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Thread: Largest 1g planet possible?

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    Largest 1g planet possible?

    My knowledge of the elemental compositions of planets and the relationship between mass, density, size, and gravity is very limited.

    I am trying to imagine a very, very large planet that has earth gravity or very close to it. It should also have earth like environment.

    My guess is that it would need to have little to no heavy metals. Would this be a problem for life? What about the core of the world... wouldnt it need a similar dynamic to earth in order to produce the EM field that protects us from many of the hazards of stellar weather? Or could this be countered by the composition of the atmosphere?

    Thanks in advance

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    Is this for a story? We don't really know that much about compression, but it there were a planet formed from mostly Carbon and Oxygen, and so the core was diamond and CO2, the density would be lower than that of an Iron core, but we don't know by how much, thanks to unknown compressibility. It might be that a large Silicon-Carbide core would hold up better and be less dense.

    If you want to allow a technological solution, it might even be possible to build something out of carbon nanotubes that is in some kind of very low density but strong configuration the size of a planet... that that a layer of rock got piled on top of it that makes the planet support our kind of environment.

    I'm not sure what you're asking for.
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    Moon certainly has very little iron.

    How much is the uncompressed density of Mars different from that of Earth? And how much of the difference is from different iron fraction, how much from different heavy rock/light rock composition?

    What does Pallas consist of?

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    Quote Originally Posted by antoniseb View Post
    Is this for a story? We don't really know that much about compression, but it there were a planet formed from mostly Carbon and Oxygen, and so the core was diamond and CO2, the density would be lower than that of an Iron core, but we don't know by how much, thanks to unknown compressibility. It might be that a large Silicon-Carbide core would hold up better and be less dense.

    If you want to allow a technological solution, it might even be possible to build something out of carbon nanotubes that is in some kind of very low density but strong configuration the size of a planet... that that a layer of rock got piled on top of it that makes the planet support our kind of environment.

    I'm not sure what you're asking for.
    More for simplu musing than a concrete story. But the information could be useful for a story.

    I am specifically looking for the possibilities allowed for in nature.

    Lack of knowledge about compression is the problem then? I understand this communities reluctance for blind speculation... you're scientists and I dig that.

    Let me turn the question inside out and see if that makes it easier.

    Is it plausible that a world with 50 times the surface of area of Earth could have an Earth like gravity and environment?

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    49 times the surface area requires 7 times the radius. Since a=Gm/r2, and you are solving for constant a, you'd need m to be 49 times bigger, and the overall density would be one seventh the density of Earth ... so slightly less dense than water on average. That would require some fairly incompressible light material.
    Last edited by antoniseb; 2012-Feb-17 at 02:51 PM.
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    The textbooks used to say that Uranus probably had 1 g at its surface, back when they thought it had a solid surface. I suppose that their radius estimate neatly compensated for the greater mass at the planetary center.

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    Quote Originally Posted by antoniseb View Post
    49 times the surface area requires 7 times the radius. Since a=Gm/r2, and you are solving for constant a, you'd need m to be 49 times bigger, and the overall density would be one seventh the density of Earth ... so slightly less dense than water on average. That would require some fairly incompressible light material.
    Hmm, ok, this is a good start.

    So the size I propose looks likes a very improbable natural occurence. What about 10 times the surface area? I wouldn't ask if I could do the math myself

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    Quote Originally Posted by ZunarJ5 View Post
    ... What about 10 times the surface area?...
    10 times the surface are would require about 1/3 the density of Earth, so a density of 2 (roughly). I'm not sure what material you could find that would hold up to a world that size and still only be a density of two.

    You might get a density of 3 by having a Silicon Carbide core, or 3.3 for a diamond core. Pure Aluminum core could give you 2.7, but I don't know how nature would give you a planet made of that. A density of 3 gives you about 4 times the surface area of Earth.
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    Exactly what were the compressed and uncompressed density of Vance´s Big Planet?

