Got Moons?
There’s nothing in the definition of a planet, or a dwarf planet about having a moon or having dozens of moons. In fact, anything goes when it comes to moons.
Currently, there are 169 known moons orbiting the eight planets in our solar system and 4 orbiting the three dwarf planets. Dozens more moons orbit other small solar system bodies. The moon-count continuously increases as telescopes and spacecraft discover new ones that have been difficult to see.
The giant gas planets have the most moons. Jupiter currently has 63 and Saturn is close behind at 60. Uranus is next with 27 moons, followed by Neptune with 13. The inner planets have fewer moons: Mars has two tiny moons while our planet Earth has one large moon. Mercury and Venus have no moons at all.
Among the dwarf planets, Ceres has no moons (although other objects in the asteroid belt do), Eris has only one known moon and Pluto has three moons; two tiny ones and one almost as large as Pluto!
What does it take to be a moon? A moon is a natural satellite that orbits another body larger than itself. Moons come in all different sizes. The largest moon is one of Jupiter’s moons Ganymede with a diameter of 5,276 kilometers (3,280 miles)! That’s larger than the planet Mercury and the largest dwarf planet Eris. The Cassini spacecraft discovered tiny moons about 3.2 kilometers (2 miles) in diameter in orbit around Saturn. There may very well be moons less than a mile in diameter hidden in the rings of the other giant planets.
How did moons come to be? The larger moons most likely formed along with the parent planet, out of coagulating matter – as they got bigger, they attracted smaller fragments close by them, sweeping their orbit path clean. The smaller moons are probably captured asteroids or comets which wandered too close and got caught by the planet’s gravitational field.
In the case of Pluto’s large moon, scientists think that a massive body slammed into Pluto early in its history, sending massive amounts of debris into orbit around the planet. Over time, the rocky debris accreted and formed Charon. It’s likely that Earth’s moon formed this way as well.
Something to ponder….are there moons of moons? None are known but by definition, a moon could have a smaller moon orbiting around it. The question is the long-term stability of the orbits. In most cases, the gravity of other nearby objects (most probably the primary planet) would perturb the orbit of the moon’s moon until it broke away or impacted the primary. Ouch!