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Zanizaila
2010-Dec-22, 08:09 PM
I am making a fictional world in the genre of plausible/hard science-fiction, and since I'm more into biology and other things rather than astronomy, I need some help.

Some time ago, I googled about planet gravity, mass and such, and came across this thread: http://www.bautforum.com/archive/index.php/t-6708.html

That's the way I found my way to this board then, today.
I already knew that gravity is directly connected to the planets mass, but then about the density... Earth seems to have 1.4 times the density of Mars, for example, and about 4 times the density of Jupiter, which of course is mainly gas, while Mars is terrestrial.

My question is - how much bigger than Earth can a planet be, while still keep the same gravity, and have density enough to support life like what we find here on Earth?

Please be nice, when I asked on an other board, they bashed me about how I can do worldbuilding when I "don't even know this". :shifty:

astromark
2010-Dec-22, 09:06 PM
I think I can help you...

" My question is - how much bigger than Earth can a planet be, while still keep the same gravity, and have density enough to support life like what we find here on Earth? "...

I would say Earth like conditions require a Earth like environment. For the sake of 'This' question I will call the Earth the Standard Unit.
On the surface of Earth we have a useful atmosphere and pressure. A gravity we measure as 1g. The Mass of the Earth dictates a gravity effect. increase it any by more than a 0.5 (half) of 1g and everything changes... The planet could be larger but the density would need to be less. Fiddle to much with that and we have a environment that becomes different... Different is not conducive to being the same... So the point I am attempting to tell...'Its got to be nearly like this, or it's not going to be like this...' But,. If you are doing a work of fiction. You are in charge.
Whatever you say, is. The science can be different for fiction. I draw your attention to the 'Star Trek' stories... They have some sort of gravity unexplained... warp speed... Photon canons...Mater transporters... you get my drift ? and might I welcome you here...mark.:dance:

antoniseb
2010-Dec-22, 09:14 PM
how much bigger than Earth can a planet be, while still keep the same gravity, and have density enough to support life like what we find here on Earth?
You could potentially have a planet which has a much larger fraction of Carbon, and much less of Silicon and Iron. I don't know how compressible the various compounds would be under extreme pressure, but you could plausibly have a habitable planet with a density less than 2 gm/cc.

Ken G
2010-Dec-22, 10:53 PM
And a useful formula is that if you keep the basic internal structure the same but simply modify the density (rho) and the radius (R), then the surface gravitational acceleration scales like rho*R. So if you want to stay at 1 g, then if you can drop the density by a factor of 2, you can hike up the radius by a factor of 2 also. It would seem to be pretty hard to do much more than that without changing things.