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I was looking into creating a planet with an icy ring around the equator while also having ice at the poles and bands of warm/tropical areas in between.

The way I thought to do this would be having the planet with a thick planetary ring (or rings) orbiting a star with either no axis tilt or a very small percentage tilt just enough to cast a wider shadow near/on the equator that only fluctuates slightly each year from the tilt, keeping at least half of the shadow covering the equator at all times.

Would this be constant shadow cool the equator enough to form ice?

--- EDIT ---
Removed second scenario since it would be far too difficult to figure out.

With the new information brought forward in a few of the answers, it has lead me to ask about an addition to the scenario.

Sticking with the idea above, if I added scattered mountain ranges along the equator of the planet capable of having ice/snow year round, and lets say the planet had an early snowball period, but the snow/ice melted between the equator and the poles first due to elevation + wind & ocean currents, would the albedo of the ice/snow maintain cold temperatures in the valleys/gaps between mountain ranges? Given of course that the albedo between the equator and poles absorbs heat.

Also, I'm not trying to get a 100% perfectly provable outcome, it just has to be close enough that it could be considered possible enough to be believable.

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    $\begingroup$ @ARogueAnt. would having a mountain range along the equator throw off the orbit/spin or prevent it from becoming habitable? $\endgroup$
    – Zethnos
    Aug 11 at 1:51
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    $\begingroup$ I'd note that planetary climates are transient on geologic timescales. To have tropical mountain ice now, the planet doesn't have to have been a snowball planet earlier in its development so much as it had to have an ice age recently, which is why we still have glaciers (for now) in temperate zones. $\endgroup$ Aug 11 at 16:56
  • $\begingroup$ @jeffronicus Yeah that is true. An ice age would work as well, I'm just not sure if the light/heat reflection as a result would be enough to keep it cold all along the equator and not just on the mountain tops. $\endgroup$
    – Zethnos
    Aug 11 at 17:20
  • $\begingroup$ You're now asking multiple questions and risk shifting the focus of the question as to orphan existing answers. $\endgroup$
    – rek
    Aug 11 at 17:50
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Not With Natural Rings

A number of deeply unlikely things would have to happen, combining to make the circumstance so unlikely as to qualify as "impossible".

  1. The rings would have to be in the same plane as the planetary orbit to be constantly edge-on with the sun. If formed by either impact or orbital capture, this is deeply unlikely. Saturn's rings range over 26.7 degrees over the course of its year. Though I don't have the math for it, I would also question the stability of a ring system that had the sun constantly deforming the ring in the same vector.
  2. The rings would have to be entirely rocky. Assuming a habitable planet, the planet wouldn't be able to hang on to icy or gaseous rings because of proximity to the sun. It's really hard to assemble such a ring.
  3. The rings would have to be distributed in a fashion that maximized reflection. Without ice, your albedo isn't going to be fantastic, so it's a safe bet that a rocky/dust ring would be deorbited in the process of reflecting light away from the planet. You don't have enough mass to waste any.
  4. I'm not even sure if this last qualifier is possible, but discarding the other issues, I can't see any way that the insolation near the shadowed equator, in atmosphere, wouldn't keep the region ice-free regardless of the ring.

This could be an artificial construct, with stationkeeping and mirrors, of course, but it's difficult to conceive of this as a natural formation.

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Highly unlikely. Keep in mind rings are only a few metres thick at their densest, reflect light that otherwise wouldn't have reached the surface, and don't stop heat convection in the atmosphere (from adjacent sun-lit latitudes) simply by casting a shadow.

Here's another approach: on Earth ice forms at the equator at elevations above ~4500 metres:

Latitude and glacial formation graph Source: Landscape development of the Himalayan–Tibetan orogen: a review, Lewis A. Owen

(Lower if your planet is in a particularly glacial mood.)

Now it's arguably unlikely you'll get an equatorial ring of unbroken mountains and plateaus, but at least that would realistically sport ice caps and permanent snow.

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  • $\begingroup$ Question: If I went this type of route where I had various equatorial mountain ranges (not necessarily connected, but scattered along the equator) mixed with something like, "in early development it became a snowball planet and as it warmed back up the ice melted between the poles and equator first, due to elevation + wind & ocean currents" could I use the albedo of the ice on the mountains to maintain this effect between the gaps in the mountain ranges if the albedo between the equator and poles absorbs the heat? $\endgroup$
    – Zethnos
    Aug 11 at 13:16
  • $\begingroup$ Are you asking if albedo would keep neighbouring regions cold? I don't think so. It wouldn't offset ocean/atmo heat transport to the low-lying spaces between ranges. $\endgroup$
    – rek
    Aug 11 at 17:57
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The Good, the Bad, and the Ugly

Gravity is not your friend. The effect of a lot of objects (like a ring) orbiting is that they spread out and thin out. For example, while some of Saturn's rings are 2 miles thick, the majority are only 30 feet thick. Those rings do cast a shadow on the planet thanks to the planet's axial tilt, but while the shadow can be seen, it's not nearly as opaque as you might assume. In other words, the ring isn't solid, so there's still plenty of light getting through.

So, a science-based answer is no, you can't do that.

But a science-fiction answer is, why not? The universe is blowing our minds every day! Anyone with the guts to say it can't be done is just setting themselves up to be the cul de la blague du jour. So I say describe a set of rings that's unusually thick and unusually dense and never look back. To do this on the equator, you'd need an axial tilt of zero degrees.

The brown dwarf solution is more complicated, because the physics of how a planet is heated is still in play. Anything star-related that would let the equator be a snow-zone would freeze the rest of the planet because everywhere else is getting even less light and heat.

But let's look at a science-fiction answer again: what if you had a binary star system where the planetary eccliptic was perpendicular to the secondary star ecliptic? In that case you'd have very warm poles, much cooler equators, except when the planet and that other star happen to come together — in which case the equator would get cheerfully warm. OK, scrap that. It's not quite what you're looking for.

Off the top of my head, I can't think of a way to do this with a star.

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  • $\begingroup$ Thanks! The setting I'm trying to build will ultimately be used for many different purposes involving fantasy + science fiction, that's why I was hoping for a more science-based answer, so I could use it in both settings. I didn't really consider just saying "eh because science, probably" and coming up with a description that makes sense without technically being possible. I was concerned with people picking it apart as not being possible if I didn't have at least a really close to possible answer. $\endgroup$
    – Zethnos
    Aug 11 at 2:43
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    $\begingroup$ @Zethnos Worry less about people picking it appart and more about telling a good story. When Larry Niven wrote his first Ringworld book, he had (as I recall) the students from MIT visiting the convention he was at marching through the halls chanting "The Ringworld is Unstable!" Did he take it personally? Nope, he used their insight to modify how the Ringworld worked and wrote that into the next book. There will always be some bozo out there just aching to find a flaw in your creativity - toss them the proverbial vulgar hand gesture and ask if they have a published book? $\endgroup$ Aug 11 at 21:19
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An equitorial ridge: similar to that on Iapetus:

enter image description here

Iapetus is one of the oddest moons in the solar system. Not only is half of it pure white, and half is nearly black, it also has a 13km high ridge that goes around the equator. The ridge may be the result of the moon spinning rapidly when it was young, and slowing down (somehow) or it might be the result of the accretion of material from ring around the moon that has fallen onto the moon's surface.

Never the less, a line of mountains on Earth of equivalent height, let alone proportionate height (that latter is probably not possible) would be covered in permanent ice and snow.

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