8
$\begingroup$

I’m trying to design a Planet that is as stereotypically “Alien” as possible. That means an Alien Ecology, twin suns, and this view from the surface which I’m sure you’re all familiar with:

enter image description here

How can I achieve this effect without the world being a moon of a gas giant? Because that entails all sorts of issues with radiation and tides that I really don’t want. Is there such a thing as a ringed moon?! Help!

$\endgroup$
3
  • 2
    $\begingroup$ Beautiful image ! To me your image looks like a view from Titan, one of Saturn's moons. Does this need to be a moon in your story, or could it also be a neighbouring gas giant on a larger distance ? $\endgroup$
    – Goodies
    Nov 3, 2022 at 10:13
  • $\begingroup$ Have the rings be artificial constructs circling the moon like a Dyson ring. Why I will leave up to you. $\endgroup$
    – Joe Smith
    Nov 4, 2022 at 0:56
  • $\begingroup$ The moon in the image is so large, it may be bigger than the planet itself :D $\endgroup$
    – Haukinger
    Nov 4, 2022 at 8:12

4 Answers 4

11
$\begingroup$

Is there such a thing as a ringed moon?

The main problem here is that in order to form a nice big ring, your moon would need to have had its own moonlet. Capturing a big icy asteroid or comet is very tricky, and given the relatively small gravitational sphere of influence of the moon relative to its parent the chances are that the moon wouldn't be doing the capturing anyway. Having a moonlet generally requires your moon to be quite large, and therefore your planet to be larger still, which probably renders it uninhabitable.

The chances of the moon appearing that big in the sky in the first place are also slim. Even a binary companion planet might not look as good as that.

moon of a gas giant... that entails all sorts of issues with radiation and tides that I really don’t want.

This doesn't need to be the case. Jupiter has a pretty hostile set of radiation belts, but the same isn't true of Saturn. The magnetosphere of the parent world can be tuned to your needs.

You would certainly be stuck with a tidally-locked world however. There are various other questions on this site going back many years about having non-tidally-locked moons, and the answers generally seem to be "probably not" but it isn't entirely outside of the bounds of probability. However, in order to avoid tidal locking your world has to be quite a long way from its parent planet, so the view won't be nearly as dramatic.

On the bright side though, a close enough orbit for a good view is going to be a relatively short period orbit, so you don't have to deal with nights that are hundreds of hours long.

Moreover, stable close orbits around binary stars are awkward, and more distance orbits can be chilly. The radiated heat from a very big gas giant parent (potentially even a brown dwarf) might go some way to keeping things warmed up.

$\endgroup$
5
  • 2
    $\begingroup$ To reiterate something I've said before, it's always nice when Starfish reaches these questions before me, because they say everything I would have said, except with greater clarity. $\endgroup$
    – jdunlop
    Nov 3, 2022 at 19:01
  • 3
    $\begingroup$ A tidally locked moon isn't necessarily a problem the way a tidally locked planet would be. Sure, on a planet where one side faces the sun all the time, you'll have one side baked and the other frozen. A tidally locked moon, however, still gets mostly equal doses of day and night as it orbits its planet. Sure, people on the far side aren't getting that cool view of the planet like those on the planet side, but they still get day and night with respect to the sun, provided the planet isn't eclipsing it all the time. Tilting the moon's orbital plane could alleviate that issue. $\endgroup$ Nov 3, 2022 at 20:28
  • $\begingroup$ One can even have a stable orbit around a binary star, this is what the L4/L5 Lagrange points are for. Just take care to have the necessary mass ratios and more or less circular orbits. $\endgroup$
    – fraxinus
    Nov 3, 2022 at 22:21
  • 1
    $\begingroup$ Even Jupiter's radiation belts aren't a deal-breaker. Radiation levels on Callisto are tolerable, and once you get much closer than Callisto, Saturn-style rings would take up too much of the sky to be dramatic. $\endgroup$
    – Mark
    Nov 3, 2022 at 23:27
  • 1
    $\begingroup$ @fraxinus Ahabitable planet would probably not be in a trojan obrit in a binary system. The mass ratios betwen the three objects necessry for long term stable orbits would probably b extremely inconsistent with the mass range of stars. Any posible stars, let alone stars which could be in the same system as habitable planets, would be too similar in mass for a trojan relationship. $\endgroup$ Nov 4, 2022 at 6:31
5
$\begingroup$

It all depends on how long you want that view to be available.

A significant meteor impact on our moon would, for example, create a ring of ejecta around it (and part of it would probably shower us, too).

The problem would be that

  • moon's gravitational field alone has very few long term stable orbits: most of the ring would fall back to the surface within years. If your moon has the same situation, you will have the same problem
  • the proximity of the main planet means that the ring would be highly perturbed and, again, have troubles being stable for long period of time.

Therefore, if you are fine with having the ringed moon only for some years (and getting as a bonus a meteor shower on your planet), it's totally plausible.

If you want it for geological times, you need to come with some other mechanism, like a volcanically active moon, with volcanic activity constantly replenishing the ring and keeping a dynamic balance with the material lost due to gravity.

