A planet that spins faster than orbital velocity?

[Rdivine]

Is it possible for a planet to form such that it's rotational period is less than, or equal to the orbital period at 0m elevation? [velocity of the rocks at sea level would be orbiting the planet] If such a planet exists, the materials at sea level would essentially be orbiting the planet itself.

Beside's that, what would happen to such a hypothetical planet's oceans?

Is it possible where it's atmosphere is also moving at close to orbital velocity as well?

What would happen to it's shape? The poles of the planet would experience the same weight force, but the equators would be weightless. Hence, is it possible that it's shape remain a sphere, or something similar?

 

The existence of such a planet sounds crazy, but recently i saw this video that postulated the existence of doughnut-shaped planets.

 

 

Edited March 13, 2016 by Rdivine

[Tex_NL]

No. If a planet would spin that fast centrifugal force would exceed gravity. Objects would literally fall up. The planet would rip itself apart.

[LN400]

1 hour ago, Rdivine said:

Is it possible for a planet to form such that it's rotational period is less than, or equal to orbital velocity at 0m elevation? If such a planet exists, the materials at sea level would essentially be orbiting the planet itself.

Beside's that, what would happen to such a hypothetical planet's oceans?

Is it possible where it's atmosphere is also moving at close to orbital velocity as well?

What would happen to it's shape? The poles of the planet would experience the same weight force, but the equators would be weightless. Hence, is it possible that it's shape remain a sphere, or something similar?

 

The existence of such a planet sounds crazy, but recently i saw this video that postulated the existence of doughnut-shaped planets.

 

If I understand the post correcly: The first question (highlighted) can not be answered, or even asked. It's like talking about a 60 Joules lightbulb. So I assume you're asking if a planet can have a surface velocity less than or equal to the planet's orbital velocity at 0m altitude.

Less than, absolutely. Earth is such a planet. Surface speed at Earth's equator is something like 460 m/s which is less than orbital velocity at 0m altitude at some 7900 m/s.

Equal to, now that is different. Like Tex_NL said, such a planet would tear itself to pieces if it was formed in the first place. Only way such a planet could have existed would have been if it started out with a lesser surface velocity, then somehow some forces sped up the surface velocity and then it would tear itself up.

Edited March 13, 2016 by LN400

[kerbiloid]

This planet will transform into an ellipsoid cloud of dust and stones orbiting an iron ball of its core.
Several centuries later the friction forces will dissipate energy in form of infrared radiation, then all this cloud will condensate again into a planet (not too fast, though).
Of course, all lite substances (air, water) will be lost because of several thousand kelvins of temperature.

Edited March 13, 2016 by kerbiloid

[DanDaAwesome]

The strongest force of 'gravity' would be inside the ring (i think).

Gravity is in quotations 'cause its actually centrifugal force!

[RainDreamer]

I can't imagine what could be holding things together at that point. Unless artificially by some technology.

[DanDaAwesome]

15 minutes ago, RainDreamer said:

I can't imagine what could be holding things together at that point. Unless artificially by some technology.

Massive metal spokes. It would look like a bike wheel

[cantab]

No, by definition of "planet". If the equatorial surface speed is equal to or greater than the orbital speed at that height then I believe the object will not be in hydrostatic equilibrium.

Now a small body, held together by its own rigidity, could indeed spin that fast. I wonder what craters in the equatorial region would look like?

[LN400]

This whole topic got me thinking, so I did a little reading around the internet. Although this is not a planet, it shows that some objects can have an absolutely ridiculous spin so what I found was

PSR J1748-2446ad, a pulsar that has a calculated equatorial rotational speed of around 70,000,000 m/s. Now, being a pulsar it has a gravitational pull far greater than Earth and I have absolutely no idea what the orbital velocity would be at 0 m altitude but it shows that rotational speeds for large objects can be pretty insane.

So, feel free to calculate the orbital speed neccessary for that bad boy.

