Gravity of Planets

[untier]

Is it always correct that the bigger and higher density a planet is and has, the greater gravity of the planet is? And therefore animals on the planet would be shorter and broader rather than taller and lankier?

[Mr Skeptic]

The strength of gravity depends on mass and distance. At large distances, the density of the planet is irrelevant. However, the denser a planet is the closer you can get to its center, so that the surface gravity is stronger.

 

And yes, based on our own larger animals, they will be pretty much as you described. However, they will likely also be smaller. Aquatic creatures won't care either way. Flying creatures might have an easier or harder time, depending on the density of the air as well.

[toastywombel]

Is it always correct that the bigger and higher density a planet is and has, the greater gravity of the planet is? And therefore animals on the planet would be shorter and broader rather than taller and lankier?

 

Well, if a planet is bigger, it does not exactly mean it has a greater gravitational pull, nor does the planets density. Gravity is really determined by the planets mass (amount of matter) and your distance from the planet. The greater the mass, the greater the gravitational pull, the closer you are to the surface, the greater the gravitational pull

 

While gravitational pull would have an effect on the size of the animals on the planet (greater the pull, the smaller the animals). It would not be a set rule.

 

Say if Venus for example was an inhabitable planet, it has a weaker gravitational pull than earth, and therefore, according to what you said lifeforms on Venus would be larger. However, the atmospheric pressure on Venus is much greater than the atmospheric pressure on Earth. So even though the gravitational pull is weak, the pressure is very strong, thus making it hard for large lifeforms to form.

 

The point is, there are many variables that would would determine the size of lifeforms on a given planet and gravity is just one of them.

 

Hope this helps.

[Moontanman]

Well, if a planet is bigger, it does not exactly mean it has a greater gravitational pull, nor does the planets density. Gravity is really determined by the planets mass (amount of matter) and your distance from the planet. The greater the mass, the greater the gravitational pull, the closer you are to the surface, the greater the gravitational pull

 

While gravitational pull would have an effect on the size of the animals on the planet (greater the pull, the smaller the animals). It would not be a set rule.

 

Say if Venus for example was an inhabitable planet, it has a weaker gravitational pull than earth, and therefore, according to what you said lifeforms on Venus would be larger. However, the atmospheric pressure on Venus is much greater than the atmospheric pressure on Earth. So even though the gravitational pull is weak, the pressure is very strong, thus making it hard for large lifeforms to form.

 

 

No, the atmospheric pressure would not make larger creatures more difficult any more than deep sea animals are smaller due to pressure. Gravitational pull at the surface would be the primary impediment to size, greater air pressure might, if it was great enough, allow larger animals via being supportive much like water does in the oceans of the earth.

[toastywombel]

No, the atmospheric pressure would not make larger creatures more difficult any more than deep sea animals are smaller due to pressure. Gravitational pull at the surface would be the primary impediment to size, greater air pressure might, if it was great enough, allow larger animals via being supportive much like water does in the oceans of the earth.

 

I just do not understand your logic here?

The idea that there are smaller animals at the bottom of the ocean (because of greater pressure) would imply that the greater the pressure, the harder it is for large lifeforms to form.

[Moontanman]

I just do not understand your logic here?

The idea that there are smaller animals at the bottom of the ocean (because of greater pressure) would imply that the greater the pressure, the harder it is for large lifeforms to form.

 

If it were true that deep sea animals are smaller you would have a point but the fact is that deep sea animals are often quite large, food supply being the limiting factor in size not pressure.

[toastywombel]

Furthermore, to say that gravitational force at the planets surface would be the primary impediment to size is not true. You could have a really low mass planet that has very little natural resources to support life. Even though the gravity would be weak at the surface, It wouldn't make sense for large lifeforms to form because there is would not enough resources for such a lifeform to survive.

[Moontanman]

Furthermore, to say that gravitational force at the planets surface would be the primary impediment to size is not true. You could have a really low mass planet that has very little natural resources to support life. Even though the gravity would be weak at the surface, It wouldn't make sense for large lifeforms to form because there is would not enough resources for such a lifeform to survive.

 

It is indeed the primary reason, that does not imply it would be the only reason. Even on a planet with a small food supply large creatures would be physically possible. The cube square law is the primary reason the food supply would be secondary.

