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How to design a weird but comfortable planet

  1. Mar 22, 2010 #1
    Hi all,

    I'd like to write a story whose setting is a planet that is, on the one hand, hospitable to earth-like flora and fauna, while on the other hand being as different as possible from Earth in as many respects as possible. In other words, which parameters can I fiddle with without greatly affecting surface conditions. Not exactly a novel idea, per se - one might say one has to look hard for sci-fi in which extrasolar planets don't have an extra sun or moon or two... but I'd like to go a bit deeper than that, while approaching the matter as scientifically as possible.

    What I'm looking for are both new ideas and feedback on the ones I've come up with so far.

    1. The basic physical characteristics (size and density) of the planet are more or less fixed, it seems to me, by the requirement that composition, gravity and atmosphere have to be earth-like.

    2. As concerns orbital and rotational dynamics, temperature (both average and variations) is of course the main concern as far as habitability is concerned. The following assumes that the sun is the dominant source of energy for the planet, which seems inevitable to me. In the case of Earth solar radiation contributes almost ten orders of magnitude more than internal effects like radioactive decay, and it seems unlikely that any non-negligible process that Earth is lacking could compete.
      • Rotational period: The length of the day determines the temperature fluctuations between day and night. Assuming similar heat buffering and heat transfer processes as those on Earth, I'm thinking I can't make the day significantly longer than the terrestrial one. However, shortening it doesn't seem to present any immediate problems. What would be the limiting factor here? Coriolis storms? Tides (depending on satellites, obviously)?
      • Axial tilt: Main driving force for seasonal change. Decreasing it from Earth's value towards zero makes things more and more boring. What are the effects of increasing it, in detail? If I've got this right, the extreme case of a 90-degree-inclination should mean that the annual average temperature at the poles and at the equator is more or less equal: The former still experience a single "day" and "night" each year, but the sun approaches the zenith during that day, which would make a radical difference. The latter experiences mild but definite winters during the polar days, as the sun doesn't climb far above the horizon. How about intermediate latitudes and inclinations?
      • Orbital period: Fairly tightly bounded by the solar mass, which determines the solar luminosity, hence the habitable zone, and hence the orbital radius. Obvious limits here are imposed by standard stellar evolution - too heavy, and the sun is too short-lived for planet formation, too light, and nuclear fusion never ignites at all. Are there narrower limits? Is a Sun-like ("yellow") spectrum particularly suitable for earth-like life?
      • Orbital eccentricity: This one seems quite interesting to me. A planet with small axial tilt and large orbital eccentricity would have a global summer during perihelion and a global winter during aphelion, accompanied by a visible change in solar size. I'm not quite sure how to figure this one out quantitatively, though. My back-of-the-envelope calculation went like this: Earth's temperate latitudes receive something like four times as much incident solar energy during summer as during winter (days twice as long and sun twice as high), which by the black body law predicts a 40% change in temperature (J ~ T^4). The actual change in temperature is at most 10% (30 K) for coastal and 20% for continental climates, so heat buffering and transfer processes must be taken into account. At least the latter of these would work very differently for eccentricity-driven seasons, though, so... non-trivial.
        I found http://discovermagazine.com/2002/nov/featcircles" [Broken] which gives some modelling results, at any rate.

    3. One of the more obvious considerations is to use a binary star system rather than a single sun. It can either be a distant binary in which the planet circles one of the two stars, but that option doesn't seem all that interesting: The second star has to be quite far away for the planetery orbit to remain stable, so it would primarily be a visual addition to the sky with little influence on the planetary conditions. Or, it can be a tight binary in which the planet circles the centre of mass of the system. Is it feasible for a planet to survive the death of one its suns? If so, the tight binary could consist of a normal star and a compact object, with mass transfer between the two via an accretion disk, etc. Though I suppose one would have to look at the radiative conditions in such a system, which might not be all that healthy... magnetic fields to the rescue, maybe? Other problems with this scenario?

    Okay, that's where I'm at, for now.

    TIA for any constructive replies! :)

    ETA: x-posted @ http://www.astronomyforum.net/gener...-how-design-weird-but-comfortable-planet.html, http://cs.astronomy.com/asycs/forums/t/45709.aspx
    Last edited by a moderator: May 4, 2017
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  3. Mar 22, 2010 #2


