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I need to build a planet. Care to help?

  1. Jul 24, 2014 #1
    I know, it sounds a little more epic than it really is. It is true, however. I am in the process of writing a story and would very much like to have the specifics of the other planet described at one point to the reader so they can get a grasp on the world part of the story is taking place in. Now while I could just use the the "it's magic" to describe the planet, I think a lot of people would get a kick out of getting a small glimpse on how a planet like this might be able to exist right here in our own universe.

    I know jack all about cosmology though. Except that most planets may or may not have something that resembles a rock like substance. Unless it is made of something else, then it wouldn't be made of rock anymore. Some if you have the time, patience or imagination, I would appreciate the help.

    Here is a list of specifics I would like to see in this world, along with some general questions.
    → This planet is roughly the size of Jupiter
    → This planet has an Earth like environment that is capable of supporting all kinds of life from mammal, reptilian, avian, microscopic, bacterial, etc etc.
    → This planet does have large bodies of water, land masses and expansive crystaline structures and outcroppings, some even large enough to see from long distances.

    Some general questions I would like to know.
    → What would the general day and night cycle be?
    → How many hours would be in a full day?
    → What would be the general length of the seasons if the seasons adhered to the standard four being: spring, summer, fall, winter?
    → Where in the solar system would this planet have to be placed to maintain it's size, but still be able to support life like Earth does?
    → How many moons would it need to have to affect the tides much like our moon does, but not yank everything off the planet due to gravitational shears?

    With most or all of those questions answered, and being backed by science and the imagination, I think I could construct a planet both wonderful and realistic for the story to take place on. So, any help would be appreciated.
  2. jcsd
  3. Jul 24, 2014 #2


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  4. Jul 25, 2014 #3


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    I don't think such a thing is feasible, but it doesn't matter for science fiction writing I guess.

    A planet the size Jupiter would need to a low density to support vertebrate life. I'm not sure how this could be accomplished.

    The planet would need to rotate fast enough to support a dynamo effect, and not expose any one side to wild tempature differences.

    You would need an axial tilt for seasons, a large satellite would help stabilize such a thing.

    Not sure.
  5. Jul 25, 2014 #4


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    You need to look up the DENSITIES of the various solar system bodies. As I recall the density of Earth is around 5, moon and mars are more like 3 (three times density of water)

    I think it would be hard to have an Earth-like environment if it didn't have an iron core like earth. and was mostly rock. You need a magnetic field to protect you from the solar wind, which otherwise ionizes and gradually strips away your atmosphere and moisture.

    Nothing but rock would mean a density of around 3 (like moon and mars).

    rock plus some iron core would mean more like density 5 (like Earth).

    You should decide first of all on the average density of your planet. That would be my suggestion.

    Once you have decided what the density is, then different sizes will have different strength gravity.

    For example, if you decide to have the same density as Earth, then doubling the radius will double the strength of gravity at the surface. Tripling the radius will triple the strength of SURFACE GRAVITY, so things will weigh 3 times as much.

    so once you have chosen a density for your planet then you can play around with different sizes, different radii or diameters, and see what you like by way of sizes.
    Last edited: Jul 25, 2014
  6. Jul 25, 2014 #5


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    What marcus described boils down to this simple equation for gravity(g), density(ρ), radius(R) relationship:
    All values are fractions of Earth's. "1" means the same as on Earth.

    For example, you start with radius being about ten times that of Earth(size of Jupiter), R=10. If you want the gravity to be the same as on Earth, g=1, then you get ##1/10=ρ##, so the density needs to be 1/10th of Earth's average density. That's about half the density of water, so you can't have a rocky surface.
    If you keep the size of Jupiter(R=10), and set the density to Earth's(ρ=1), you end up with gravity being 10 times higher ##g=1*10##
  7. Jul 25, 2014 #6
    If you want a Jupiter-sized planet with the possibility of an Earth-like environment you could have a supramundane planet - a habitable shell built around a gas giant.
  8. Jul 25, 2014 #7
    Seasons depend on axial tilt, and a high eccentricity would also cause season-like effects, though they would be global. The rotation of the planet would result in a day-night cycle.

