I would like to know, if it is possible for a planet to orbit a star with one pole (let's say the north pole) facing away from the sun throughout its orbit?

What I am thinking of is: while the Earth's axial tilt gives us seasons and midnight sun during arctic summers, could another planet with a similar axial tilt "lean outwards" during the entirety of its solar orbit?

I'm too much of a layman (and too lazy I guess) to figure out if angular momentum would prohibit such an orbit...

Thank you in advance!

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Do you mean like Uranus with an axial tilt of 97.7º ?

I don't think that would work: the pole has to keep facing in the same absolute direction, which will only be directly away from the sun at one point in the planet's orbit.

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How about a planet that is tidally locked to the parent star?

If we ignore its magnetic field, it would fit the criteria. Otherwise, I say +1 for Uranus.

It was established (IMHO) in the "Can an object in space have many axes of rotation?" thread that such an orbit would be highly unstable.

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If we have a rapidly spinning planet tilted like Uranus, it would take a colossal amount of torque from heaven only knows where to make it precess rapidly enough to keep that spin axis pointed toward the Sun.

We can get virtually the same lighting and heating effects by simply having the planet tidally locked with its rotation axis perpendicular to the plane of the orbit. The difference would be the lack of a strong Coriolis effect in the atmosphere.

Thank you very much, that helps a lot.

I wasn't thinking of anything as drastic as Uranus (which I understand is practically lying on its side), and a tidally locked planet won't do for my purposes either. (A bit of speculative fiction I had in mind)

--- Edit ---

I was wondering: could the sort of orbit I'm talking about exist for a short period?
a1call mentioned that it would be highly unstable.
So could something (a large impactor for instance) temporarily cause such an impact? And how long would it last?

Thanks again!

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A colossal impact of a substantial mass could role a planet into a orbit as you describe. I do not think it could sustain that position for more than a half revolution. However the pole facing the star would half a revolution later be facing away...To tidally lock a pole toward a star would be against the orbital momentum of the axial energy. Play with a gyro and all will be clear.
I hope I helped...

I see now that I read between the lines and imagined an extreme Uranus-like tilt when you said nothing of the sort. My bad.

My basic opinion stands. A colossal amount of continuous torque would be needed to make the planet precess that rapidly, and I cannot imagine a possible source. If we take some sci-fi liberties and create such a planet, I would expect the climate to stabilize with the northern hemisphere much colder than the southern hemisphere.

Hi El_Thorsen,

Firstly please note that I have no qualifications to comment on Orbits and the likes.
Keeping that in mind here is what I think. Such an orbit would be the result of the combination of two axial rotations.

*- One equal to one revolution per orbit and around an axis perpendicular to the plane of the orbit. Our moon has exactly one such rotation.
*- Plus a 2nd rotation/wobble around an axis perpendicular to the 1st axis such that it remains towards the center of the orbit.

This combination is unstable because it subjects the planet/moon to elastic deformations which result in energy deception of the angular momentum. The imperfections of the planet/moon will cause the wobble axis and primary axis to rapidly approach each other and constitute a single axis (eventually).
The amount of time such an extreme wobble could last will be shorter the faster the wobble and longer the more massive the planet/moon is.

Bottom line: An impact of an asteroid on the front/back border of the Moon could cause it to rotate/wobble such that it would satisfy the OP. If the Moon was perfectly homogeneous and spherical and infinitely rigid this would last for ever. In reality though it will change quickly towards a closer axis wobble. I can not make any valid estimates as to the expected duration of such an orbit.

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"They reasoned that an object situated at the center and related equally to the extremes in every direction can have no impulse to move in any specific direction. In fact, they compared the situation of such an object with that of a man violently but equally hungry and thirsty, standing at the same distance from food and drink and unable to decide in which direction to move." - Aristotle

So did I, actually. Yet, re-reading the OP it becomes clear that he said nothing of the sort...

Must have been something in the air that day.

Though I admit to not knowing a lot about orbits, I think it's impossible, or nearly so, for two reasons:

1.) The gyroscopic effect of the planet's rotation would act to keep it "pointed" in one direction, à la Uranus.

2.) The very act of keeping a hemisphere's rotational axis toward its star would necessitate rotation in *another* axis to maintain it, thus resulting again in a planetary axis more or less perpendicular to its orbit.

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Can the star be a magnetar (one with an intense magnetic field)? That might provide the force we'd need for the planet's orientation to be constantly changing like that, if the planet had the right composition, mass, and distance. Messing with the planet's angular momentum like that could make it fall in or float away and screw up its day length (and maybe tectonic stresses too), but it could at least take longer than the mere fraction of one year that gyroscopism alone would take to permanently orient the axis without any such external force at work. (The planet would also have a radiation problem if anyone's supposed to live on it, though.)

I should have mentioned that I was looking for something more earth-like than Uranus (when I was planning the post I had it in mind, but must have forgotten about it...)

Guess it can't be done.
Thanks again!

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The short answer to both questions is no. The longer answer is that no planet will ever keep its pole facing the star under any circumstances, in the absence of an outside torque. You might be able to stabilize such a system with an electromagnetic torque for a short time, but it won't last long, if it is even possible.

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If we build SSP = space solar power in GEO orbit; We will likely spin the solar panel aray, one revelution per year, so it's rotational axis points continiously at the Sun. This will require some station keeping energy, but not much as the mass will be much less than a planet. Neil