I'm attempting to create a world that has very brutal, hard swinging weather and environments, leaving much of the planet desert-like. The world I've designed is a moon of a gas giant.

The Parent Planet

This planet is a gas giant orbiting a sun smaller and cooler than our own. It's closer than the goldilocks zone of this star, so it is quite warm. It has a wide band of rings, and a single moon orbiting closer to the planet than the rings.

The Moon World

The moon is earth sized, not tidally locked, and experiences days similar in length to earth days, and its orbit is about 40 of these days. Most of the year, due to the rings, this planet has a band at the equator of essentially arctic tundra, never seeing sunlight. However, due to the axial tilt of the parent planet, and the moon's slightly erratic orbit, each hemisphere spends a quarter of the year (Each year being about four orbits, or 160 days) more and more in the shadow of the rings, making for one very, very cold orbit (-40F, -40C at night). On the other side of the year, with one hemisphere out of the shadow of the rings entirely, the surface becomes blazing hot, reaching regular temperatures of 120F (49C) During the day. During the other two orbits, with some shadow from the outer rings, temperatures are more stable and around 80-60F (27-16C) high and 60-40F (16-4C) low.


Is this proposed moon possible? Would it act how I expect it to? If not, can I fix it? Bonus points for pointing out any cool and unexpected visual effects.

  • 1
    $\begingroup$ tidally not locked moon of a giant planet is possible only if it is very far from it. But then optical effects of the rings are negligible. $\endgroup$
    – Gangnus
    Commented Sep 21, 2018 at 21:59

3 Answers 3


(In answer to the original question: Is it possible to have a planetary moon closer than rings?)

It doesn't really make sense.

Rings and moons aren't unrelated features that just happen anywhere around a planet. Rings are what happen when a moon is too close to its parent planet, i.e., when it is inside its Roche Limit

It's theoretically possible, if you had a really, really dense moon (e.g., pure osmium) orbiting just inside the rings that resulted from a really, really light moon (water ice), but it would be a very contrived situation.

  • $\begingroup$ Thanks! I'm ok with it being bordeline impossible... As long as it is. The real concern is the proposed calendar. I'll edit for clarity. $\endgroup$
    – Skyler
    Commented Sep 21, 2018 at 20:15
  • $\begingroup$ Any close satellite initiates a ring closer to the planet. This fact was found in 80-ties. So, even if there will be an inner heavy satellite, there will be one more ring inside its orbit. Rings APPEAR according to Roche radiuses, but they later move to other places, according to the satellites scheme. $\endgroup$
    – Gangnus
    Commented Sep 21, 2018 at 21:54
  • $\begingroup$ @Gangnus So it would technically be possible to have older rings that have an ascending orbit, and have a moon be formed closer than the rings? $\endgroup$
    – Skyler
    Commented Sep 21, 2018 at 22:28
  • $\begingroup$ @Skyler 1. Each close satellite creates a stable zone for a ring UNDER it. 2. A satellite can exist ABOVE Roche radius only. 3. If a satellite goes under the R.radius, it fells apart and can become a ring. 4. Processes on low orbits go faster. .. How can the innermost satellite not to have its ring? Oh! Its ring can be eaten by atmosphere! $\endgroup$
    – Gangnus
    Commented Sep 21, 2018 at 23:16
  • $\begingroup$ An edit and an upvote! ;-) $\endgroup$
    – Fabby
    Commented Sep 22, 2018 at 1:53

So... the moon's existence might be possible given the planet's own position and composition. But for the smaller details like the tidal lock and the temperature, I'm not so sure about.

Planets form from leftover debris from the formation of a star; rocks and solids usually orbit closer to the star as further out the gravitational field would be too weak to lock them in orbit. However, gas giants can only form within the larger clouds of gas and ice further out as only there are the materials abundant enough to make them 'giant'; like the gas giants within our own solar system.

There would have had to be some kind of push from another celestial body to nudge it that close to the star. And yeah, there's a precedent for this happening: http://hubblesite.org/hubble_discoveries/discovering_planets_beyond/how-do-planets-form

So now you need a large celestial body to give a gravitational nudge to the planet. And there you might be able to explain some of the rings; some pieces from a nearby asteroid belt were thrown away by its gravitational field and nudged it ever closer to the star while others were trapped in the field and began orbiting it as rings.

The moon could be what's called a "Shepherd Moon"-- one that orbits in the very inner or outer circle of the rings. After some more research I don't think it's possible for a moon to form any closer than the shepherd moons in the rings (which keep them from clumping and collapsing onto the planet), there's a lot that can happen in space and the possibility it could occur might be very unlikely but still possible with some extreme circumstances and a bit of glossing over. https://www.iflscience.com/space/how-saturns-shepherd-moons-herd-its-rings/ https://physics.stackexchange.com/questions/26643/why-arent-saturns-rings-clumping-into-moons https://en.wikipedia.org/wiki/Ring_system

It's not the best explanation, but this could suffice as a placeholder until something better comes along.


There is no 'shadow of the rings'

Saturn's majestic A ring is 10-30 meters thick; overall the rings of that planet range from 10 meters to 1 km thick.

The asteroid belt is a 'ring' around the Sun. That, too, is not dense enough to block the sun. In this answer, I show that if the asteroid belt was ground down into particles 100g in mass, each particle would have 1530 km$^2$ of space to itself; each particle would be about 14 km from the next particle.

Rings just aren't thick enough to create an appreciable shadow.

  • $\begingroup$ Yes, but these rings wouldn't be casting a straight - on shadow. Saturn's rings are about 282,000km across, if they were casting a shadow on only half the planet, the diagonal of the rings would be much, much thicker. $\endgroup$
    – Skyler
    Commented Sep 21, 2018 at 20:30
  • $\begingroup$ In fact, with some quick trig, considering the width of the rings (282,000) and the half-width of an earth-like planet being only 6371km, the angle of the rings would be 88.6, if they perfectly tilted to cover half the planet, making the percieved width of the rings almost the entire width of the rings... certainly enough to cast an appreciably thick shadow. $\endgroup$
    – Skyler
    Commented Sep 21, 2018 at 20:42
  • $\begingroup$ @Skyler Yeah, but they are 10 meters thick, and they are not solid. Density is like 0.02 g/cm^3; barely more than a gas. The only way they can shade the sun is end-on; and in that case they are too thin. $\endgroup$
    – kingledion
    Commented Sep 21, 2018 at 20:45
  • 2
    $\begingroup$ That is objectively incorrect based on observation of our own solar system. quora.com/Does-Saturns-ring-cast-shadows $\endgroup$
    – Skyler
    Commented Sep 21, 2018 at 20:48
  • 1
    $\begingroup$ @Skyler Those are long exposure pictures; the long exposure heightens the contrast between light and shade. The luminosity difference is nowhere near close enough to make a tundra zone. $\endgroup$
    – kingledion
    Commented Sep 21, 2018 at 20:52

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