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Thread: 24 Superhabitable Exoplanets

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    24 Superhabitable Exoplanets

    scitechdaily

    Earth is not necessarily the best planet in the universe. Researchers have identified two dozen planets outside our solar system that may have conditions more suitable for life than our own. Some of these orbit stars that may be better than even our sun.

    A study led by Washington State University scientist Dirk Schulze-Makuch recently published in the journal Astrobiology details characteristics of potential “superhabitable” planets, that include those that are older, a little larger, slightly warmer and possibly wetter than Earth. Life could also more easily thrive on planets that circle more slowly changing stars with longer lifespans than our sun.

    The 24 top contenders for superhabitable planets are all more than 100 light years away, but Schulze-Makuch said the study could help focus future observation efforts, such as from NASA’s James Web Space Telescope, the LUVIOR space observatory and the European Space Agency’s PLATO space telescope.
    paper

    Abstract:
    The fact that Earth is teeming with life makes it appear odd to ask whether there could be other planets in our galaxy that may be even more suitable for life. Neglecting this possible class of “superhabitable” planets, however, could be considered anthropocentric and geocentric biases. Most important from the perspective of an observer searching for extrasolar life is that such a search might be executed most effectively with a focus on superhabitable planets instead of Earth-like planets. We argue that there could be regions of astrophysical parameter space of star-planet systems that could allow for planets to be even better for life than our Earth. We aim to identify those parameters and their optimal ranges, some of which are astrophysically motivated, whereas others are based on the varying habitability of the natural history of our planet. Some of these conditions are far from being observationally testable on planets outside the solar system. Still, we can distill a short list of 24 top contenders among the >4000 exoplanets known today that could be candidates for a superhabitable planet. In fact, we argue that, with regard to the search for extrasolar life, potentially superhabitable planets may deserve higher priority for follow-up observations than most Earth-like planets.
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    Prepare The Starships!

    Let's go.

    May as well start the plannin now.
    Last edited by John Mendenhall; 2020-Oct-06 at 07:03 PM. Reason: typo

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    Time To Leave Low Earth Orbit

    Quote Originally Posted by John Mendenhall View Post
    Let's go.

    May as well start the plannin now.
    More seriously, 24 potentially superior exoplanets is the good news of a lifetime. There must be many many more. What spectacular goals for future generations!

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    Quote Originally Posted by John Mendenhall View Post
    Let's go.

    May as well start the plannin now.
    Well, better for life does not mean better for human life, we evolved for Earthly conditions. We probably couldn't compete with super-natives.
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    I Ain't Young, But I Can't Pass This Up

    Quote Originally Posted by Noclevername View Post
    Well, better for life does not mean better for human life, we evolved for Earthly conditions. We probably couldn't compete with super-natives.
    NCN, thank you, good point.

    I don't want to hijack this thread. I need to do a little research, then I'll start another thread, and I will post link here, I hope, if all goes well.

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    Quote Originally Posted by Noclevername View Post
    Well, better for life does not mean better for human life, we evolved for Earthly conditions. We probably couldn't compete with super-natives.
    Heh. This superhabitability concept made me think of a web story series (free with different authors) that has a fun conceit, but overall isn’t that great (in my opinion at least). Essentially, most inhabited worlds have largely compatible biochemistry, but compared to Earth, most planets are gardens of Eden. Earth is considered a deathworld. Humans are physically stronger and tougher than almost all aliens. We’re also better at fighting. Viruses and bacteria that don’t hurt us will quickly kill aliens (which have far less sophisticated immune systems). Humans can eat almost anything from other planets, but aliens would be poisoned by various toxins in plants and animals that humans have evolved to eat. Just the pollen or fragrance from various plants that are pleasant or at worst would cause a mild allergy in humans are extreme biohazards to aliens.

    There is one story where a woman is captured by aliens and gets free on an alien planet. For reasons I don’t recall, she finds out there is a base or station thousands of miles from her current location, so she sets out to walk there. Food (both plant and animal based) is plentiful and tasty, weather is always pleasant, gravity is lower than Earth’s, carnivores are weak and pose no danger to humans, and animals are easy to catch. There are no deserts, no harsh areas, the world is superhabitable.

    But what she doesn’t realize is that after she has moved on, at every place she relieved herself all life in the area starts dying because the native life has no defense against gut bacteria. She is a WMD for that world. Humans in another ship eventually show up and can tell from orbit where she has been by the path of destruction. Ultimately, she inadvertently destroys almost the entire ecology, and humans take the planet over as a colony world, terraforming it, since no aliens could live there without heavy protection.