[toastywombel]

If it were true that deep sea animals are smaller you would have a point but the fact is that deep sea animals are often quite large, food supply being the limiting factor in size not pressure.

 

Okay when we talk about large, are we talking size, or mass? Also, deep sea animals are not quite large comparative to the many other animal on the planet. Unless you could point out a large animal that lives in the Hadalpelagic Zone.

[Moontanman]

Okay when we talk about large, are we talking size, or mass? Also, deep sea animals are not quite large comparative to the many other animal on the planet. Unless you could point out a large animal that lives in the Hadalpelagic Zone.

 

Underwater or in a very dense atmosphere neither size or mass matter as much as in (standard) air, and yes deep sea animals can be quite large, generally speaking the food supply keeps deep sea animals smaller, giant tube worms, large clams and huge colonies of other creatures around black smokers where the food supply is large is a good example of large deep sea animals.

 

Huge salps, free swimming worms, giant squids, gigantic jellyfish, many large animals live deep in the sea, whales are an example of how a supporting medium allows large size. There are also very large sharks that live only in very deep water, the amount of food that drifts down from above is what limits the size of deep sea animals not the pressure.

 

Gravity limits the size of animals in a very primary way, the higher the gravity the smaller animals will tend to be unless there is some way around the cube square law. Internal skeletons is one way around the cube square law compared to external skeletons but gravity is still the primary reason land animals are smaller than oceanic animals in general.

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BTW the Hadalpelagic Zone is a very small part of the ocean and the pressure there is far more than the pressure at the surface of Venus. But none the less the primary impediment to life there is lack of food.

Edited March 6, 2010 by Moontanman
clarity

[untier]

According to my assossiation and sixth sense i think in a water or gas circumstance if you scale a given thing up, don't change its structure and material, the bigger form would be able to sustain higher pressure, wouldn't it? Why? What's the laws in that case?

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Though i know the trend isn't a straight line. There must be vertice or vertices and wave on the graph. Because the thing has its own gravity, when it's larger the gravity is greater, at a curtain size it will collapse itself...

[Moontanman]

According to my assossiation and sixth sense i think in a water or gas circumstance if you scale a given thing up, don't change its structure and material, the bigger form would be able to sustain higher pressure, wouldn't it? Why? What's the laws in that case?

 

The reason pressure is an issue for humans or mammals or any animal with hollow spaces inside is indeed the hollow spaces. As long as there are no hollow spaces to compress or as long as the internal pressure is equal to the external pressure the pressure is not an issue. (there is some ways pressure can change the shape of some very complex molecules but life has adapted to that in our oceans depths)

 

So it's not correct to think of the pressure being sustained or resisted, the pressure is simply the same inside as out. When a human scuba dives we have to breath air that is compressed to the same pressure as the outside water, if the pressure wasn't equal we would be crushed as would any animal, even a whales lungs compress when it dives. The issue is gravity and weight not pressure.

 

Though i know the trend isn't a straight line. There must be vertice or vertices and wave on the graph. Because the thing has its own gravity, when it's larger the gravity is greater, at a curtain size it will collapse itself...

 

yes, the cube square law covers that.

[Martin]

... greater than the atmospheric pressure on Earth. ...the pressure is very strong, thus making it hard for large lifeforms to form.

...

 

How does increasing pressure (other things being equal) obstruct the development of large lifeforms?

 

If the earth had sealevel air pressure of 10 atm, instead of 1 atm, but other things equal (e.g. temperature) would that have made it hard to have animals comparable to whales, elephants, giant redwoods, sharks, rhinoceros etc. etc.

 

Especially large plant lifeforms. Why would a tenfold increase in pressure have obstructed their development? Curious about your reasoning.

[Radical Edward]

I just do not understand your logic here?

The idea that there are smaller animals at the bottom of the ocean (because of greater pressure) would imply that the greater the pressure, the harder it is for large lifeforms to form.

 

that isn't really true. There is a decent variation of sizes right through the ocean, but one thing you have to remember is that near the surface, there is a lot more energy, and thus food available because of the sun. There are still massive animals deep down, like giant squid and the kraken.