3
$\begingroup$

In a comment to this question imallett brought up the idea of a gas giant spinning so fast that the air is moving at near orbital velocity.

In Mission of Gravity Hal Clement has Mesklin spin so fast that it’s strongly oblate and has 700G at the poles and 3G at the equator. That’s most of the way to 0G due to spin! Clement did not offer any explanation for how his planet came to be that way.

If planet were a gas giant or ice giant or something like that (as opposed to rocky) you can get differential rotation, too. Would the gas envelope rise so easily due to centrifugal effects that it would fly off before it could achieve this speed or could it spin with a speed that’s within the speed of sound of the orbital velocity?

The main thing is: what would make it spin so fast? I know other bodies like stars can be spun up to high speed. What kind of history or astrophysical situation could possibly result in a planet spinning so fast?

I’d entertain ideas featuring other kinds of “planets” as long as it has a substantial atmosphere. So brown dwarfs etc. are OK.


Edit: Note that a catastrophic origin is OK. After all, look what happened to our own planet to form the moon. I don’t mean a planet has to be given this rotation without wrecking it; I mean it formed that way or settled down millions or billions of years after being wrecked.

$\endgroup$
  • $\begingroup$ The force required to spin up a rocky planet to such speeds would likely liquefy it. Even if kept solid, the surface of such a world would be absolutely unlivable as the constant super-hurricane-plus force winds would strip and polish it to bedrock. $\endgroup$ – Michael Richardson Jul 27 '16 at 15:10
  • $\begingroup$ @MichaelRichardson why wouldn’t the atmosphere rotate with the planet? $\endgroup$ – JDługosz Jul 27 '16 at 15:12
  • $\begingroup$ The atmosphere would also be rotating. However, as there is no firm connection between land and air, there is nothing that would keep them rotating together. If the rotational speed is fairly low, the difference in speed will not result in a large relative difference. On a planet with such a large rotational speed, even a slight difference in speed between land and air will result in extremely high winds. $\endgroup$ – Michael Richardson Jul 27 '16 at 20:19
  • $\begingroup$ I am so glad you didn't ask about how Clement's planet Mesklin was spun up. That would be agonisingly difficult to conceptualise. Do not think that was a suggestion for a possible WB question, because it most definitely wasn't! $\endgroup$ – a4android Jul 31 '16 at 5:43
  • $\begingroup$ Our own planet is spinning 1000 miles per hour at the equator and is subject to differential heating and atmospheric tides, yet we have horse latitudes. I just don’t see why you would expect supersonic wind because the air is not rigidly attached to the ground! $\endgroup$ – JDługosz Jul 31 '16 at 15:59
3
$\begingroup$

Consider that the impact of a Mars sized protoplanet on the Earth caused the spin to increase to today's 24hr and created the moon.

splat

So to radically increase the spin of a planet, you would need to apply far more energy. Sadly, the sorts of astronomical objects which could provide energy on these scales tend to be the sorts of things which would destroy planets (dwarf stars, neutron stars and black holes). As well, the mechanical stress of spinning so fast will also cause the planet to melt, or if the rotational energy exceeds the gravitational binding energy, to disperse into a rapidly expanding cloud of gravel.

The reason a neutron star spins so rapidly is it has taken the angular momentum of an entire star and shrunken it into a tiny volume of space, much like a figure skater bringing her arms in to speed up her spin. This isn't going to be possible with a rocky planet, and a gas giant planet would need the core to collapse (something which could only happen artificially, such as the monoliths in 2010 turning the metallic hydrogen into neutronium to initiate fusion).

The only way it seems possible to spin up a planet without destroying it would be to somehow artificially spin it up but at a controlled rate. Freeman Dyson (the same guy who developed the idea of the Dyson Sphere and the ORION nuclear pulse drive, among other things) came up with a planetary spin motor, which couples the magnetic field of the planet to an external influence to drive the rotation (sped up or slowed down, depending on what you want):

plan sketch

In real life, the theory of a planetary spin motor was thought up by Freeman Dyson and published in a scientific paper in 1966.

He proposed that a planet could be destroyed by accelerating it's [sic] rotation about its axis until centrifugal forces become greater than its internal cohesive forces. When the planet's period of rotation dropped to about one hour, it would be ripped apart.

In order to achieve the necessary acceleration of the planet's spin, he suggested wrapping it in a metal grid. This grid would be charged by a powerful electric current, creating an electromagnetic force, which would speed up the rotation of the planet.1

References ↑ Freeman J. Dyson (1966), "The Search for Extraterrestrial Technology", in Perspectives in Modern Physics (Essays in Honor of Hans Bethe), R. E. Marshak (Editor), John Wiley & Sons, New York. ↑ "How to Disassemble a Planet", Space Archaeology, http://spacearchaeology.org/?p=105

$\endgroup$
  • $\begingroup$ While I have great admiration for the creative genius of Freeman Dyson, he was not the originator of nuclear pulse propulsion concept. That was Stanislaw Ulam in about 1945 or 1946. Certainly he refined and developed the concept further, and has been a foremost promoter of NPP. He also was part of the General Atomics team that need actual research and experiments into NPP, essentially proof of concept work. Doubleplus good for mentioning the Bethe festschift. $\endgroup$ – a4android Jul 28 '16 at 2:14
  • 2
    $\begingroup$ BTW the original spin of the (final) Earth was more like 10 hours! $\endgroup$ – JDługosz Jul 28 '16 at 7:22
1
$\begingroup$

Stars and planets all form from insterstellar clouds. The model we currently have for stars and planets formations states that as they form, they should generally keep the same angular momentum as the cloud initially had, unless something happens to them (as with Uranus, for example, which is tilted 98 degrees).

