What are planets and what are they made of?

Answer 1

To paraphrase the IAU definition of planet, it is a celestial body that

1. is in orbit around a star or the remnant of a star;
2. is sufficiently massive to have attained a nearly spherical shape;
3. has substantially cleared its orbit of debris.

The definition remains controversial among experts. The exact definition accepted by the IAU is a little different from the above, but according to the official IAU definition no bodies observed outside of our solar system are planets because the definition refers only to bodies orbiting our Sun specifically and not to any star or star remnant. We know now that there are hundreds of planets outside of our solar system

The inner planets, Mercury, Venus, Earth and Mars, are typically called rocky planets, because they are made of accumulated rocky materials from the original accretion disk that orbited the sun in the early days of planetary formation, up to roughly 5.5 billion years ago. The outer planets, Saturn, Jupiter, Uranus and Neptune, are called gas giants because they are made mostly of gas, with small rock/metal cores deep inside. Uranus and Neptune are sometimes called ice giants instead, because although they are made substantially of gas, the planets are so cold that their gases have frozen into slushy ice. The current understanding is that rocky materials in the accretion disk maintained their orbit around the sun, and the solar winds pushed gaseous materials farther out from the sun, where they eventually coalesced into the planets we know.

You would not be able to stand on the 'surfaces' of Saturn or Jupiter; in fact they don't have surfaces in the usual sense. Being able to 'stand' on a celestial body is not a requirement for it to be a planet. Jupiter is by far the largest and most massive of our planets.

Note: It is the third part of the definition that excludes Pluto from the list of planets. Pluto spends most of its orbit in the Kuiper Belt, with the countless other objects, some larger than Pluto, called Kuiper Objects. They are rocky and/or icy chunks of material that exist beyond the orbit of Neptune. By the way, Gigantic Neptune is NOT considered a piece of debris that tiny Pluto has not yet cleared from its orbit. Their orbits are such that they will never collide, barring some huge and unexpected disturbance from outside of the system.