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    Maybe you can try to modify those universal dimensionless constant such as the fine-structure constant?
    Keeping such constant the same, but slightly changing the parameters associated with it, like making the electrostatic charge stronger, at the same time increasing the speed of light. The constant remains unchanged, the world is still allowed to survive, but it won't be the same :)
  4. Mar 23, 2010 #3
    I really know next to nothing about astrobiology, but I'm assuming neither does the rest of the human race considering there is no hard evidence to suggest there is life elsewhere. However, would it be conceivable to assume that life on planets with a slower rotation and longer orbital period would have longer lifespan? If life evolved according to the rotation of Earth and there were planets with longer rotations, would they live longer? Obviously, if the rotational period is too long they would bake during the day and freeze at night. However, we know that life on earth can survive in very extreme conditions.
  5. Mar 23, 2010 #4

    i'm relatively certain it doesn't work that way, at least not for mild variations. actually, humans, especially in the ages under 30, prefer a 25 hour day. (i knew psych 105 would come in handy one day!)
  6. Mar 23, 2010 #5
    You mean if you transplanted life from Earth to a planet with a longer day/year, yes? My guess would be that most higher life forms would live shorter, if anything, because their biological rhythms are (more or less, see previous post) adjusted to Earth's periodicities, so major changes would be unhealthy. The statement should really read "life on Earth can evolve in very extreme conditions" - most of it doesn't cope well at all with conditions that lie outside its accustomed range, whatever that range happens to be.
    An exception might be creatures that habitually die of other factors than old age, in one form or another, like an insect species that mates a fixed number of days after hatching, with the males dying immediately afterwards. Again, though, I'd guess that one quickly reaches a point where some direct or indirect consequences of such a change become too major for the organism to adapt.
  7. Mar 23, 2010 #6
    Sorry I see why that could be confusing. Actually, I was using Earth as a reference in that life on a planet would evolve according to the planets rotation, such as being active during the day and sleeping at night or vice-versa. So if planet had a dramatically different rotational period, would this still be the case? I can see why having shorter or longer days wouldn't necessarily affect total lifespan, though.
    Last edited: Mar 23, 2010
  8. Mar 23, 2010 #7
    That's a very good question that I've briefly thought about before without coming to a conclusion either way. In trying to imagine what evolution would produce given different planetary conditions as a playing field, one always has to walk the fine line of not taking anything for granted on the one hand and not letting the imagination run amok on the other. Vision, as in an organism's ability to detect EM radiation near the peak of the solar spectrum, seems such an obvious advantage that it's hard to imagine higher life forms developing without it. And a selection pressure similar to that of Earth then naturally leads to diurnal and nocturnal behaviour, it seems to me.
    I don't see any problems with adaptation to a somewhat longer day in that respect, staying awake for a week and then sleeping for a week shouldn't even require major physiological changes. In the other direction, things are more interesting IMO. It seems questionable that e.g. predators of Earth-like size could structure their life according to a 1-hour day, for example, because hunting is a complex activity that simply requires a certain minimal amount of time. And while smaller creatures would be able to live faster, higher organisms do require a certain minimal size.

    That's as far as my thinking went on this matter, anyway...
  9. Feb 18, 2012 #8
    The idea of a global winter and summer is interesting. I wonder though if the CO2 levels would make this difficult though.

    Remember how Appollo 13 one of the big problems was the CO2 levels rising? It only takes around 1000 ppm CO2 I think for humans to start feely stuffy, I forget what the leathal dose is. When I ran some numbers myself, I was surprised how quickly the CO2 levels reach uncomfortable levels with a human in an enclosed envirnment. The figure I found gave 1 kg / person / day.

    Biosphere 2 also noted that the CO2 levels steadily rose during the night (photosynthesis stops but animal and human respiration continues) and then steadily dropped during the day as the plants caught back up. (Watch out for those cloudy days!) With a global winter, would you have a similar problem with less photosynthesis during the winter months? If the atmosphere was large compared to the plant & animal life, it could be a sufficent buffer.
  10. Feb 19, 2012 #9
    A figure-8 orbit around both stars of a binary system? Impractical. I once tried looking for one, but I could not find a stable one. The planet would either escape or orbit only one of the stars.

    However, a planet's orbit will be stable if it is relatively close to one of the stars or relatively far from both. For the stars being separated by R and the planet's orbit size by r, then orbiting one star is stable if r <~ R/3, and orbiting both stars is stable if r >~ 3R.

    The weirdest environment that can nevertheless have an Earthlike planet is, I think, the planet being a satellite of a more massive planet, like a Jupiter-like planet. The planet will likely become tidally locked to it, with one side always facing it. This is what has happened for most of the larger planetary satellites in the Solar System, like the Earth's Moon.

    The planet will show phases, just as the Moon does from the Earth.

    If the planet has rings, then those rings will be seen edge-on, where they will make a thin line. However, the rings will cast shadows on the planet, and with thick enough rings and a large enough spin-orbit inclination, those shadows should be easily visible.

    The planet will eclipse the star, and the satellite might cast a visible shadow on the planet.

    The planet will likely have several satellites that are visible from this Earthlike satellite, making for some interesting celestial phenomena.
  11. Feb 20, 2012 #10


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    Aanar and lpetrich thanks for your input but this thread is nearly two years old and thus too old to revive.
  12. Feb 20, 2012 #11

    Vanadium 50

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    And been done before, by Brian Aldis in Helliconia Spring. Note that this thread is two years old, and the OP has been gone for over a year.
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