Why is Ganymede Not Considered a Planet?

Why is Ganymede not considered a planet? Is it just because it revolves around another planet (Jupiter)? It has a larger diameter than Mercury. Being that Jupiter is a gas giant, is it possible that Ganymede (as well as other moons of Jupiter) was once a terrestrial planet that was pulled into Jupiter's gravity? Just curious as to any thought's, ideas or theories.

Ganymede is orbiting Jupiter, and therefore it is a satellite of that planet. If it was orbiting the Sun, then it would easily be large enough to be called a planet.

It's worth noting that Titan is also larger than Mercury (although considerably less massive), and Callisto is only slightly smaller.

reply to post by Mogget



Yes, I get that point. So if it orbits a planet, then it's a moon. But moons,although orbiting planets, still orbit the sun along with their parent planet. Perhaps Jupiter, along with Ganymede and Titan, should be looked at as a cluster of planets rather that a planet with 2 planet sized moons.

Clearly, these moons were not born out of Jupiter, rather, they were pulled into Jupiter's gravity. Did they once have a place in the solar system as planets? were they rogue planets from outside the solar system?

Although this would NEVER happen...suppose Earth was pulled into Jupiter's gravity and became a satellite of Jupiter. Would we still consider Earth a planet because it originally was classified as such, or would Earthlings then have to refer to our home as Moon Earth rather than Planet Earth?

reply to post by no name needed


I don't know if we would be alive if Earth moved that much...
The definition of a planet is still vague. Technically, most planets cannot be considered planets under the current definition, Earth included. I would expect Earth to be always considered a planet just based of it being Earth our home. I wouldn't worry about any what if situations, just what we have now. Anything orbiting Jupiter or Saturn, or any other planet..is simply considered a satellite of that object.

Who knows, maybe one day if Jupiter decides to condense into a small star, then perhaps its moons will be called planets then, maybe even able to support life. Kind of like in the movie 2010.

Clearly, these moons were not born out of Jupiter, rather, they were pulled into Jupiter's gravity. Did they once have a place in the solar system as planets? were they rogue planets from outside the solar system?

It is highly probable that Jupiter had a ring of material around it when it was forming, and the Galilean moons (Io, Europa, Ganymede, and Callisto) were all formed from this ring of material.

The chances of four large objects being captured by Jupiter's gravity in the early days of the Solar System are so vanishingly small that we can safely disregard it. It is exceptionally difficult for a planet to permanently capture a solar orbiting object. It would probably require numerous decelerating encounters in succession to achieve. An extremely unlikely scenario.

There is no chance whatsoever that they are rogue planets from outside the Solar System.

Who knows, maybe one day if Jupiter decides to condense into a small star, then perhaps its moons will be called planets then, maybe even able to support life.

Jupiter will never become a small star. It has barely more than 1 per cent of the mass required to compress its core to such an extent that nuclear fusion reactions begin. Don't listen to any of these wild theories about Jupiter being almost massive enough to ignite as a star. It isn't even close.