a question about planet size..

[rohmenari]

hello there, <br /><br />Just curious as to whether or not it is possible for a planet the size of Saturn or Jupiter to exist with an atmosphere and climate just like that of earth. <br /><br />I am writing a science fiction story and I would like to keep it believable to those in the world of science.

 

[jurgens]

It probablly would, but it would depend completely on it's density I guess and would have to be made out of solid materials.

 

[rohmenari]

and how do you suppose that would affect the general make up of the planet?

 

[Saiph]

not really possible. The reason is the gravity is strong enough to retain hydrogen and helium. If you can keep those you drastically change the makeup of the atmospehre to mostly hydrogen and helium, as they are the most abundant elements by far. Hydrogen makes up 75% of the universe, helium 25%, and what little bit is overestimated in those two, make up <i>everything</i> else.<br /><br />That's why the gas giants have that composition, if you get big enough to keep those, that's what you get, there really is no mechanism to alter that. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

 

[jurgens]

I dunno about that Saiph, if you had a planet the Size of jupiter but with only about 1/3 of the mass, it would have about the same gravitational attraction as Earth does at sea level.

 

[nexium]

Perhaps Jurgen ment to say 1/3 density instead of 1/3 mass. If you increase the diameter of an Earth like planet by ten times you have to expect nearly everything to change at least slightly. That is a surface area increase of 100 times and a volumn increase of 1000 times. If the average density is 5.5/same as Earth, then the mass increases by 1000 times. This produces a much stronger gravity on the surface, which compresses the atmosphere into a narrow layer. If air pressure is one bar at 1000 feet, it will be perhaps two bar at sea level, and 1/2 bar at 2000 feet.<br /> The only possibilities I can think of is a hollow planet, with a vacuum inside, or very hot hydrogen, which may be why Saturn has normal Earth gravity at it's cloud tops. With a vacuum inside. the crust would need to be made of a material far stronger than any know/ even stronger than CNT = carbon nano tubes to have normal Earth surface gravity. Normal Earth surface gravity would permit an atmosphere and climate much like Earth has at present, with some additional tweeking of details. Neil

 

[rohmenari]

ok, lets just say the planet is 5.5 times that of Earth. <br /><br />Would that make it possible for life to survive on the planet, human lifeforms that is, and would the planet retain the basic properties of earth? <br /><br />Also with the gravity being much stronger, would that not make it harder to access space? The reason I ask is because the people on this planet will eventually have fairly powerful technology, but I have been having a hard time justifying keeping them stuck on the planet. If they were unable to escape the pull of gravity and reach space, that would solve my problem for the most part.

 

[nexium]

I doubt that land creatchers taller than Man could adapt to triple gravity. To fall down would mean damaged tendons, ligaments, muscels and/or bones. Average density 1/3 (if 1/3 is correct) that of Earth might be possible at 151 times the volumn of Earth = 5.5 cubed. A core of mostly metalic hydrogen and a mantle of mostly lithium perhaps? The ocean would likely be shallow and no hill tops more than 2000 feet above sea level, would be likely as the plate tetonics would have to lift 3 times the weight. A moderately compressed atmosphere might produce only moderate problems. There would likely be no flying creatures. Aviation might not progress beyond hot air and gas filled balloons.<br />If the sun had a very strong solar wind 4 billion years ago, most of the hydrogen and helium would have blown away in spite of the much stronger gravity, so that could solve the Saiph problem. Neil

 

[jurgens]

heh? what are you talking about dude, your making my head hurt.<br /><br />Gravity and Density don't have a linear relationship, if you wanted a planet that was 5.5 times bigger then earth but with the same density, The planet would be about 166 times heavier then Earth... But it would also have a gravitational acceleration 5.5 times greater then on earth(just plug it into a= GM/r^2, pretty simple really, you just want delta M/delta r^2 to equal one so it has the same acceleration as Earth). On the other hand if you wanted the planet to have an Earth like Acceleration it would need to be 30 times more massive then the earth.<br /><br />Now, what you would need is a solid that had a density of around (Mass of hypothetical Planet)/(Volume of Hypothetical Planet)<br />(30 * mass of earth in kg)/(4/3 * pi * (5.5* radius of earth in m)^3)<br />(30 * 5.9742 × 10^24kg)/(4/3 * pi * (5.5*6373000m)^3)<br /><br />993kg/m^3<br /><br />which is about .993 g/cm^3, for comparison, the density of water is 1g/cm^3, and the density of graphite is 2.25g/cm^3<br /><br />So what that most nearly means is that No. The Material wouldn't be dense enough, of course though, if you increase the mass of the planet 2x, you COULD do it, but then gravity might end up being strong enough to keep a very very thick atmosphere, so no again. Also if your planet had 2x the mass, most likely it would end up compressing on itself which would decrease the radius and increase the gravitational acceleration at the surface.<br /><br /><br />What I wonder though is, would it be possible to live on the upper atmosphere of these planets? I mean they have all the hydrogen you could ever want to power a fusion reaction, plus probablly many more resources just floating in the air. Not to mention huge magnetic fields which could be used to also generate power. And you wouldn't have to do much to stay on the surface, since the density of the atmosphere would be so great. Kind of like Cloud City from Star Wars.

 

[nexium]

Hi JurgenS: So 5.5 times bigger diameter and same density = 166 times mass of Earth? I got 151, so I was low.<br />5.5 times bigger and 30 times the mass of Earth would yeild Earth normal surface gravity, and an average density of 0.993? Warm water and ice have that density.<br /> Lithium has a density of 0.534@ 20 degrees c, so it could offset other elements which have higher density at high pressure such as the mantle of the planet. <br /> I think Cloud city is within easy grasp of present technology, but it may not be cost effective in Earth's upper atmosphere, and would likely cost a trillion dollars to do in Saturn's upper atmosphere in 2040 (starting a major effort in 2005, unless we get a major break though or two. The other gas giant planets have uncomfortablely high gravity at their cloud tops. Venus has 55 degrees f if you can keep your cloud city at that altitude. Other Venus altitudes are too hot, without a massive air conditioning budget. Neil

 

[kelle]

Lithium is a very rare element in the universe, heavy elements like silicon, iron and aluminum (which in combination with oxygen are the most common elements on earth) are thousands of times more common. So it wouldn't exactly be very probable that a rock planet formed naturally that had so much lithium that it would make the planet float in water. Maybe the planet could be some kind of light metal construct made by a very advanced species long gone? <img src="/images/icons/smile.gif" />