Can a planet's moon have a mini moon of it's own?

Answer 1

I would add a third precondition to the above two suggestions (oversized moon and planet at a long distance away): the planet, though larger, be of lower density than the moon and therefore of similar mass to it and similar gravitational pull.

Interesting idea but I suspect that the planet being larger and with more gravitas would usually have more of a gravitational pull and would make the minimoon rotate around it rather than around the moon.

Some 80 minor planets (asteroid belt and Kuiper Belt objects) have moons around them (eg the asteroid 87 Sylvia has two such moons.) but I am not aware of any of the 162 moons of the 8 planets having any minimoons orbiting them.

Which is not to say it cannot happen but we lack an instance of it in this solar system to date,

But taking this as a working hypothesis that there could be such mini-moons ...

Where should we look?

JUPITER: The very outermost moon of Jupiter may perhaps be a candidate on the second score (distance) if not on the first (oversized moon), S/2003 J 2 is about 2 kilometres in diameter, and orbits Jupiter at an average distance of 29,54 Gm (million km) in 980 days,

Perhaps the outermost rings of the three planets with rings (Neptune Uranus and Saturn) might contain material than an outer moon might capture and cause to orbit it?

URANUS: Uranus' 3 outermost rings with distances from Uranus and the moons associated with them:

Epsilon Ring 51,149 kms (shepherded by Cordelia and Ophelia)
R/2003 U 2 ~66,000 kms (near Portia)
R/2003 U 1 97,734 kms (at Mab)

NEPTUNE: The most prominent of Neptune's rings is the narrow outer Adams ring. It includes several longitudinal "arcs" spanning 4-10° each along their length that are much brighter and much more opaque than the remainder of the ring.

Known as Liberte, Egalite and Fraternite, these are unique in the solar system, and the method of their longitudinal confinement has been a matter of lively scientific investigation. It now appears that resonances with the satellite Galatea are largely or wholly responsible for the confinement, although the details are still somewhat uncertain.

This is highly speculative but has Galatea captured some of the ring material between the segments?

SATURN: The B Ring is the innermost of the two largest, brightest rings.

Its thickness is estimated as 5 to 10 metres, its mass as 2.8e19 kilogram (about three-quarters of Mimas)

The outer edge of the B Ring is maintained by a 2:1 resonance with the moon Mimas. Ring particles at this location orbit twice for every one orbits of Mimas. The resonance causes Mimas' pulls on these ring particles to accumulate, destabilizing their orbits and leading to a sharp cutoff in ring density.

There is also the bright and narrow Titan Ringlet centred at 77,883 kilometres from Saturn's centre. This ringlet is eccentric; that is, it is slightly elliptical rather than circular. The Titan Ringlet is so named because it is in a resonance with Titan.

In this case, the time period of a ring particle's apsidal precession is equal to the time period of Titan's orbital motion, so that the outer end of this eccentric ringlet always points towards Titan.

Then there is the Encke Division, also historically called the Encke Gap, which is a perceived gap within the A Ring.

The division is centered at a distance 133,580 kilometers from Saturn's center, and has a width of 325 kilometers. It is caused by the presence of the small moon Pan, which orbits within it.

And then there is the Keeler Gap which is a 42-kilometre-wide gap in the A Ring, approximately 250 kilometres from the ring's outer edge. It is named after the astronomer James Edward Keeler. The small moon Daphnis, discovered May 1, 2005, orbits within it, keeping it clear,

And most interesting of the lot is the final sentence in this sketch: The F Ring is one of the outer rings of Saturn. It is located outside the larger rings, just beyond about 3000 Km, the A Ring. It was discovered in 1979. It is very thin, just few hundred of kilometers wide, and is held together by two shepherd moons, Prometheus and Pandora, which orbit inside and outside it.

Recent closeup images from the Cassini probe show that the F Ring consists of one core ring and a spiral strand around it. They also show that Prometheus's gravitational attraction creates kinks and knots in the F Ring as the moon 'steals' material from it.

EXOPLANETS: with asteroid belts being discovered in 2004 and 2005 around the star Tau Ceti (11 light years away) and the star HD 69830 (41 light years away) (being respectively ten times as massive and 25 times as massive as our own) who knows? the evidence may be there for us to photograph from rockets in the future,

Answer 2

I think this possible but unlikely. The gravity of the planet would disturb the orbit of the mini moon greatly.

I don't think it would happen as a result of accretion. But, it might happen as a result of capture. And then, I think it would happen only if there were a an oversized moon orbiting a planet at a large distance.

Answer 3

Yes, and to clarify: As the mass of the mini moon approaches the mass of the 'moon', they will orbit each other about a point midway between them. In fact, ALL orbiting bodies (including planets around the sun) orbit around a point not coincident with either center of mass.

Answer 4

yes they can, any object can have a smaller natural satellite or "moon" orbiting them. Many smaller objects such as our moon may not have as much gravity to hold something in orbit long enough. The pull of the earth will probably pull it out of the moons orbit into our atmosphere, like wise with any other moon out there since to have a moon you will have to have a strong gravitational force.

Answer 5

Sure. It's all about gravity. There's asteroids in the asteroid belt with smaller "moon" asteroids orbiting them. There could even be a moon orbiting the moon of some other moon's moon.

Answer 6

Maybe, but by definition no. A moon orbits a planet.

Answer 7

Yes, if the moon is massive enough it would absolutely be able to capture a smaller object in its orbit.

Answer 8

well,by definition a satellite orbits a planet,and doesn't have anything else orbiting it,but I suppose in reality,it's possible!!!

Answer 9

if the earth revolve around the solar and a moon revolve around the earth and if u evaluate the earth as a "moon" of the solar, then it rather is available that a moon could revolve around a "moon".

Answer 10

Yes..the often call them satellites