Astronomy question or help me Science!

[Kyanbu The Legend]

So I have a question about astronomy and I wasn't sure it would belong in the sticky'd science thread.


How much of a difference in gravity would a planet like Earth in terms of eco system only 3 to 4 times its size in mass/atmosphere have compared to Earth?

[Aldurin]

Originally Posted by Kyanbu The Legend

How much of a difference in gravity would a planet like Earth in terms of eco system only 3 to 4 times its size in mass/atmosphere have compared to Earth?

Probably about 3 to 4 G's of gravity if the mass by itself fits that proportion, atmosphere has a much smaller effect in comparison. Of course the idea of an earthlike ecosystem in that kind of gravity is a potentially long stretch scientifically, since stronger gravitational forces would result in different geological behavior, a harsher environment for life to survive because the organisms have to be reinforced to keep the gravity from ripping them apart (we can barely withstand a few G's of force before feeling negative effects, so life on such a planet would have to be tough enough to resist that kind of constant strain), and more debri being drawn in, though that may or may not be mitigated depending on the atmosphere's density.

It's a lot of hypothetical science, especially since we don't have effective means of simulating increased gravity for long periods of time. We also don't have much on the specifics of planets forming, so we can't be sure if it enough matter would actually solidify in that amount, or become a gas planet.

[Kyanbu The Legend]

Ah I see, thank you for the answer, Aldurin.

[Professor Smarmiarty]

Everybody would have monkey tails.

It's hard to really say what kind of life would develop in higher gravity. Microbial life can survive in crazy shits (one of the proposed origins of life is deep-sea vents) and they can survive under higher pressure but whether large animals with like muscles and bones and stuff (which have reasonably hard-coded strength limits) can develop is another question. Do whatever you want.

[Kim]

just make all the creatures like super deep see life

[Arcanum]

[McTahr]

Uh. Density and radius are probably more liable to get a scientific response. Going purely off mass could lead to any number of things, such as a huge, low density gaseous planet, or a highly dense burnt out former star-esque hunk. It has a lot to do with history, how it developed, and what was available while developing.

I'm assuming you're talking of a planet with roughly four times the mass of earth, of similar density/makeup? Here's a good example: Gliese 581 g

(For those who don't care to read down:
Gravity is estimated around 1.1 to 1.7x earth gravity, because while gravitational force increase with mass, yes, it also decreases with radius, and necessarily a larger planet will have a surface farther from its center of mass, leading to surface gravities that are often surprising. (Jupiter is only 2.5x earth gravity.)
No seasons, and it's tide locked. Permanently light on one side, dark on the other, etc. etc.)

Re: Ecology / Evolution.
Estimates put it as anything from an extremely dense atmosphere to fairly thin.
For a higher gravity, obviously a heart will need to work harder to keep blood pumping in all the right directions, so either more streamlined cardiovascular systems or stronger striated cardiac muscle tissue could be expected.
For a denser atmosphere, whatever gas is preferred for breathing (CO2, H2O, and O2 are all likely to be found at this density, so you could reasonably approximate for fiction's sake similar biology to earth) will be far more available, thus less efficient use of that gas once in the bloodstream is likely to be seen, while the opposite is true for thinner atmospheres (studies have found people at higher altitudes live longer, theories mostly revolve around more efficient cardiac/respiratory processes) and more efficient bodily usage of the gas would be likely.

Also, without proper atmospheric distribution of heat (one side permanently being light kinda gets toasted without it) you might see particularly evolved species capable of dealing with extremely high or low temperatures, but that's really getting into janky sci-fi speculation at that point.

[Sithdarth]

a = G*M/r^2

So double the mass and increase the radius by the square root of two and nothing changes. So lets say we want to keep the same over all planetary density. So:

D = M/V or
D ~ M/r^3 (since volume of a sphere contains some constants and an r^3)

Thus if D is to remain constant r must increase by (M/M_0)^(1/3) where M_0 is the original mass. So a planet 8 times more massive than Earth needs a radius only twice as big to maintain the same density. Putting that into the very first equation

a = G*8*M_0/(2*r_0)^2 = 2*a_0

So taking the naive assumption that the density of earth is about average a planet 8 times the mass of the Earth only has twice the acceleration due to gravity. Now this isn't very exact. Obviously planets aren't homogeneous and rocky planets tend to have iron cores. So the density of the planet could increase if a greater percentage of its volume is iron. It could also decrease if there is a smaller percentage of iron by volume. In fact a decrease in density by half coupled with that 8 fold increase in mass and a doubling of radius means no change in acceleration due to gravity.

So the basics are that in order to know what the gravity of a fictional planet might be you need to know what its core is made out of, how much of the planet by volume is core, what the rest of the planet is made out of, its volume, and the density of all the things its made of. Once you have an idea of the structure of the planet you can then work out its radius and mass. Once those are known you can calculate the acceleration due to gravity on the surface.

TLDR: Planetary science is complicated.

[Aerozord]

oh while we are at it lets get really complicated and calculate the time dilution caused by the altered gravity. Sure it would be so minor you wouldn't notice it, but I like the idea of compelling someone to calculate it

[Sithdarth]

For the 8 times the mass at constant density case the dialation is sqrt(1-4*(2GM/(Rc^2))). Plug in the mass and radius of the Earth as well as the speed of light then divide by sqrt(1- (2GM/(Rc^2))) (which is the value for Earth) and you have your answer.