    As for a planet the size of Jupiter having Earth-like life... Very improbable. If an astronomical object has enough mass, it will retain primordial hydrogen and helium, which is what makes bigger planets gas planets. If a rocky planet were to be as big as Jupiter, the result would be the planet having a massive gravity, far greater than that of "our" Jupiter.

    A planet could also be made out of another very common substance: water, but since it is far less common in the universe than H2/He I don't think such worlds would reach Jupiter-size very often. The world in question wold also be panthalassic, i.o.w it would be a world spanning ocean with a minimum depth of thousand of kilometers...

    Look up the concept "supramundane planet". It is an artificial construct, so it wouldn't occur naturally, but it gives the theoretical possibility to have world far bigger than the Earth but with Earth-like gravity, atmosphere, surface, life... I'm particularly fond of the pressure-shell variety, which though it would require herculean efforts and a massive industrial capacity doesn't require unobtainium nor technology approaching magic.
  9. Jul 26, 2014 #8
    Shouldn't those moons be in a resonance?
  10. Jul 26, 2014 #9
    Lots of great information here for sure. Thanks, guys, for helping me out! All of this info will come in handy at some point.
  11. Aug 20, 2014 #10


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    I had similar problems a few years back when I was designing a planet for a race in a story I was writing. That planet was only 4 times the size of Earth and yet there was no way I could get it to work.

    At 10 times the size of Earth it would still have an oppressive gravity regardless of what substance you made it out of. If you think about it, Jupiter's composition is mostly Hydrogen, extremely low density and yet its gravity and pressure are murderous to life as we know it.
    On Jupiter the pressure is so enormous at the core that the Hydrogen actually takes on metallic properties. If the planet were composed mostly of silicates, I cant even speculate what that much pressure and heat would do to them. Certainly the elements of that planet would be transformed into something unlike anything we have on Earth. I cant imagine how a life sustaining environment could evolve in a place like that.

    On the other hand the idea of an artificial shell has lots of possibilities. The inhabitants could have evolved and lived there for millions of years and only in their scientific age do the residents of that world realize that their environment was constructed. this leads to plenty of drama and speculation for the characters to speculate on who built their world and why...
  12. Oct 23, 2014 #11


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    Hmm, sounds kind of like ringworld. You could envision a mostly hollow shell the size of Jupiter, held together by some strong materials. These are probably imaginary materials stronger than we can currently think of, but I haven't gone through the physics of what the material stresses would be.
  13. Oct 25, 2014 #12
    When fictional premise is impossible like this, one solution is to find something so far outside of human experience that real science can't touch it:

    A black hole tore itself out of space/time due to contact with another universe. It left an empty timeless spaceless void-sphere where the event horizon had been. Mater that contacts this sphere simply collides with itself and can't go anywhere, but the first millimeter of dust coating the shell creates the illusion of a jupiter-sized sphere of ordinary solid matter that somehow has very low gravity. This would preferentially collect only heavy elements until an earth-like crust formed around it.

    A star that once had a black hole around it would have strange characteristics, and would likely be rather old. Moons around the void-sphere would have to be the result of capture or collision - unlikely since the former black hole would have swept the area clean. Alternatively, the star exploded and sent the sphere into another solar system where it collided with a terrestrial planet, providing mass and perhaps a moon. I'm not sure how you'd get a protective magnetic field around this though. If you had enough mass to melt the innermost layer contacting the void, would that create one?
  14. Oct 25, 2014 #13


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    Blasphemer! :mad:
    In fantasy, it doesn't matter. In Science Fiction, it's critical that the science be correct or at least within the bounds of extrapolation of known laws. Hence the difference between "Star Wars" and "2001: A Space Odyssey". Both genres are enjoyable, but they are in no way interchangeable. They can, however, be combined into things such as "Star Trek".
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