# Are all satellites of all planets in the same plane?

I'm aware of the fact that all planets are almost in the same plane as the Sun and the Earth(ecliptic) plane except Mercury and Pluto(which are more tilted). Are all satellites of all the planets also in that very plane or the same plane as one satellite is for a planet but (maybe) not in the same plane as other planets are with the Earth-Sun plane?

• I tried to search on Google for a lot of time, I didn't get a satisfactory answer, right now if you search, the most accurate answer is this page itself, if you have links for the pages that answer this better, refer them to me, I'll delete this question. Sometimes, the link is not indexed and user is unable to find in search engines and these sites are to refer to those links as well. – messy_tech Dec 21 '20 at 14:57
• Comments are not for extended discussion; this conversation has been moved to chat. – called2voyage Dec 21 '20 at 18:14
• Although earth's orbit is approximately elliptic, the corresponding plane is called the ecliptic. – das-g Dec 22 '20 at 10:42
• Thank you @das-g – messy_tech Dec 23 '20 at 22:17
• @JCRM I would have, but with my rep level I can't make edit suggestions that change fewer than 6 characters. – das-g Dec 24 '20 at 9:45

Most large regular satellites orbit in the equatorial plane of their planet. Every planet spins, and thus has an equatorial bulge caused by centrifugal forces that over time aligns the satellites' orbits over the equator due to the non-uniform gravitational field.

There are only 3 exceptions for large satellites: Our Moon, Iapetus, and Triton.

• Our Moon's orbit is more aligned with the ecliptic plane (the plane of Earth's orbit) rather than Earth's equatorial plane because the perturbations by the Sun are more significant than the perturbations due to Earth's equatorial bulge.

• Iapetus is the only large regular moon of Saturn to have a significant inclination to Saturn's equator (≈16°). The reason why remains a subject of debate but being the farthest large satellite of Saturn likely has something to do with it.

• Triton is harder to explain being closer to Neptune and the perturbations from the Sun being even smaller but it being a captured object likely has something to do with it.

Regarding the moons of Uranus: they are much less massive than the moons of the other solar giant planets. The combined mass of the five largest moons is less than half the mass of Triton. per Wikipedia:

The orbits of the regular moons are nearly coplanar with Uranus' equator, which is tilted 97.77° to its orbit. Uranus' irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at large distances from the planet.1

Keep in mind there are plenty of outer satellites than do not orbit in their planet's equatorial plane. A satellite close to its planet will orbit in its equatorial plane. Satellites further away will, in general, orbit around the planet's orbital plane due to Solar perturbations.

• Is the difference between rocky and gas composition a major contributing factor to the significant of the equatorial bulge? – chrylis -cautiouslyoptimistic- Dec 21 '20 at 4:49
• Moot why specifically? Does the increased bulge from a fluid composition not proportionally increase the centering effect? – chrylis -cautiouslyoptimistic- Dec 21 '20 at 5:29
• @chrylis-cautiouslyoptimistic- Rocks aren't rigid at planetary scale: gravity pulls them into approximately the same shape they'd have if they were fluid. The highest mountains on Earth are only a bit over 0.1% of Earth's radius above the geoid. – John Doty Dec 21 '20 at 12:36
• I think it's worth adding Jupiter, besides its larger moons, has a myriad of small satellites, a lot of them in completely random orbits. – SF. Dec 21 '20 at 14:54
• I think that this answer could be improved by explicitly addressing the moons of Uranus, and whether or not they're in the same plane as its axis of rotation, which is rotated about 90 degrees from the plane of the elliptic. – nick012000 Dec 21 '20 at 15:39

Are all satellites of all planets in the same plane?

Arguably... No, not even for the satellites around a given planet, not even if their orbits were nearly coplanar.

Depending on how you look at it, retrograde moons with inclinations close to 180° could be successfully argued to be not in the same plane.

Their angular momentum vectors are in the opposite direction to those of prograde satellites, they would precess in the opposite direction, and their sidereal periods relative to prograde satellites would be

$$\frac{1}{T_S} = \frac{1}{T_1} + \frac{1}{T_2}$$

rather than

$$\frac{1}{T_S} = \frac{1}{T_1} - \frac{1}{T_2},$$

but if one were being hairsplitingly pedantic one would insist that if two orbits are in the same plane, then they are in the same plane even though from an orbital-mechanical, physical and astronomical perspective there are in opposite planes that simply happen to coincide.

According to the Wikipedia article on irregular moons, retrograde orbits further out from a planet are more stable than prograde

Retrograde satellites can be found further from the planet than prograde ones.

Not necessarily. Those native satelites formed out of the debris cloud of the parent planet during it's formation will tend to have orbits more or less aligned with the planet's equatorial plane. However, any satelites captured from the surrounding environment can have any orbital inclination imaginable.

• It "can have any orbital inclination imaginable" for a short time, but over a longer period of time aren't there effects that tend to move moons away from some inclinations, and towards others? – uhoh Dec 23 '20 at 12:00