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    The idea behind 'superhabitability' seems to be that we should be able to find a number of planets in orbit around K-type stars which are a little bit larger a little bit warmer and a little bit damper than Earth. I don't think these words would be paradise-like; indeed, they'll have heavier gravity, and the atmosphere would be thick, warm, and damp. But what they would have as an advantage over Earth is longevity. Our complex biosphere is likely to last a billion years, maybe a bit longer; the complex biosphere on a superhabitable world could persist for tens of billions of years.

    Humans probably wouldn't thrive on a superhabitable world without some fairly radical genetic, technological or medical adjustments, but we are a very adaptable species.

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    Quote Originally Posted by Van Rijn View Post
    Heh. This superhabitability concept made me think of a web story series (free with different authors) that has a fun conceit, but overall isn’t that great (in my opinion at least). Essentially, most inhabited worlds have largely compatible biochemistry, but compared to Earth, most planets are gardens of Eden. Earth is considered a deathworld. Humans are physically stronger and tougher than almost all aliens. We’re also better at fighting. Viruses and bacteria that don’t hurt us will quickly kill aliens (which have far less sophisticated immune systems). Humans can eat almost anything from other planets, but aliens would be poisoned by various toxins in plants and animals that humans have evolved to eat. Just the pollen or fragrance from various plants that are pleasant or at worst would cause a mild allergy in humans are extreme biohazards to aliens.

    There is one story where a woman is captured by aliens and gets free on an alien planet. For reasons I don’t recall, she finds out there is a base or station thousands of miles from her current location, so she sets out to walk there. Food (both plant and animal based) is plentiful and tasty, weather is always pleasant, gravity is lower than Earth’s, carnivores are weak and pose no danger to humans, and animals are easy to catch. There are no deserts, no harsh areas, the world is superhabitable.

    But what she doesn’t realize is that after she has moved on, at every place she relieved herself all life in the area starts dying because the native life has no defense against gut bacteria. She is a WMD for that world. Humans in another ship eventually show up and can tell from orbit where she has been by the path of destruction. Ultimately, she inadvertently destroys almost the entire ecology, and humans take the planet over as a colony world, terraforming it, since no aliens could live there without heavy protection.
    That's a refreshing change from Kal-El and Spock and all the super-aliens found in the usual SF clichés. Writers so want to make their ETs "cool" that they always end up making humans the Scrappy Underdogs.
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    Quote Originally Posted by Noclevername View Post
    That's a refreshing change from Kal-El and Spock and all the super-aliens found in the usual SF clichés. Writers so want to make their ETs "cool" that they always end up making humans the Scrappy Underdogs.
    That's actually something I thought about recently, similar to the novel "Gulliver's Travels" I imagined humans visiting different worlds only to discover they were bigger, stronger more powerful than the aliens on one alien world. Then find themselves smaller, weaker and less powerful on the next.
    If we ever discover life on other worlds it will be interesting to see if there would be potentially lethal bio/chemical hazards to life as we know it but find the alien life thriving in that environment, obviously this assumes that we only consider bio chemical life.

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    Easing A Heavy Burden

    Quote Originally Posted by eburacum45 View Post
    The idea behind 'superhabitability' seems to be that we should be able to find a number of planets in orbit around K-type stars which are a little bit larger a little bit warmer and a little bit damper than Earth. I don't think these words would be paradise-like; indeed, they'll have heavier gravity, and the atmosphere would be thick, warm, and damp. But what they would have as an advantage over Earth is longevity. Our complex biosphere is likely to last a billion years, maybe a bit longer; the complex biosphere on a superhabitable world could persist for tens of billions of years.

    Humans probably wouldn't thrive on a superhabitable world without some fairly radical genetic, technological or medical adjustments, but we are a very adaptable species.
    And a little bit less dense.

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    Super-Habitable is a misnomer IMO. It assumes there is an absolute standard of "habitability".

    There may be absolute physical constraints needed for the initial biogenesis (or not), but everything after that is evolution. Life either adapts to conditions or it doesn't. And we only have one example of both biogenesis and a habitable environment, and our life works specifically on Earth. A world with other life, would have life evolved specifically for those conditions. Which would, as all things do change over time.

    Our planet was a hell world for most of its existence... by the standards of current oxygen breathing life forms. Now, it's super habitable... for us. Life evolved to meet the changes.

    We flow like water into new shapes of container. There's no ideal shape of water jug. There are a few bad ones with no way to get the water out, but there's no "best" super-jug.
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    Quote Originally Posted by eburacum45 View Post
    The idea behind 'superhabitability' seems to be that we should be able to find a number of planets in orbit around K-type stars which are a little bit larger a little bit warmer and a little bit damper than Earth. I don't think these words would be paradise-like; indeed, they'll have heavier gravity, and the atmosphere would be thick, warm, and damp. But what they would have as an advantage over Earth is longevity. Our complex biosphere is likely to last a billion years, maybe a bit longer; the complex biosphere on a superhabitable world could persist for tens of billions of years.
    I've read arguments like this before, this seems to amount to finding actual candidates for it. It's been argued that K dwarf stars are ideal for life, being less temperamental than M dwarfs and allowing wider orbits for habitable planets than M dwarfs, but are longer lived than G dwarfs, so allowing longer habitability times for planets. F dwarfs on the other hand would have short habitability times for planets, with the star's luminosity increasing much more rapidly than our sun's.

