90 degree axial tilt--habitable?

[YankeeGamer]

I was thinking of long days and nights--this IS the solstice--and had an oddball idea for a planet--is it even possible?

I'm envisioning a habitable planet that has an axial tilt of nearly 90 degrees. On the equator, it has normal days and nights in summer and winter, increasing to a time when ther's perpetual near twilight as the sun seems to be skimming the horizon. As you go away from the horizon, the days get longer in the summer, AND the sunlight at the peak of summer is both perpetual and direct...extreme heat.
In winter, the smae pole is BITTER cold, since there is no sun anywhere near the pole.

Could this be habitable near the equator? Or is any iteration of this concept simply unable to have the familiar oxygen-nitrogen atmosphere?

[Phantasm]

Look at Uranus, which has a similar tilt (97.77°; or 82.23° and a retrograde rotation).

We have to remember that the poles will not point directly at the star full-time unless the planet was also tide-locked. However, this means that you'd be looking at areas of midnight sun around the summer solstice and no sun at all for a few days around the winter solstice. During the equinoxes, the equator would be pointed at the sun directly, but during that time the days and nights would be normal length. The autumn equinox would have dawn while you're on the "bottom" of the planet, have the sun directly overhead at midday, and dusk while you're on "top" of the planet; the spring equinox would be the reverse.

Putting this oddball planet in the life zone... I'm hard-pressed to say exactly where the temperate areas would be. It probably wouldn't have polar ice caps, and the wind patterns and tides would be different from what we're used to. I'd be willing to accept an Ocean planet, and I could conceive of an oxygen cataclysm happening to give us a breathable atmosphere...

What percentage of a hydrosphere did you calculate?

[Henchman99942]

So you are assuming the axis continues to point toward the same point in the sky, and as the planet orbits it's star, one pole is baked while the other is frozen, then they switch.

I think a planet that is straight up tidally locked would fair better, with a band of habitable zone maintaining a fairly constant environment.

In the case you present, the habitable zone may shift during the year, forcing everyone to follow that band as it moves - which could produce a perpetually migratory society. But I think the extreme heating and cooling would create too much extreme weather and other side effects.

[YankeeGamer]

With a near 90 degree axial tilt, most of a hemisphere will be in darkness for a half year, and light for a half year. The equator gets light year round--day and night in autumn and spring, sun circling near the horizon in summer and winter.

Earth has a moderate tilt, and some areas see many days of perpetual sun or night--more extreme here. At the peak of summer, not only does a pole get permanent daylight, but it's coming in directly--inferno.

I haven't done any calculations--don't have any resources to do hat sort of calculation.

[YankeeGamer]

Originally Posted by Henchman99942

So you are assuming the axis continues to point toward the same point in the sky, and as the planet orbits it's star, one pole is baked while the other is frozen, then they switch.

I think a planet that is straight up tidally locked would fair better, with a band of habitable zone maintaining a fairly constant environment.

In the case you present, the habitable zone may shift during the year, forcing everyone to follow that band as it moves - which could produce a perpetually migratory society. But I think the extreme heating and cooling would create too much extreme weather and other side effects.

The equator should get a fairly steady amount of sun.

I was looking at this simply because I'd never seen it done

[Rupert]

The story The Man Who Counts, also published as War of the Winged Men by Poul Anderson is set on such a world. Most non-aquatic animals can fly and migrate with the seasons. The planet has an unusually low density, but has retained a fairly dense atmosphere, and thus the fliers can get quite large, thus allowing roughly human-sized sapient fliers.

[edk926]

One thing to take into account is that, if one side is baking and the other side is frozen, you are going to create some crazy winds due to differences in pressures caused by the widely varying temperature.

[AlexanderHowl]

A 90 degree axial tilt is not going to inherently make a planet uninhabitable. It will just have very long days and nights.

[Anaraxes]

Originally Posted by AlexanderHowl

A 90 degree axial tilt is not going to inherently make a planet uninhabitable. It will just have very long days and nights.

Yes. And "just" having very long days means higher temperature variation during that long day and long night.

The Earth heats up all day, while the sun is shining on that part of the surface. Then that side cools down all night, while it's radiating heat away with nothing incoming. The result is about a 15 C temperature variation on average (though the possible range is much higher). On the Moon, with its two-week-long days and nights, the temperature range is about 273 C (+100 C to -173 C).

Yes, the atmosphere has something to do with that difference. But if you're talking a nighttime lasting half a year (assuming an Earth-like orbit), then the temperature on the night side is going to drop quite a lot when the axis is pointed at the sun. Three months around, the planet's rotation (if any) will give you a more Earth-like variation, with a 12-hour day and night. But as the axis gets closer to the star, the days and nights get longer, and the temperature extreme widens further and further.

For Earth, it'd take less than a week of no sun for the average surface temperature to be below freezing, while the hot side average is higher than Earthly record temperatures in places like Algeria, Oman, or Death Valley . A few months would put it somewhere between -50 C and -100 C, while the light side might start boiling liquid water.

A "very long day and night" might not make life impossible, but it would certainly be very hard, with any life needing special adaptations for the extreme (lots of CP in Temperature Tolerance) or high mobility. It's not merely dark for a while. with no other changes.

As was mentioned upthread, that energy difference is going to get evened out. If there's anything fluid, like atmosphere or oceans, it's going to tend to move from the hot side to the cold side. The axial seasons will be extremely stormy (in a way that makes Earthly hurricanes look like a spring shower). So just migrating to the equator doesn't solve all your problems.

[RyanW]

Originally Posted by YankeeGamer

With a near 90 degree axial tilt, most of a hemisphere will be in darkness for a half year, and light for a half year. The equator gets light year round--day and night in autumn and spring, sun circling near the horizon in summer and winter.

Well, half the year is day and half night for every point on Earth, it's just a matter of how it's broken up. At exactly 90 degrees, the poles will see a six month night and a six month day, while the equator will see a pretty normal day cycle with the exception of the sun's zenith getting closer and closer to the horizon as it approaches the solstices (at which, the sun just circles the whole sky right at the horizon). Anywhere in between has a similar day night cycle interrupted by a "summer" of constant day and "winter" of constant night, with the duration of the long day/night being determined by latitude.