Why are there different sizes and types of planets?

Question

And does every sun have to have planets? Why/ why not?

Answer 1

1. there are two TYPES of planets, but many CLASSIFICATIONS for a planet.

types:

terrestrial- small, rocky planets like earth, venus, mars and mercury. and pluto if it was considered a true planet.

jovian/gasseous- planets made mostly out of gas it the liquid or gaseous state, and are larger than terrestrial planets. the jovian planets in our solar system are jupiter, saturn, uranus, and neptune.

Classifications:

in order for any celestial body to be considered a planet, it must meet the following qualifications:

- it must orbit a star

- it must be the largest object in its orbital shell

- it must be in the shape of a sphere as a result of its own gravity

- it must have an orbital path clear of asteroids, or other small celetial bodies. in other words, it has to have clean orbit.

if a planet meets these qualifications, then it is divided into two main different classifications.

-true planet (such as earth, venus, mars and mercury). these planets must be atleast the size of mercury and must not be in an asteroid belt. these planets are usually closest to their local star than the other non-planets which i'll talk about next.

-dwarf planets/plutoids (such as pluto, eris, sedna, qouoar, and ceris). these planets are small, and usually lie in asteroid belts beyond the last major planet of the solar system. in our solar system, ceris is an exeption. it lies in the asteroid belt between mars and jupiter. dwarf planet would be the official classification for these planets outside our solar system, but these planets in our solar system fall under "plutoid". the definition of a plutoid is a celestial body that meets all of the qualifications of a planet, but lies in an asteroid belt, or beyond neptune and is not greater than the size of mercury.

*size doesn't matter so much as long as the planet is large enough to shape itself into a sphere as a result of its own gravity.

2. the definition of a "sun" is a star that is the center of orbit for multiple planets, and atleast one asteroid belt. it is also known as a "local star". our sun is named "sol", but we refer to it as "the sun". thats why i prefer to use the term "local star" and not "sun" to describe other stars that are part of a solar system. a sun has to have planets. if it didn't it would just be a star. planets that orbit their sun are part of that star's solar system. in order for there to be a solar system, there has to be one star in which all planets orbit, as well as an asteroid belt, although that standard is still being debated upon.

if a star does have planets and an asteroid belt orbiting it, it is called a sun because it is the center of that solar system. if it doesn't have planets or an asteroid belt orbiting it, it is not the center of a solar system, and therefor not a local star/sun.