Image from NASA
Have you ever played a fairly complex new game where every time you thought you had figured it out, they told you about a new rule that made it more difficult? Maybe you thought it would be an easy game, but by the time you found out about all of the rules, it seemed impossible to win. Looking for planets (or more accurately exoplanets – planets orbiting another star) that might host advanced life seems like this. How many exoplanets in our galaxy or the universe are habitable depends directly on how many requirements one places on the criteria for being “habitable”. Does habitable mean that the planet can potentially support bacteria? If so, perhaps many, many are “habitable”. If by habitable, we mean that it is stable enough and with all of the vital characteristics necessary for advanced carbon-based (multi-cellular) life, that is another matter.
NASA reports “The standard definition for a habitable planet is one that can sustain life for a significant period; based on our solar system, life requires liquid water, energy, and nutrients.” Perhaps in more detail, Wikipedia reports, “In its astrobiology roadmap, NASA has defined the principal habitability criteria as “extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism”.” That would perhaps allow bacteria to thrive, but more advanced and more complex life is more demanding.
We need the right elements and temperature range. Let’s imagine we have a planet with liquid water and all of the other right chemicals. It even has a plate tectonic system to keep the water liquid. Does that mean that the planet would support advanced life? If some sort of process driven development is expected, then the exoplanet needs to have been at least somewhat stable for billions of years. There are just so many ways for the planet to become inhospitable for advanced life. Scientists recognize that advanced carbon-based life like our planet has in abundance would never flourish under some climate conditions. Climate study is a hot topic for scientific study on this planet. It also is an additional constraint of the habitability of exoplanets. Many circumstances might cause an exoplanet to have climate conditions that would be inhospitable for advanced life.
Macdonald, et al., 2022 reported that they have developed simulation models that can test some of the constraints that we can predict would limit an exoplanet’s habitability (Macdonald et al. 2022; Sci-News 2022). In particular, they model “M-Earths”, rocky planets of roughly Earth size that rotate around dwarf stars. This configuration gives astronomers a better chance to observe them and learn at least some characteristics of them: the planet’s mass, radius, and atmospheric composition. Given these, we might be able to infer more about an M-Earth. This might in turn help us infer more about other types of exoplanets. The Macdonald article proposes that they may eventually be able to infer more about the exoplanet. Would it be possible to further constrain such an exoplanets habitability?
A Mind behind the Universe and a Deck of Cards
They report,
“In this study, we systematically vary dayside land fraction and configuration on an M-Earth using a large set of 3D climate simulations. We find that land has little effect on the general circulation, but can cause large changes in temperature and humidity. Our work highlights the need for detailed surface conditions in M-Earth climate models”
They also say:
” We find that the amount and configuration of land can change the planet’s globally averaged surface temperature by up to ∼20 K, and its atmospheric water vapour content by several orders of magnitude. The most discrepant models have partial dayside land cover with opposite continent configurations. Since transit spectroscopy may permit observations of M-dwarf planets’ atmospheres, but their surfaces will be difficult to observe, these land-related climate differences likely represent a limiting uncertainty in a given planet’s climate, even if its atmospheric composition is known.”
Large variations in temperature and humidity mean that some configurations between land and water will be inhospitable for complex life. This is just one more way that limits the number of exoplanets that might host advanced life.
Science is at a real disadvantage because they really have no model for how life originates, even under the ideal conditions of a laboratory, let alone in nature. Advanced life, from a naturalistic standpoint, would require conditions far different from some hypothetical condition for abiogenesis even if we understood what that was. Intelligent life of any possible form would have changed the conditions further. It is difficult to envision a scenario where we will learn enough details to confirm advanced life or its absence on a significant number of exoplanets. Even more limiting than our ability to learn details about the planet is just how rare such a planet would be given what we know about the requirements that such a place would have to meet to even be an option. We continue to find new ways that the universe can be dangerous and inhospitable to life. How many exoplanets would we have to study before we would happen on one that could host advanced life? Perhaps all of them.
We continue to see how special Earth must be. The contrast between Earth with its extravagant variety of life in virtually every environment with the barren harsh surfaces of the planets we know is huge. Work like this study shows that even if there is abundant liquid water on the surface, many things can go wrong that would make it impossible for life to flourish there.
References:
Macdonald, Evelyn, Adiv Paradise, Kristen Menou, and Christopher Lee. 2022. “Climate Uncertainties Caused by Unknown Land Distribution on Habitable M-Earths.” Monthly Notices of the Royal Astronomical Society 513 (2): 2761–69. https://doi.org/10.1093/mnras/stac1040.
Sci-News. 2022. “Study: Location and Amount of Land on Planet’s Surface Can Significantly Impact Its Habitability.” Sci.News: Breaking Science News. July 12, 2022. https://www.sci.news/astronomy/terrestrial-planet-habitability-10992.html.
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