    Likewise, superearths would have active geology longer than Earth, could maintain a magnetic field for a longer time, and could better retain atmosphere both because of the magnetic field and mass.

    I understand it isn't like the story reference I mentioned, it just reminded me of the conceit in the stories (if anyone is curious, look up "The Deathworlders" - there are a number of authors with varying story quality and I think the idea is better than the execution, but it can be amusing at times).

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    Hi-grav worlds with thick, warm, damp atmospheres strike me as being rich sites for decay organisms. Yeast-analogs, parasitic plant-analogs, fungus-analogs, scavengers, so-on and so-forth. These places might persist for billions of years, but they may be icky places to live.

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    So they would be like the "Golden Age" Venus?
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    Apart from the higher gravity, I suppose so.

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    Quote Originally Posted by Noclevername View Post
    Well, better for life does not mean better for human life, we evolved for Earthly conditions.
    Very important distinction!

    Quote Originally Posted by Noclevername View Post
    Super-Habitable is a misnomer IMO. It assumes there is an absolute standard of "habitability".

    There may be absolute physical constraints needed for the initial biogenesis (or not), but everything after that is evolution. Life either adapts to conditions or it doesn't. And we only have one example of both biogenesis and a habitable environment, and our life works specifically on Earth. A world with other life, would have life evolved specifically for those conditions. Which would, as all things do change over time.

    Our planet was a hell world for most of its existence... by the standards of current oxygen breathing life forms. Now, it's super habitable... for us. Life evolved to meet the changes.

    We flow like water into new shapes of container. There's no ideal shape of water jug. There are a few bad ones with no way to get the water out, but there's no "best" super-jug.
    Don't different environments support different levels of biomass and biodiversity? Isn't that a basis for distinguishing between marginally habitable, medium habitable, and super habitable?

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    Quote Originally Posted by Noclevername View Post
    Super-Habitable is a misnomer IMO. It assumes there is an absolute standard of "habitability".

    There may be absolute physical constraints needed for the initial biogenesis (or not), but everything after that is evolution. Life either adapts to conditions or it doesn't. And we only have one example of both biogenesis and a habitable environment, and our life works specifically on Earth. A world with other life, would have life evolved specifically for those conditions. Which would, as all things do change over time.
    "Or it doesn't" seems like the standard of habitable though, right?

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    Quote Originally Posted by Noclevername View Post
    Life either adapts to conditions or it doesn't. And we only have one example of both biogenesis and a habitable environment, and our life works specifically on Earth. A world with other life, would have life evolved specifically for those conditions...

    Life evolved to meet the changes.

    We flow like water into new shapes of container.
    That is not accurate. Earth has deserts and ice fields and tundras with little or no life. Even a grassland has more life than those, and life in grasslands is relatively suppressed compared to a forest, especially a rainy one. So the variety of ecosystems on this planet alone shows not only the fact that some conditions are easier or harsher for this planet's life to adapt to (otherwise it would be equally well adapted to them all), but also an immediate idea of what some of the biggest restrictors are. The main limiting factors here are cold temperatures and shortage of water. That means that a planet that's otherwise like Earth but without such cold cold spots or such dry dry spots would have more forest & swamp area, and thus not only more life but also more diversity of life. (And just making the land masses smaller would have that effect all by itself, without even thinking about messing with stuff like the total size, star type, or orbit.)

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    Quote Originally Posted by eburacum45 View Post
    Apart from the higher gravity, I suppose so.
    Ya, I'm not getting the part where higher planetary mass is expected to help.

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    Quote Originally Posted by Delvo View Post
    Ya, I'm not getting the part where higher planetary mass is expected to help.
    With habitability? It has to do with how long a world could remain habitable. A world like Mars, at an ideal distance from its star, doesn’t have the mass to retain a thick atmosphere long term. It also loses internal heat more rapidly, so has a shorter period of active geology. One effect is that it can’t maintain an active dynamo so will lose a magnetosphere, which also affects how an atmosphere is retained. But another factor is that many elements that are needed for life don’t get recycled by the geology, and just end up buried at the lowest elevations, so life would become much more sparse over time even if the atmosphere and water remain. A planet like Earth has some of the same issues, it just takes longer. In our case, the sun’s increasing luminosity will likely be more important in affecting future habitability.

