Habitability of a Low Axial Tilt Planet with a Planetary Ring

Question

So, I'm creating a world for a science fantasy setting. It has magic and such, but since it's mainly taboo to use it, I want the world's climate to be based on hard scientific facts.

My world has an axial tilt of twelve degrees, creating much milder seasons than on Earth, and expanding the temperate zone considerably. What’s complicating things is that this planet has a silicate planetary ring. For six months, the tropics experience normal light and heat energy for a planet's equator. However, for the other six months, the rings block out some sunlight, creating an average temperature similar to more polar regions. This makes the area prone to flooding, and limits what vegetation can grow there.

Now, based on my research, the shadow of the ring would create a strong front of cold air that blows towards the side of the tropics that still experience normal sunlight, creating a massive band of storms that spans the whole tropical band. It also creates currents that vary between tropical and polar temperatures, bringing about stark seasonal differences in those lands the currents are near. What I want to know is, would this planet still be mostly habitable? Or have I created a scenario where things would be far too unstable for anything to survive? I'm most worried that the planet might just be full of storms at all points, making it too turbulent for humanoid life to be sustainable. Any help you guys can provide is much appreciated!

Answer 1

I suspect that life would still be possible on such a planet, however many species as we know them would not survive and those that remained would have to be very adaptable. It would depend to some extent on the origin of the ring. If life had evolved with it then it would have adapted to it. If the ring “arrived” at some later point there would likely be a large mass extinction due to climatic disruption.

One of the key drivers of the climate are Hadley cells: https://groups.seas.harvard.edu/climate/eli/research/equable/hadley.html These affect the mass movement of moisture and heat in the atmosphere north to south as well as the jet stream, rain fall patterns and more besides. If the Intertropical Convergence Zone was converted to a pole it would reverse the atmospheric circulation pattern, however it would probably take months to establish a pattern by which time it would reverse again. https://en.wikipedia.org/wiki/Atmospheric_circulation

I would predict a chaotic climate with all manner of extremes of wind, rain, snow, heat, cold and draught at irregular intervals. Sea life would probably fair best as the oceans would act as some form of heat buffer, but it could easily leave the planet devoid of all or much higher life forms on land.

Answer 2

To be clear, if axial tilt is 13 degrees (and ring is not too distant), every point within your tropics will have a time of year within the shade of the ring. More extreme latitudes (towards the poles) will be unshaded.

When "shaded" you will have part of each day under the ring and part not. The closer you are to the equator the more extreme the shade is (the equator will be shaded 24/7 at Autumn/Spring meridians, making them coldest in a syncopated pattern to the dominant global seasons).

My Geophysics PhD thinks that you will see a cooling from the shade, and a moderately strong enforcement of convection around the shadow. Air in the upper atmosphere will cool and sink, leading to an outflow away from the shadow at ground level. Net impact will be a dry season when under the shadow, with strong winds.

The rotation rate of your planet will impact the telecommunication range of effects. A faster planetary spin means smaller hadley cells and more localised effects. A slower spin means the change in atmospheric dynamics will be well communicated very far from the equator (context: the Earth Hadley cells are three from equator to pole, spanning equator-to-Sahara in the first cell)

PS.

My world has an axial tilt of twelve degrees, creating much milder seasons than on Earth

If you are going down this rabbit hole, you should be aware that there are two sources of seasonality:

1. Axial tilt: creating hemisphere-based seasons based on the 6-months when the hemisphere "points" at the local star;
2. Orbital ellipsis: Creating global temperature changes based on when the planet is literally closer to, or further from, the star in its orbit due to the elliptical nature of the orbit;

and you will need a more circular (than Earth) orbit to dampen seasonality.

Answer 3

You need more numbers, like how large are the rings, how opaque they are, and how far they are from their planet. However, I can say that I’ve found a useful method for figuring out how much of the total solar energy a planet receives, as shown in How to Determine Planetary Extremes of Temperature from Average Global Temperature?

The answer provides a useful way to find the amount of radiation a specific latitude receives, using trigonometry.