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To add on to the above, it is such features as this that cause "high-grav" or "low-grav" worlds, which gave us the Ogryns and the Squats of 40K.

Then you have all the many, many Death Worlds in the Imperium that are inimical to human life for one reason or another... and, yet, many of them do have human populations.

A planet could, conceivably, be extremely high-gravity... but have no atmosphere to speak of. It may lie too close to (or too far from) its star, or have lost its atmosphere due to some celestial cataclysm (or the action of Xenos). This will not change the gravitic measure of the planet to any appreciable degree. If Earth's atmosphere suddenly vanished, we wouldn't go drifting off into space, the planet's mass is what causes its gravity, not its atmosphere.

You fail to even grasp the very simple point I'm making. Mass causes gravity, mass comes in some degree from size, some degree from what the core is made of. But in the end, big planets has big atmospheres, small planet has small atmospheres. Size matters, its just that simple.

Read the Horus heresy novels... in plenty of novels these cruisers get shot out of the sky.. Plenty of refenrance!

 Beaviz81 wrote:
You fail to even grasp the very simple point I'm making. Mass causes gravity, mass comes in some degree from size, some degree from what the core is made of. But in the end, big planets has big atmospheres, small planet has small atmospheres. Size matters, its just that simple.

No, I understand what you're saying, but what you're saying is just incorrect. The planet's size has very little to do with the size (or presence) of its atmosphere.

 Psienesis wrote:

 Beaviz81 wrote:
You fail to even grasp the very simple point I'm making. Mass causes gravity, mass comes in some degree from size, some degree from what the core is made of. But in the end, big planets has big atmospheres, small planet has small atmospheres. Size matters, its just that simple.

No, I understand what you're saying, but what you're saying is just incorrect. The planet's size has very little to do with the size (or presence) of its atmosphere.

You have yet to disprove me in any way except stating I'm wrong and not forking up any facts.

 Beaviz81 wrote:

 Psienesis wrote:

 Beaviz81 wrote:
You fail to even grasp the very simple point I'm making. Mass causes gravity, mass comes in some degree from size, some degree from what the core is made of. But in the end, big planets has big atmospheres, small planet has small atmospheres. Size matters, its just that simple.

No, I understand what you're saying, but what you're saying is just incorrect. The planet's size has very little to do with the size (or presence) of its atmosphere.

You have yet to disprove me in any way except stating I'm wrong and not forking up any facts.

It has been shown multiple times. Look at the formula for gravity. It only cares about mass, not physical size.

Yes, being bigger(all other things equal) will result in higher mass. But not all things are equal. Composition of different planets will be different, thus their gravity will be mostly determined by their mass.

Jupiter is tens of thousands of times bigger than Earth, but its not tens of thousands of times the gravity.

You showed zero source. So I assume you just made it up.

 Beaviz81 wrote:
You showed zero source. So I assume you just made it up.

You think I would make up a well known and documented scientific equation?

I linked the wikipedia page on gravity, which has the equation. Here it is again. http://en.wikipedia.org/wiki/Gravity

And I really shouldn't have to source a very simple equation like that. Its common knowledge and anyone can verify it.

I think you misunderstand at least partly. A planet must have a certain size in order to have an atmosphere. Just look at Mars for example of a planet a bit too small to achieve that. And it can't be too large or the atmospheric pressure would crush anything on the ground.

 Beaviz81 wrote:

 Psienesis wrote:

 Beaviz81 wrote:
You fail to even grasp the very simple point I'm making. Mass causes gravity, mass comes in some degree from size, some degree from what the core is made of. But in the end, big planets has big atmospheres, small planet has small atmospheres. Size matters, its just that simple.

No, I understand what you're saying, but what you're saying is just incorrect. The planet's size has very little to do with the size (or presence) of its atmosphere.

You have yet to disprove me in any way except stating I'm wrong and not forking up any facts.

I feel like I'm getting suckered into doing all the research for a 7th grade science class report.

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 Beaviz81 wrote:
I think you misunderstand at least partly. A planet must have a certain size in order to have an atmosphere. Just look at Mars for example of a planet a bit too small to achieve that. And it can't be too large or the atmospheric pressure would crush anything on the ground.

No, the real reason Mars doesn't have much of an atmosphere is because it lacks a magnetic field to keep solar radiation from blowing the atmosphere away.

And sure, on a large enough planet we'd have an atmosphere too dense for humans. But it would have to be a HUGE planet.

