Need second opinion on a hypothetical planet

[pcw27]

So I'm working on this story.

I wont get into too much detail but basically the planet I designed was struck by another protoplanetary object at an angle perpendicular to the rest of its gravity plane, creating a second rotational axis. The result was a planet tidal locked to its star and rotating on a horizontal axis in a manner similar to a top which has fallen on its side and kept rolling.

I was told the dual axis would destroy the planet. If this is true it might require a lot of rewriting to fix. Is this orientation impossible? If so should I care or just follow the Nucky Thomson doctrine of "Never let the truth get in the way of a good story"?

 

[ghostrunner01]

First: to the Forum.
Second: I'm no scientist, but I would think it would survive if the planet was massive enough. The tidal forces caused by the orbit and rotation would probably cause bulges on the planet, but probably not enough to destroy it. Good luck on the story!

 

[RisingFury]

What do you mean by "dual axis"?

As far as tidal locking goes... while an impactcould strip the planet of most of it's angular momentum, that in itself won't create a tidal lock. A tidal lock of a planet will be created by it's star. The star's gravity stretches the planet and as the planet rotates the tidal bulge gets pushed ahead. Since the tidal bulge closer to the star feels a bit stronger gravity then the one facing away from the star, this creates a situation where torque is applied to the rotating body thus slowing it down.

The closer the planet is to it's parent star and the more massive the star is, the faster tidal lock will be achieved

 

[pcw27]

Fury,

Basically the orbit was synchronous and the impacting object caused the planet to rotate on a horizontal axis.

I'll just elaborate to say that the story required the planet to always have the same hemisphere pointing towards its star and the other away. I know that's pretty rare as far as planets go and figured since the moon helps keep the Earth on a stable axis rotating perpendicular to the gravity plane, a moon which orbits perpendicular to the hypothetical planet could maintain the orientation it needs.

 

[tblaxland]

What do you mean by "dual axis"?

I think what he means is that the planet will have a large torque-free precession. I doubt that it is likely to occur for a planet size object. At least, it wouldn't last for very long nor would the precession be very large. It would not "destroy the planet", and any effect would be way less than other geological effects of the impact. An object the mass of a planet will return to a symmetrical shape pretty quickly and the torque-free precession will largely disappear. That property is one of major things that allows us to call it a planet. The collision could result in a significant change in the direction and magnitude of the spin of the planet though, and that would be preserved long term.

 

[pcw27]

So to take it further. Could the debris from the collision leave a moon orbiting on the perpendicular axis?

 

[tblaxland]

It is unlikely. You might get a high-ish inclination (< 20° :shrug but any higher is unlikely. I think the orbital angular momentum of ejected debris is likely to be in roughly the same direction as the new angular momentum of the body. At higher inclinations, [ame="http://en.wikipedia.org/wiki/Kozai"]Kozai instabilities[/ame] are likely to make accretion of a moon difficult.

For a real world example of both the axial tilt change and moon formation scenario, you might consider Uranus. The highest inclination of it large moons is ~4° to the equator. http://en.wikipedia.org/wiki/Moons_of_Uranus#Large_moons

 

[Urwumpe]

So to take it further. Could the debris from the collision leave a moon orbiting on the perpendicular axis?

Since Uranus exists, yes. But why Uranus axis is 98° tilted to the ecliptic plane, is not really known yet, a strong impact is just the leading candidate.

 

[pcw27]

It is unlikely. You might get a high-ish inclination (< 20° :shrug but any higher is unlikely. I think the orbital angular momentum of ejected debris is likely to be in roughly the same direction as the new angular momentum of the body. At higher inclinations, Kozai instabilities are likely to make accretion of a moon difficult.

For a real world example of both the axial tilt change and moon formation scenario, you might consider Uranus. The highest inclination of it large moons is ~4° to the equator. http://en.wikipedia.org/wiki/Moons_of_Uranus#Large_moons

What if the moon isn't formed from accretion and is rather more of a large chunk left by the impact?

Since Uranus exists, yes. But why Uranus axis is 98° tilted to the ecliptic plane, is not really known yet, a strong impact is just the leading candidate.

Yes but Uranus isn't tidal locked to the sun. I was told the tidal locking would result in forces that would eventually split the planet in two by constricting the meridian.

 

[Urwumpe]

Yes but Uranus isn't tidal locked to the sun. I was told the tidal locking would result in forces that would eventually split the planet in two by constricting the meridian.