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    The idea of foamed rock with water filling the pores might be of interest. A hailstone type formation with layers might allow for interesting uses--lots of places to hide. A true Magog warren.

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    Centrifugal force

    There is an other way to have a 1 G gravity within a very big planet , it is when you take into account the centrifugal force.

    If the planet is rotating fast enough , you could get only one 1G at the equator when it is much more at the poles.

    I think there is a novel from Hal Clement about this kind of planet. And in our own solar system Planet Saturn is in this case except it is a gazeous planet.

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    You may enjoy this excerpt:

    Although I had often wondered about Jupiter, I had never hoped nor cared
    to visit it because of the inhospitable conditions which earthly
    scientists assure us pertain to this great planet. However, here I was,
    and conditions were not at all as the scientists had described.
    Unquestionably, the mass of Jupiter is far greater than that of earth or
    Mars, yet I felt the gravitational pull far less than I had upon earth.
    It was even less than that which I had experienced upon Mars. This was
    due, I realized, to the rapid revolution of the planet upon its axis.
    Centrifugal force, tending to throw me off into space, more than
    outweighed the increased force of gravitation. I had never before felt so
    light upon my feet. I was intrigued by contemplation of the height and
    distances to which I might Jump.
    -- Edgar Rice Burroughs, "Skeleton Men of Jupiter" (1942), Chapter Four.

    John Carter, the Earthman on Mars has great strenghth there due to the lesser gravity, and can thus beat the daylights out of fifteen-foot Green Martians. Now, one would think that he'd be reduced to a slug when kidnapped and taken to Jupiter. However, a pulp writer of the forties can explain anything to benefit the tale, and Burroughs was one of the best.

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    As the others suggested, more than one g surface gravity at the poles, if more than three times the surface area of Earth, with any presently known natural material.
    Saturn has one g at the cloud tops, but Saturn is mostly hydrogen with about 10% helium and it is hot near the center, which reduces the density. At incredible cost we can fly a trillion or so giant hydrogen filled balloons, Preferably cube shaped, at the cloud tops of Saturn, add a tarp that covers all of Saturn, then a thin veneer of dirt to make it much like Earth. Add an oxygen-nitrogen atmosphere, the weight of which will likely burst some of the balloons, as would even shallow oceans. If the tarp and balloons are strong enough it would behave like a giant arch, and not collapse even after the balloons lost some of their lift. If the people there can breath mostly oxygen at about 4 psi = not much nitrogen etc that will reduce the weight of the atmosphere by about 3 times. The main problems are that heat loss will be reduced, so the cloud tops will get warm, possibly hot, after a few thousand years, causing the planet to expand perhaps 5%, and solar concentrators are needed to get enough sunlight for efficient photo synthesis. There however is plenty of light for human vision. The surface area would be about 50 times that of Earth, I think. The air would be very cold until the cloud tops got hot, if they got hot, but other solar systems likely have a planet similar to Saturn in the goldilocks zone. Neil
    Last edited by neilzero; 2012-Feb-18 at 08:46 PM.

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    In recent news, GJ 1214b was found to be a "water world", with a radius of 2.7x Earth and a mass of 7x Earth. That makes its surface gravity around 1 gee. This planet is currently too hot for life, at least on the day side, but it shows how a large 1g world could naturally occur.

    http://www.space.com/14634-alien-pla...d-gj1214b.html

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    Quote Originally Posted by antoniseb View Post
    10 times the surface are would require about 1/3 the density of Earth, so a density of 2 (roughly). I'm not sure what material you could find that would hold up to a world that size and still only be a density of two.

    You might get a density of 3 by having a Silicon Carbide core, or 3.3 for a diamond core. Pure Aluminum core could give you 2.7, but I don't know how nature would give you a planet made of that. A density of 3 gives you about 4 times the surface area of Earth.
    Sorry for the late replies.

    Hmm.. looks like I'll have to get more creative

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    Quote Originally Posted by publiusr View Post
    The idea of foamed rock with water filling the pores might be of interest. A hailstone type formation with layers might allow for interesting uses--lots of places to hide. A true Magog warren.
    I was kind of thinking along those same lines. Stellar pumice?