 Beaviz81 wrote:
You fail to even grasp the very simple point I'm making. Mass causes gravity, mass comes in some degree from size, some degree from what the core is made of. But in the end, big planets has big atmospheres, small planet has small atmospheres. Size matters, its just that simple.

but it really doesn't

case in point, the Planet Venus

Very similar size to the earth: 95% of the size of earth
Similar Gravity: 91% of earth's
Similar Mass: 81% of earth (less dense)
Drastically different atmosphere: 9000% greater atmospheric pressure

Now part of that pressure difference has to do with temperature. Venus is the hottest planet in the solar system, despite being much further out than Mercury. Most of it just has to do with Venus having more gas on it than earth.

Composition matters a lot more than raw size or even mass.

To capitalize on what Exergy's said, consider Mercury and Mars.

Mars is significantly larger than Mercury, and is even masses more. But Mercury has a gravity field that's almost identical to that of Mars... despite the fact that Mars is both larger and "heavier".

Could it perhaps be that the composition/density of Mercury is the reason for that?

The spinning iron core that creates our magnetic field makes our atmosphere possible. Which means that liquid aluminum core would not generate a magnetic field. Also gravity is directly proportional to mass, and mass is dictated by the amount of matter in a object, not its size (we call that volume). Mass and its relation to volume are dictated by a principle called Density (how compactly the matter is packed into a finite space), for instance, neutron stars, the most dense objects known in the galaxy short of black holes have gravity millions of times higher than earth because the density of the stars are huge, it is the same mass as a star packed into something a few hundred miles across. A table spoon of it would weigh as much as several hundred tons... so yea size of a planet relative to density dictates gravity, but size alone doesn't.

 GKTiberius wrote:
The spinning iron core that creates our magnetic field makes our atmosphere possible. Which means that liquid aluminum core would not generate a magnetic field. Also gravity is directly proportional to mass, and mass is dictated by the amount of matter in a object, not its size (we call that volume). Mass and its relation to volume are dictated by a principle called Density (how compactly the matter is packed into a finite space), for instance, neutron stars, the most dense objects known in the galaxy short of black holes have gravity millions of times higher than earth because the density of the stars are huge, it is the same mass as a star packed into something a few hundred miles across. A table spoon of it would weigh as much as several hundred tons... so yea size of a planet relative to density dictates gravity, but size alone doesn't.

It's not pure Iron, it's a mixture of Iron and Nickel largely with heavy elements mixed in to create something far denser than a normal Iron Nickel alloy.

Regardless of its actual composition, however, the fact that it's comprised of fairly heavy metals contributes to the total mass of Earth, which is where our gravity comes from. If our core were, say, silicates and aluminium, we'd have a low-gravity planet, even if it were the same size.

 Psienesis wrote:
Regardless of its actual composition, however, the fact that it's comprised of fairly heavy metals contributes to the total mass of Earth, which is where our gravity comes from. If our core were, say, silicates and aluminium, we'd have a low-gravity planet, even if it were the same size.

And even with an iron core, the day it stops spinning it doesn't matter what the mass of Earth is. The atmosphere simply gets blown off.

Theoretically, but not definitely. The spinning of the planet is not what creates its gravitic pull. All bodies of mass generate gravity; the more mass, the more gravity. While the centrifugal force of the planet's rotation is responsible, in part, for our weather patterns, it's theoretical whether or not it would somehow float off if we stopped spinning.

The main question on that is "why?". There's not another body within range to overcome Earth's own gravity well that would pull the atmosphere off.

 Wyzilla wrote:

 Psienesis wrote:
Regardless of its actual composition, however, the fact that it's comprised of fairly heavy metals contributes to the total mass of Earth, which is where our gravity comes from. If our core were, say, silicates and aluminium, we'd have a low-gravity planet, even if it were the same size.

And even with an iron core, the day it stops spinning it doesn't matter what the mass of Earth is. The atmosphere simply gets blown off.

We'd still have some atmosphere. Mars actually still has a, very weak, magnetic field despite having no molten core. And if it had the correct composition it would still be breathable. Of course solar radiation would be a huge concern for life, and this is the biggest reason a strong magnetosphere is needed. Atmosphere retention is a close second in terms of concern.

So any terraforming attempts on Mars would succeed in creating a breathable atmosphere, but it would always remain thin and you couldn't spend long periods outside without protective gear. Only the hardiest plants and animals would be able to survive.


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 Psienesis wrote:
Theoretically, but not definitely. The spinning of the planet is not what creates its gravitic pull. All bodies of mass generate gravity; the more mass, the more gravity. While the centrifugal force of the planet's rotation is responsible, in part, for our weather patterns, it's theoretical whether or not it would somehow float off if we stopped spinning.

The main question on that is "why?". There's not another body within range to overcome Earth's own gravity well that would pull the atmosphere off.

He meant the Core spinning, not the entire planet.

And without the strong magnetosphere our atmosphere would become thinner, probably still breathable. But it would wreak havoc.