You are told wrong. If "tidal locking" could result in a planet fragmenting into a debris cloud (not just in half, planets aren't that solid on a stellar scale), it is inside the Roche lobe of the star.

Tidal locking is outside the Roche lobe, it is caused by the planet being slightly deformed towards the star.

If Uranus would be tidal locked, it couldn't rotate that way.

Next: Adding two rotations does not result in a rotation in two axes, but actually a more complex motion, a precession. The force of the sun (via the tidal bulge) on such a perpendicular rotating planet does not make it spin around two axes, but rather tumble around a major rotation axis.

This one might be interesting for you:
http://arxiv.org/PS_cache/arxiv/pdf/0912/0912.0181v1.pdf

 

[Rtyh-12]

There are many objects in the solar systems that are tidally locked and they don't break into two. The Moon is one of them. Most moons are. Of course, they are just moons, but there is no reason for them to break apart, unless they are really really close, but in that case they would break apart even if they're not tidally locked.

I'll just elaborate to say that the story required the planet to always have the same hemisphere pointing towards its star and the other away.

If it's tidally locked then that condition is fulfilled. No need for a second spin.

Yes but Uranus isn't tidal locked to the sun.

Doesn't matter.

 

[pcw27]

The claim was that a tidal lock in conjunction with a rotation perpendicular to the gravity plane would tear the planet apart, not the tidal lock alone.

Imagine a top that's fallen over on its side and is now rolling along a table in a circle. That's the planet (and possibly moon) with the star being at the center of the circle. Is that possible?


Is tidal lock dependent on proximity to the parent body? That was another claim. I was told the planet was too large to achieve a tidal lock without being so close to its star that it has no atmosphere.

 

[Urwumpe]

The claim was that a tidal lock in conjunction with a rotation perpendicular to the gravity plane would tear the planet apart, not the tidal lock alone.

Again, that claim is pretty stupid. You can't get tidal lock AND a rotation perpendicular to the orbit plane. That is both exclusive.

Gravity plane is nonsense. gravity is everywhere. Orbit plane is what you mean, the plane in which the planet orbits the sun.

 

[pcw27]

So then the planet I'm describing couldn't exist?

 

[Urwumpe]

So then the planet I'm describing couldn't exist?

No. Tidal locked means, the rotation axis is perpendicular to the orbit plane, the axial tilt is near zero.

you could bring a planet with 90° axial tilt into the Roche lobe of the star, destroying it into a cloud of smaller particles, but that is something geologic and can only happen during the formation of a solar system.

 

[Rtyh-12]

I still don't understand why you'd want two axes of rotation, but anyway. The planet itself could exist, but slightly different... And about that thing with the planet, proximity and atmosphere... what star does the planet orbit? (as in red dwarf, blue giant, yellow star etc.)

 

[pcw27]

In my ignorance i thought the second axis would make the planet more consistently oriented with a single pole always pointed towards the sun.

I think the star I chose is a red dwarf but that's immaterial. It needs to be a star hot enough to make one side of the planet way too hot for anything other then extremophiles while the other is too cold for anything other then extremophiles. It needs at least half an atmosphere of atmospheric pressure. The key is the tiny band in the middle where the two extremes mix. The mass of the planet can vary by about half an Earth mass. Too massive means my characters wont be able to walk, and too small reduces danger from falling and the like. Since the story's based on suspense and a sense of danger it kind of ruins it for the characters to be able to fall from great distances without being hurt or jump 10 feet in the air to avoid danger.

 

[Rtyh-12]

Did you read my mind here?

If the star is a red dwarf, then the planet needs to be closer to have the same temperature. Close enough to be tidally locked.

The planet can also be the size you want it to be.

'Tidally locked' means that one pole of the planet will always be towards the sun while the other away. Just like our moon, with respect to Earth of course. You shouldn't worry about one side of the planet being too hot, one too cold and the band in the middle just fine. Physics does that automagically for you!

 

[pcw27]

Oh the middle band isn't really "just fine" Its wracked by horrible storms.

How long would a year typically be on a planet like this?

 

[Rtyh-12]

[ame="http://en.wikipedia.org/wiki/Gliese_581_g"]Gliese 581 g[/ame] is an example of such a planet. It seems to be right what you're looking for. Its year is 36 days and a half. It orbits at 0.14 AU, which is in its star's habitable area. The planet is believed not to exist, but it's a good example.