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    Quote Originally Posted by DonM435 View Post
    You may enjoy this excerpt:



    -- Edgar Rice Burroughs, "Skeleton Men of Jupiter" (1942), Chapter Four.

    John Carter, the Earthman on Mars has great strenghth there due to the lesser gravity, and can thus beat the daylights out of fifteen-foot Green Martians. Now, one would think that he'd be reduced to a slug when kidnapped and taken to Jupiter. However, a pulp writer of the forties can explain anything to benefit the tale, and Burroughs was one of the best.
    I've been working my way through many of the main stream sci-fi classics. Barsoom has been looming.

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    Quote Originally Posted by neilzero View Post
    As the others suggested, more than one g surface gravity at the poles, if more than three times the surface area of Earth, with any presently known natural material.
    Saturn has one g at the cloud tops, but Saturn is mostly hydrogen with about 10% helium and it is hot near the center, which reduces the density. At incredible cost we can fly a trillion or so giant hydrogen filled balloons, Preferably cube shaped, at the cloud tops of Saturn, add a tarp that covers all of Saturn, then a thin veneer of dirt to make it much like Earth. Add an oxygen-nitrogen atmosphere, the weight of which will likely burst some of the balloons, as would even shallow oceans. If the tarp and balloons are strong enough it would behave like a giant arch, and not collapse even after the balloons lost some of their lift. If the people there can breath mostly oxygen at about 4 psi = not much nitrogen etc that will reduce the weight of the atmosphere by about 3 times. The main problems are that heat loss will be reduced, so the cloud tops will get warm, possibly hot, after a few thousand years, causing the planet to expand perhaps 5%, and solar concentrators are needed to get enough sunlight for efficient photo synthesis. There however is plenty of light for human vision. The surface area would be about 50 times that of Earth, I think. The air would be very cold until the cloud tops got hot, if they got hot, but other solar systems likely have a planet similar to Saturn in the goldilocks zone. Neil
    Thats a fantastic idea! Where do I sign up?

  19. #19
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    Quote Originally Posted by IsaacKuo View Post
    In recent news, GJ 1214b was found to be a "water world", with a radius of 2.7x Earth and a mass of 7x Earth. That makes its surface gravity around 1 gee. This planet is currently too hot for life, at least on the day side, but it shows how a large 1g world could naturally occur.

    http://www.space.com/14634-alien-pla...d-gj1214b.html
    Huh... 2million k? Thats pretty damn close. How would the red dwarf react to a water world of that size hitting it? Enough hydrogen to give it some flare back? Does it even work that way?

  20. #20
    Quote Originally Posted by neilzero View Post
    Saturn has one g at the cloud tops, but Saturn is mostly hydrogen with about 10% helium and it is hot near the center, which reduces the density. At incredible cost we can fly a trillion or so giant hydrogen filled balloons
    If the Saturnian atmosphere is mostly hydrogen, then why would hydrogen filled balloons float in it?

    Venus, Saturn, Uranus and Neptune all have equatorial "surface" gravities within +/- 15% or so of earth's. Only Venus is earthlike, and being slightly smaller than earth it's not surprising that it would have slightly less gravity (about 0.9g).

    But the three massive planets with earth gravity are all gas/ice giants with much lower densities than earth; there's no other way this could be.

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    Quote Originally Posted by KA9Q View Post
    If the Saturnian atmosphere is mostly hydrogen, then why would hydrogen filled balloons float in it?

    Venus, Saturn, Uranus and Neptune all have equatorial "surface" gravities within +/- 15% or so of earth's. Only Venus is earthlike, and being slightly smaller than earth it's not surprising that it would have slightly less gravity (about 0.9g).

    But the three massive planets with earth gravity are all gas/ice giants with much lower densities than earth; there's no other way this could be.
    Heat the hydrogen.

