Astronomers divide the planets within our solar system into two categories. Mercury, Venus, Earth and Mars are the so-called “terrestrial” or “inner” planets while Jupiter, Saturn, Uranus and Neptune have been classified as “gas giants,” also known as “outer planets.”
The size gap between those factions is quite considerable; although Uranus is the smallest outer planet, it’s still 15 times more massive than Earth, the largest of the inner planets. None of the other planets can compete with Jupiter in terms of sheer bulk, however. You’d need more than 300 duplicates of our puny home world to equal Jupiter’s colossal mass. It’s an absolute monster.
Now, as Isaac Newton observed, there’s a positive correlation between the mass of an object and the strength of its gravitational field. Because the gas giants are so massive, they’re able to attract more satellites.
But that’s not the only reason why planets like Jupiter have such large moon collections. Our solar system’s gas giants are relatively far away from the sun. In contrast, some stars have massive, Jupiter-like planets called “hot Jupiters.” Basically, these are gas giants which orbit in close proximity to their stars. (Imagine if Saturn switched places with Mercury.)
A 2010 paper by French astronomer Fathi Namouni argues that hot Jupiters have few, if any, moons. These planets are thought to originate in distant parts of their solar systems and then migrate inwards. Along the way, their moons get caught in a game of celestial tug of war. Gas giants may be big, but stars are much bigger. As such, they’ve got far stronger gravitational fields. So, when a hot Jupiter gets too close to its star, the star will eventually steal its moons.
Distance offsets this ability. The further you travel from the sun, the weaker its gravitational pull on you becomes. Therefore, if Namouni is correct, the real Jupiter has 92 moons and counting because it’s an insanely massive planet that’s far enough away from the sun to avoid lunar theft.