[Steel]

Just now, LN400 said:

This whole topic got me thinking, so I did a little reading around the internet. Although this is not a planet, it shows that some objects can have an absolutely ridiculous spin so what I found was

PSR J1748-2446ad, a pulsar that has a calculated equatorial rotational speed of around 70,000,000 m/s. Now, being a pulsar it has a gravitational pull far greater than Earth and I have absolutely no idea what the orbital velocity would be at 0 m altitude but it shows that rotational speeds for large objects can be pretty insane.

So, feel free to calculate the orbital speed neccessary for that bad boy.

Interestingly enough, it's orbital velocity at the surface (assuming, from the estimates on Wikipedia, 1.9 solar masses and a 16 km radius) is ~ 1.3 × 10⁸ ms^-1, meaning its orbital velocity is still 1.8 times the rotational velocity at the equator.

[LN400]

Just now, Steel said:

Interestingly enough, it's orbital velocity at the surface (assuming, from the estimates on Wikipedia, 1.9 solar masses and a 16 km radius) is ~ 1.3 × 10⁸ ms^-1, meaning its orbital velocity is still 1.8 times the rotational velocity at the equator.

Thanks for finding that info! Interesting but not unexpected.

[AngelLestat]

I think is possible.. I never thought in this possibility..  love it.

[Kerbart]

5 hours ago, Rdivine said:

What would happen to it's shape? The poles of the planet would experience the same weight force, but the equators would be weightless. Hence, is it possible that it's shape remain a sphere, or something similar?

Given that earth is, due to it’s very modest rotation period of 23 hours and 56 minutes, is no longer a sphere, I don’t think any optimism is warranted for a planet that rotates that fast to be a sphere. Not even recognizable as one, I’d dare say.

[Shania_L]

I think its possible, but not for a fully fledged planet as has already been said the material it is made of would be torn apart at such speeds. Smaller bodies like asteroids where the rotational velocity required is sufficiently low enough that the structural stress imposed would be supportable by a single piece rocky/metallic body.

For example, a small 1,000 tonne asteroid formed from a perfect spherical iceblock (because water is easier to calculate than rock). 15m diameter gives an orbital distance of ~47m at its surface which works out to 0.00009 m/sec or an orbital period of just short of 139hours, without even working out the stresses I think its safe to say you can spin a 1,000tonne iceberg around in under 139 hours.

But as you said planet, then no its not possible, anything big enough to be considered a planet would be too weak to support the rotational stress.

[cubinator]

For a planet, this is not possible. Since the rotation at the surface exceeds the force of gravity, objects there would fall upwards from centrifugal force and the planet would be torn apart. There would have to be some other force acting on it, such as rigidity. This is pretty much out of the question for a large planet, but on a small asteroid it might be possible. Phobos, for instance (not quite the same situation but similar effect) orbits closer to Mars than synchronous orbit. It is thought that Phobos is a mere lump of rubble held together by a thin crust. If Phobos didn't have that (relatively) rigid crust holding it together, it would most likely dissolve into a planetary ring around Mars (which it will probably do anyway, since it's orbit is decaying). If we had, say a very tiny asteroid with a very rigid crust and a very low gravitational pull, it could potentially be spun up (it's very unlikely it would form with >1 rotational velocity/orbital velocity ratio, as objects like these form due to gravity) so that it rotates slightly faster than it's orbital velocity. The lower the gravity, the better this works (you can easily spin a beach ball or other such object much faster than it's orbital velocity, but it doesn't work so well with Earth or the Moon or Phobos). Another thing that could cause this (still only in very small objects) is magnetism. If the hypothetical asteroid is made of highly magnetic material, it could spin 'very' (potentially faster than orbital velocity, but not ridiculously fast) quickly and still be held together by magnetic forces. This is probably a more likely way this would happen, because a lot of space rocks are magnetic and not a lot of space rocks have an accelerating rotation.

Edit: Just watched the attached video, and the narrator just references that a simulation shows it to be possible, then proceeds to assume that it is perfectly possible for a stable toroidal planet to exist and tells what it would be like if this planet was perfectly habitable to humans. She does not go into detail about the actual mechanics of this planet, which I find very unlikely to form under natural circumstances. Now, to read the referenced papers.