    Quote Originally Posted by publiusr View Post
    The idea of foamed rock with water filling the pores might be of interest. A hailstone type formation with layers might allow for interesting uses--lots of places to hide. A true Magog warren.
    The foamed rock won't stay foamy near the core; the pores will be crushed close.
    Information about American English usage here and here. Floating point issues? Please read this before posting.

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    That's right. The less massive, the less that will happen, though surface gravity wouldn't be as high. Unless... I was thinking about mirror matter. Imagine you have a planet made of mirror matter with one g surface gravity. Only you would pass through it seeing as you can't interact with the matter directly--only feeling its gravity..

    How might this be used for uber-far future habitats? Perhaps an airworld with no actual surface?

    Imagine a typical star surrounded by a shell of mirror matter, assuming that is even possible. At the cent of such a body, weightless, is the star itself. Now as heat from the star billows up, gases are lost, and they start to feel the pull of the mirror matter shell above as they near it, but pass right through.

    Could iron or other waste elements be liberated from a star using such means of harvest--resulting in a star that lasts longer?

    Having a mirror matter grid---a borg cube type lattice--phased through an active star, with mirror matter technology here and there at positions within the star. Could that be useful if some parts were suddenly changed--allowed to interact with instant heat and pressure?

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    If you change the composition, you change the "Earth-like-ness". Earth is a high-metal, low water world; if you change it up so it's larger, you change up what it's made of and the world itself will be non-Earth-like.

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    Quote Originally Posted by Githyanki View Post
    If you change the composition, you change the "Earth-like-ness". Earth is a high-metal, low water world; if you change it up so it's larger, you change up what it's made of and the world itself will be non-Earth-like.
    I should have clarified... by Earth-like I mean having conditions that could support human or earth like life even if not indiginous.

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    An artificial dynamically supported suprashell could be built around a planet or star at the right height to produce one gravity of acceleration. That makes a very large artificial structure indeed.

    Probably the largest possible structure of this kind in our galaxy would be one built around the supermassive black hole in the centre. If the Sag A* black hole has a mass of 4 million solar masses, a suprashell built around it with a surface gravity of 1 gee would have a radius of 2.3 billion kilometres. Is that big enough?

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    Quote Originally Posted by ZunarJ5 View Post
    What about 10 times the surface area? I wouldn't ask if I could do the math myself
    I wrote this web tool for doing the math a few years ago (for SF purposes). Just enter Relative Mass and Density, it calculates everything else for you.

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    How about a Hollow Planet

    Could we build a hollow planet and achieve a greater size while still maintaining the 1g requirement ... and put a layer of dirt on the outside about 50 ft thick for vegetation?

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    Quote Originally Posted by xylophobe View Post
    Could we build a hollow planet and achieve a greater size while still maintaining the 1g requirement ... and put a layer of dirt on the outside about 50 ft thick for vegetation?
    Instead of just saying that there are no conceivable materials that could maintain a hollow planet, I'll let you look up and plug into the equations. Use the formulas for an unreinforced, thin-walled sphere under external pressure.
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  29. #29
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    The 'suprashell' concept I mentioned earlier is a hollow shell held up by orbiting particles.
    Here's an image I've made of a suprashell world around an artificial black hole;
    http://www.bautforum.com/album.php?a...chmentid=12224
    the beauty of this concept (devised by Paul Birch of the British Interplanetary Society) is that it can be any size, and the black hole accepts any kind of matter; so you could fill it up with hydrogen and helium, elements that are not normally useful for bulking out inhabitable planets.

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    Heavy Reading

    Quote Originally Posted by galacsi View Post
    There is an other way to have a 1 G gravity within a very big planet , it is when you take into account the centrifugal force.

    If the planet is rotating fast enough , you could get only one 1G at the equator when it is much more at the poles.

    I think there is a novel from Hal Clement about this kind of planet. And in our own solar system Planet Saturn is in this case except it is a gazeous planet.
    "Mission of Gravity", Hal Clement. The crustacean like creatures that live at the high g poles steal the show.

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