$\endgroup$
2
  • $\begingroup$ I'm wondering, are there known instances (or at least plausible scenarios) where volcanic eruptions reach orbital velocities? $\endgroup$
    – fgysin
    Nov 3, 2022 at 7:51
  • $\begingroup$ @fgysin, Moon escape velocity is 2300 m/s, while the fastest volcanic ejecta I can find mention of were at 400 m/s. It looks like there is some space for orbital velocities to be reached. This astronomy.stackexchange.com/q/18567 gives some estimate for IO $\endgroup$
    – L.Dutch
    Nov 3, 2022 at 7:56
2
$\begingroup$

Make an artificial moon

What you want isn't something that would naturally occur because of gravity being needed for massive rings, but that's fine.

Have a moon that's fairly close to a planet which has massive artificial rings. Maybe they're satellites to generate power or something. They are maintained by advanced alien engineering.

$\endgroup$
1
$\begingroup$

Why are you worried about making your world a giant moon orbiting a gas giant planet?

Radiation belts around the planet can be a problem, but if the moon doesn't generate its own magnetic field it will be exposed to the solar wind and cosmic rays.

Unless it orbits within the magnetosphere of the giant planet which will trap charged comsic rays and solar wind particles. So long as the orbit of the mooon avoids the zones where the charged particles are trapped the Moon should be fine.

If the moon is tidally locked to the planet one side will always face away from the planet and one side will always face toward the planet. And as the moon orbits around the planet, each side will experience a day night cycle which will be the same length as the moon's orbit around the planet.

If the moon is as large as a planet it might be able to generate its own magnetic field. It is a common idea that the faster a world rotates the stonger its magnetic field will be (though it might depend on the rotation rate of the core and not that of the world as a whole.

And of course the rotation rate of a tidally locked moon would be equal to its orbital period around its planet.

So if the moon is outside the magnetosphere of the planet, it will need to rotate as fast as possible to generate its own magnetic field, and thus have as short an orbital period as possible, and thus orbit as closely to the planet as possible.

A short orbital period would also be necessary to have the daily cycle of light and dark as short as possible so that the moon doesn't get too hot in the day or two cold in the night. Opinions may vary on how long a day could be before the temperature extremes became too much for life.

If a moon orbits close to the planet, the tidal interactions between the moon and the planet will produce internal tidal heating on the moon, which will tend to warm the surface somewhat.

If the tidal heating is too great, the moon will suffer a runaway greenhouse effect, and if the tidal heating is even greater, the moon will become a volcanic hell like Io, the innermost moon of Jupiter. But if the planet and moon are too far from the star and don't receive enough heat from the star, significant tidal heating could keep the moon warm enough for life.

The habitable edge is a term for the inner limit for a moon to orbit its planet before tidal heating makes the moon too hot.

https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf

And this paper paper discusses the magnetic shielding of exomoons. It apparently concludes that a Mars sized exomoon orbiting a jupiter sized exoplanet would be in the planetary habitable zone between about 5 planetary radii and 20 planetary raddi.

https://arxiv.org/abs/1309.0811

There are two types of orbits a planet can have in a binary star system.

One is an S-type or non circumbinary orbit, where the planet orbits around one of the stars at a fraction of the distance between the two stars.

The other is a P-type or circumbinary orbit, where the planet orbits around both of the stars at a distance several tiems as great as the orbital separation of the two stars.

There are known examples of both types of exoplanet orbits in binary systems.

If you want the stars to always look close together in the sky, and if you want each star to always appear to have the same angular diameter as seen from the planet and its moon, you will have to go with the planet in P type or circumbinary orbit around the two stars.

For a circumbinary planet, orbital stability is guaranteed only if the planet's distance from the stars is significantly greater than star-to-star distance.

The minimum stable star-to-circumbinary-planet separation is about 2–4 times the binary star separation, or orbital period about 3–8 times the binary period. The innermost planets in all the Kepler circumbinary systems have been found orbiting close to this radius. The planets have semi-major axes that lie between 1.09 and 1.46 times this critical radius. The reason could be that migration might become inefficient near the critical radius, leaving planets just outside this radius.[9

https://en.wikipedia.org/wiki/Habitability_of_binary_star_systems

Suppose that the two stars are in almost circular orbits five million to twenty million kilometers apart. If the minimum distance for planetary orbits is 2 to four times that distance, the closest planets to the two stars would be at least ten million to eighty million kilometers from the two stars.

In our solar system the planet Mercury orbits the sun with a semi-major axis of about 57,909,050 kilometers or 0.387098 AU, the planet Venus orbits at about 108,280,000 kilometers or 0.723332 AU, and the Earth at 149,598,023 kilometers or 1 AU.

Tf the two stars have a total luminosity equal to that of the Sun, your planet and moon would have to orbit them at a distacnce of exactly 1 AU to receive exactly as much heat from them as Earth gets from the Sun, though of course a habitable world could receive somewhat more or less radiation from its star than Earth gets from the Sun while stil remaining habitable.

If each of the stars ha the same luminosity as the Sun, their total luminosoty would be be twice that of the sun. Because of the inverse square law, the planet and moon would have to orbit the two stars at a distance of the square root of two, 1.414213562, AU or about 211,563,552 kilometers, to receive exactly as much radiation from the stars as Earth gets from the Sun.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .