Are there any known planets whose axis is oriented such that one pole always faces its star?

  • Earth rotates around the sun and around its axis (A North, South axis) giving us days, nights and seasons.

    Are there any known planets that rotate with an axis pointing toward its sun so that one half of the planet is always facing it (the other half is always facing away from it)? Like Uranus, but not switching which side is facing the sun every 6 months.

    (Would this even be theoretically possible?)

    Earth's seasona are a result of the axus tilted by around 23 degrees. Look at Venus with almost no tilt, there are no seasons. She has another unique behaviour tho.

    @ott-- Venus is very... retro...

    Because you want the pole pointed at the star, the answer is no, Closest thing is referred to as tidally locked, it is a real thing, as a reasonable example, look at the moon relative to the earth. orbital rotation (almost)equal to axial rotation.

    There are some unlikely situations where you could make it work, like a planet that got hit by a large Iron rich meteor and as a result, it has more Iron on one side than the other. The heavy side of the planet could point towards the star, while the planet still has a rotation more perpendicular to the star. That's theoretically possible, but only if the mass imbalance from the Iron rich side of the planet was greater than the gravitational imbalance from equatorial bulge. Unlikely but possible. If you want more specifics, I can give an answer with more details.

    Some satellite orbits are sun-synchronous, tuned so that precession due to Earth's equatorial bulge has a period of 1 year. Such orbits are highly inclined but not exactly polar.

    LeHill - what you're forgetting is that: think about the "axis" of a planet. (Imagine a big stick going up and down at the top and bottom). Don't forget, **that never changes**. (OK, it can precess, but forget that for now.) You're thinking of the "string" being "attached to" that stick. That's wrong. The "string" is attached to the middle of the planet - the "stick" sort of "magically" always stays in the same orientation.

    But "Closest thing is referred to as tidally locked" ... is the exact opposite of the OP's question. A point 90 degrees away from what the OP is asking, is pointing at the star: it's an exact example of exactly not what the OP wants.

  • aventurin

    aventurin Correct answer

    6 years ago

    Would this even be theoretically possible?

    No. The law of conservation of angular momentum prevents this for a planet with reasonable inner structure in a gravitational field like that around the sun.

    The angular momentum of the planet points in a certain direction. Changing this direction requires a torque perpendicular to the rotation axis.

    If a torque existed that could force the rotation axis of the planet to always point towards the sun, the corresponding force had to have a component perpendicular to the line from the sun to the planet. The force also had to change periodically with the orbit of the planet. Obviously such a force does not exist in the planet-sun-system.

    "with reasonable inner structure" -- the implication of course is that with *unreasonable* inner structure, such as a counter-rotating core and some sort of super-science to use one to drive the other in synch with the year, you could rig something up.

    From driveby editor: "I cannot write a comment so I try this way and I hoep original poster wouldn't mind. I in part can understand your reasoning and explain ti to my self like this: The material a planet was made of was circling around the star.

    Part of that angular momentum is retained in the orbit and part in the rotation of planet. This, logicalyy cannot deviate to much form the plane of rotation of original material. But... this should be in a comment: How about if a star traps a wondering plant. This one should have any axis of rotation it likes. Am I wrong about that. Will some kiind of force, like tidal forces, align the roration of the planet?"

    The answer to that question is that it doesn't matter how the system originally came to be. Compare the angular momentum of the planet's rotation at one point in its orbit, with the opposite point in the orbit. It has the same magnitude but opposite direction, since the axis of rotation of the planet has changed 180 degrees over the course of the half year. The force required to achieve this is what aventurin is saying simply doesn't exist in orbital dynamics.

    @called2voyage, Tidal forces cannot align an axis of rotation this way. The reason is gyroscopic force (a force perpendicular to the axis of rotation results in a corresponding force 90 degrees from that). So, any force that tried to keep a rotating pole aligned to the star, would gyroscopically force the pole towards a perpendicular from the orbital plane. Thus, given enough time, all rotating orbital bodies would get their poles aligned in a perpendicular manner.

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