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What would happen if a gas giant with 25000 radius spend only 6 minutes to complete one day? And if a dense rocky planet with 15735 rotate in only 15 minutes? I've read somewhere that the faster the rotation, the gravity will be higher, and also the magnetic field. Is it true?

I'm not certain, but I think there comes a point where the planet wouldn't be able to hold itself together.

...John...

This is interesting!

Is it true that the slower a rotating planet goes, the lower the gravity is?

If so, is there a chart that will show something like that in the same vein as this: http://steve.pugh.net/fleet/stardate.html

TourqeGlare wrote:This is interesting!

Is it true that the slower a rotating planet goes, the lower the gravity is?

No. The less massive a planet is, the lower its gravity is. You'd have to be rotating ridiculously fast to get a pseudo-gravitational force pulling you to the ground because of the rotation alone.

Actually the centrifugal force would work against the pull of gravity- so a fast rotating planet would have lower gravity at the equator than at the poles.

On top of that the planet would be oblate becuse of the rotation; I believe that the increase in diameter at the equator would also lead to a lowered value for gravity (but I am not certain).

What you are asking is the Roche Limit. It tells you at what point will object break apart due to tidel forces for object of give densisty


Simple Roche Limit

L Roche Limit
R Radius of Primary
D Density of Primary
d Density of Child

L=(2.423*R*D)/d in Primary Radii

I've read somewhere about a fictional planet that have high gravity because of not only it's high mass, but also because of it's rotation, about 17 minutes!

See here:
http://rpg_ficcao.sites.uol.com.br/Espe ... linite.htm

Note: The site language is in Portuguese. If anybody here knows Portuguese, go to http://rpg_ficcao.sites.uol.com.br/ to see how many interesting things this site have. Not only about astronomy, but also fictional and RPG.

Many of the systems I have done and the add-on I'm doing now is based on the informations of this site, firstly here: http://rpg_ficcao.sites.uol.com.br/Soli ... elas02.htm http://rpg_ficcao.sites.uol.com.br/Soli ... etas01.htm

This is the best site of fictional science and astronomy I know.

If a planet were to flatten to an extreme extent - caused by very fast rotation - the gravity near the poles my be a lot higher than otherwise.
But the actual rotation does not have any positive effect on the gravity experienced on the surface.

Mesklin is very much a scifi world (it's from a Hal Clement book called "A Mission of Gravity" I think). Can't exist in reality, it'd fly apart spinning so fast!

Evil Dr Ganymede wrote:Mesklin is very much a scifi world (it's from a Hal Clement book called "A Mission of Gravity" I think). Can't exist in reality, it'd fly apart spinning so fast!

Extending science fiction beyond what we take to be a standard of science....it could be possible for a planet such as mesklin depending on its material composition. Relying on known physics however...no it probably would tear itself apart...but then our knowledge is limited to a small portion of the universe...so small in fact we actually know nothing....

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Generally though as we have seen thus far most fast rotationg objects are neutron stars....Thier mass is what makes them spin so fast I believe is the reason...And also depending on the actual material composition of these objects also depends on weither or not they are oblate....Most oblate objects are gasious in nature and an oblate object that is solid we can assume to be on the point of shattering...so I suppose if Mesklin were really to exist it would actually have to be made of a naturally occuring rubber or flexible substance....in essence alien to us....I suppose we would with our limited knowledge of the universe compute the probability of such a planet existing....Right now its rather low....But then I dont limit myself simply to the possible but rather fiddle with the improbable....Its more fun!

Rassilon wrote:Extending science fiction beyond what we take to be a standard of science....it could be possible for a planet such as mesklin depending on its material composition. Relying on known physics however...no it probably would tear itself apart...but then our knowledge is limited to a small portion of the universe...so small in fact we actually know nothing....

Er, no. We know quite a lot, actually. "Known physics" - at least in the case how well rotating objects can hold together - is very well understood.

I do notice that you have a habit of chronically underestimating our knowledge of the universe. I won't claim we know everything, but I think we know a lot more than you give us credit for.

Generally though as we have seen thus far most fast rotationg objects are neutron stars....Thier mass is what makes them spin so fast I believe is the reason...

It's mostly down to the conservation of angular momentum in a rapidly collapsing object (going from a supergiant about 5 AU in radius to a neutron star about 10 km in raidius). While most of the mass is blasted out in the supernova that creates the neutron star, there's still 1.4 to 3 solar masses compacted down into that object. That makes for a VERY rapidly spinning object.

And also depending on the actual material composition of these objects also depends on weither or not they are oblate....

Even the Earth is rotationally oblate though. What matters is the rigidity of the material. Jupiter and Saturn are somewhat oblate because their outer layers are gaseous. Earth is very slightly oblate because it's made of solid rock and metal. If its rotation was as fast as Saturn or Jupiter's, it'd be a little more oblate, but still less so than the gas planets since it's made of rock. To fly apart, Earth would have to be spinning once every two hours at least (this was, at one point, one of the theories for the origin of the moon - that earth had 'spun it off' due to rapid rotation. But that theory couldn't be supported by the facts).