    A superearth combined with a K dwarf star that can remain on the main sequence much longer than our sun could potentially remain habitable much longer than Earth.
    Last edited by Van Rijn; 2020-Oct-11 at 07:36 PM.

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    Quote Originally Posted by Delvo View Post
    That is not accurate. Earth has deserts and ice fields and tundras with little or no life. Even a grassland has more life than those, and life in grasslands is relatively suppressed compared to a forest, especially a rainy one. So the variety of ecosystems on this planet alone shows not only the fact that some conditions are easier or harsher for this planet's life to adapt to (otherwise it would be equally well adapted to them all), but also an immediate idea of what some of the biggest restrictors are.
    But life still adapts to those harsh conditions, and suffers an imbalance outside them until it adapts again. For instance, invasive species seem to do well in their new environment, but they overpopulate and use up resources that would otherwise be sufficient. They can damage or destroy an ecosystem. It's not all about quantity.
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    Water isn't the only volatile that needs to be conserved for a habitable planet. Carbon dioxide also needs to be present in some quantity in order to support Earth-like plants and photosynthesis. In less than a billion years the carbon dioxide levels on Earth wiill become so low that plants will have great difficulty finding carbon to extract from the atmosphere. I suppose a superhabitable planet might have much greater carbon dioxide levels at various points in its history, but the levels would probably diminish over time just like on Earth. I wonder what the carbon cycle would look like on such a world.

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    Quote Originally Posted by eburacum45 View Post
    Water isn't the only volatile that needs to be conserved for a habitable planet. Carbon dioxide also needs to be present in some quantity in order to support Earth-like plants and photosynthesis. In less than a billion years the carbon dioxide levels on Earth wiill become so low that plants will have great difficulty finding carbon to extract from the atmosphere. I suppose a superhabitable planet might have much greater carbon dioxide levels at various points in its history, but the levels would probably diminish over time just like on Earth. I wonder what the carbon cycle would look like on such a world.
    I thought the argument for Earth was that co2 would need to drop with increasing luminosity or temperature would go up? Eventually I believe it’s thought the Earth would become like Venus, because there is plenty of carbon that could go into the atmosphere, and eventually it would no longer be controlled by photosynthesis. Presumably a planet in orbit around a K dwarf would see luminosity increase more slowly, so the problem would take longer to occur. Is there something I’m missing?

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    I thought the argument for Earth was that co2 would need to drop with increasing luminosity or temperature would go up?
    Yes, that's the argument, the Gaia Hypothesis. But there's no real causal link between the presence of life and the maintenance of a comfortable temperature - it is just a happy circumstance which need not persist into the deep future.

    CO2 levels will diminish to nearly useless levels, then the seas will evaporate away despite the absence of carbon dioxide (this time due to the increasing luminosity of the Sun), and finally the carbon dioxide will be released once more after the Earth is dry. These are the conditions a superhabitable world needs to avoid somehow.

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    Quote Originally Posted by eburacum45 View Post
    But there's no real causal link between the presence of life and the maintenance of a comfortable temperature - it is just a happy circumstance which need not persist into the deep future.
    [citation needed]
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    One problem with the carbon cycle on Earth is that any carbon subducted into the mantle can 'settle' towards the bottom of the mantle, and become separated from the crust layers and the atmosphere in general. So the amount of carbon dioxide that is outgassed will reduce over time, eventually making plant-life problematic. Even if the GAIA hypothesis has been reliable in the past history of the Earth, there is no guarantee that this will continue to operate in the future, and it probably won't.

    If a planet is to be be considered 'superhabitable', it will need to have active mixing between the different layers of the mantle, soi that carbon dioxide is outgassed for a much longer period.

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    It would be good to get more evidence of these and other potential planets in this category to help inform theories about the type of life which may or may not thrive on them. We only have basic details even on these 24. Only 2 of them are actually confirmed planets! And the authors specifically state their point is not to identify targets for follow up observations but to illustrate the principle that superhabitable worlds may have already been detected.

    Maybe it would be good to identify a top maybe a top 5 or 10 potential targets and focus on them. Or is it still too soon even for that...?

    We caution that we do not have any observational signatures of life from any of these planets. In fact, only Kepler 1126 b (KOI 2162) and Kepler-69c (KOI 172.02) are statistically validated planets (Morton et al., 2016). The other objects are unconfirmed Kepler Objects of Interest (KOIs), some of which may turn out to be astrophysical false positives. Even Kepler-69c, whose planetary status has been statistically established, will likely not be the target of future follow-up observations with the James Webb Space Telescope or its successor, potentially LUVIOR. At a distance of almost 2000 light years, it is simply too far away. Our point here is not to identify potential targets for follow-up observations but to illustrate that superhabitable worlds may already be among the planets that have been detected.

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