers of land, or basically slightly smaller than Eurasia (and about the size of North America, South America, and Australia together).

    Unless it’s in that planet’s version of a supercontinent, too, odds are that the greater oceans will heavily moderate climate on the existing land, making a milder and easier world for life.

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