So I am thinking that for a very fast rotation (in comparison to our own system), a star system may have been formed from a very fast spinning cloud. What caused the cloud to spin so fast may have been a couple novas reaching it early on in its history, from opposing directions and just the right angles, giving it a push from either side. Or maybe the cloud is the remnant of a nova and has been hit by another nova shockwave.

$\endgroup$
0
$\begingroup$

You asked for catastrophic, you've got catastrophic! Assume a gas giant planet minding its own business is orbiting its primary star when two runaway neutron stars travelling in opposite directions make a 'synchronised' fly-by of the planet's equator.

For convenience both neutron stars are of equal mass and moving through space at 30 km/s, which is a typical velocity for runaway neutron stars, relative to this planetary system. Their velocity vectors will match the direction of rotation of the gas giant planet.

As both neutron stars pass by the gas giant planet enormous masses of its atmosphere will be gravitationally attracted to its nearest neutron star. Two huge plumes of atmosphere will be drawn out of the bulk of the gas giant. This will accelerate the atmospheric plumes and radically increasing their velocity. Provided both neutron stars perform their flyby at the correct distance away from the gas giant the velocity increase will not exceed the planet's orbital velocity.

There will, of course, be a considerable amount of gravitational perturbation to the primary star and other planets in this system. In fact, the now spinning gas giant planet may be ejected from its planetary system. But the fact that the two neutron stars are approaching the planetary system from opposite directions may go some way to ameliorate the gravitational perturbation effects.

There will be some who will argue that this is an exceedingly low probability. Quite correctly too, but this of itself does not make this scenario impossible especially if our universe is a Type 1 multiverse. A Type 1 multiverse is spatially and temporally infinite which means all possible configurations of matter and energy will occur and subsequently repeat themselves across this infinite manifold. So while spinning up a planet in this way is an exceedingly low probability event it will occur an infinite number of times in a Type 1 multiverse.

$\endgroup$
  • $\begingroup$ I'd still like to see a measure of torque, rather than just “sounds like a lot of energy”! $\endgroup$ – JDługosz Jul 31 '16 at 15:52
  • $\begingroup$ Me too! This is a conceptual model. The real test comes with crunching the numbers. Frankly my mathematical skills have atrophied. This is something I'd have to persuade one of my mathematician friends to give it a try. $\endgroup$ – a4android Aug 1 '16 at 5:07
0
$\begingroup$

As has been said in other answers the problem is the spin-up event is not survivable. That means we need to spin it up by some means other than a high energy event.

An alien civilization has suffered a cataclysmic religious war. A large number of aliens have fled the violence by slowboat--but the other side got the last laugh: The ships were rigged to kill their crews far enough after launch that those on the losing side thought it was a route to safety--many such slowboats were launched.

The computers on the ships are fine, though--they're continuing to follow the flight path that includes using the planet in question for a gravity maneuver. However, their data was wrong, the planet is bigger than they thought. The autopilot guides them through the planned trajectory distance from the center of mass--but instead of a slingshot they go into the atmosphere and don't come back out. As the hit is off center this converts the linear momentum of the starship into planetary rotation. Each successive slowboat comes along and does the very same thing (the computers aren't smart enough to deal with it and the crews that could are dead.) The planet spins faster and faster until it reaches the point that additional energy ends up causing the highest speed particles in the atmosphere to escape and things reach equilibrium.

$\endgroup$
  • $\begingroup$ It doesn't need to be survivable any more than the formation of Earth’s moon was. However it came to be is part of the formation process, prior to settling down to a habitable state. $\endgroup$ – JDługosz Jul 31 '16 at 17:54
  • $\begingroup$ I think this is a scale error. How many ships would it take to spin up a planet? Would all those ships significantly increase the mass of the planet? And where did they come from, being they’re more than the mass of the home planet! $\endgroup$ – JDługosz Jul 31 '16 at 17:57
  • $\begingroup$ @JDługosz They don't need to be anything like the mass of the planet--I'm talking things traveling at a low percentage of lightspeed and I'm figuring hollowed-out asteroids as a base for construction. $\endgroup$ – Loren Pechtel Aug 1 '16 at 0:30
  • $\begingroup$ Can you show some actual numbers? $\endgroup$ – JDługosz Aug 1 '16 at 4:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.