Edit 2: So it seems that this theory predicts that the mass of the ring would counteract the centrifugal force, which makes sense. I still think that such a body would not form easily under natural circumstances, but it may just be possible. However, the OP is about planets spinning faster than their orbital velocity, not about toroidal planets. A toroidal planet would still be held together by gravity, and if it were spinning faster than it's orbital velocity it would still fall apart as described above.

A regular (spherical) planet would be able to spin very close to, although not quite faster than it's orbital velocity, and if it did this it would become a very oblate spheroid. Earth does this a little bit, it is about 42 km wider at the equator than at the poles, and Jupiter does this a lot, it is about 9275 km wider at the equator than at the poles.

Edited March 13, 2016 by cubinator

[Bill Phil]

I think the gas giants come close, to within a few km/s. In fact, they have a noticeable bulge due to their fast spin rates.

But the reason that planets spin is due to the conservation of angular momentum. If it spins faster than surface orbital speed than it needs a lot of angular momentum. The initial formation might not even occur, due to the sheer amount.

Edited March 13, 2016 by Bill Phil

[fredinno]

 

10 hours ago, RainDreamer said:

I can't imagine what could be holding things together at that point. Unless artificially by some technology.

It won't. Toroidal planets and hyperspin planets are generally unstable, and would colsene back down into a ball/potato.

4 hours ago, Bill Phil said:

I think the gas giants come close, to within a few km/s. In fact, they have a noticeable bulge due to their fast spin rates.

But the reason that planets spin is due to the conservation of angular momentum. If it spins faster than surface orbital speed than it needs a lot of angular momentum. The initial formation might not even occur, due to the sheer amount.

That's not close at all...

[Bill Phil]

17 hours ago, fredinno said:

 

It won't. Toroidal planets and hyperspin planets are generally unstable, and would colsene back down into a ball/potato.

That's not close at all...

The velocity they're spinning at is about 10 km/s. Orbital velocity is about 1 km/s less for Jupiter and Saturn. Cosmically, that's pretty close. Especially compared to Earth.

In any case, it's pretty close for the Giants, relative to Earth.

Edited March 14, 2016 by Bill Phil

[problemecium]

@LN400 Orbital velocities for neutron stars just above the surface generally tend to be close to the speed of light.

Edited March 14, 2016 by parameciumkid
The new post editor is eeevil.

[fredinno]

1 hour ago, parameciumkid said:

@LN400 Orbital velocities for neutron stars just above the surface generally tend to be close to the speed of light.

That has more to do with the insane gravity well.

[lajoswinkler]

Such planet can't even form, so no.

[OhioBob]

On ‎3‎/‎12‎/‎2016 at 5:53 AM, Tex_NL said:

No. If a planet would spin that fast centrifugal force would exceed gravity. Objects would literally fall up. The planet would rip itself apart.

That's only true down to a certain depth.  Although the surface may be spinning fast enough for some of the crust to be ripped away, as the planet shrinks in radius, the surface gravity increases⁽¹⁾ and the surface velocity decreases⁽²⁾.  There should be a point where centrifugal force and gravity reach an equilibrium and the planet stops ripping itself apart.  The planet would also have to reshape itself as the loss of material would occur near the equator.  Eventually though, I think a stable equilibrium would be reached without the complete tearing apart of the planet.

(1)  Surface gravity increases because the planet is much denser at its core. As we move closer to the dense core, the gravity increases despite the fact the planet has lost mass.

(2)  Assuming the planet maintains a constant spin rate, surface velocity decreases as the radius decreases.  This doesn't take into account conservation of angular momentum.  I've have done a detailed enough analysis to see how the spin rate might change as surface material is spun off.

[problemecium]

But wouldn't an object rotating that rapidly already have formed into a flat disc? I suppose the oblateness would decrease as it ablated, but I have a hunch that the disc shape would exacerbate the problem.

And yes, neutron star's orbital speeds are high due to the gravity well. What I meant to emphasize was that even huge numbers for the surface velocity are still easily below orbital velocity when the orbital velocity is near lightspeed.

Edited March 14, 2016 by parameciumkid
The new post editor is a monstrosity that should be exorcised.