This is Jupiter:
It’s a big ball of hydrogen.
This is the Sun:
It’s also a big ball of hydrogen.
Basically, if an object is primarily composed of hydrogen, it will start undergoing fusion once it reaches a certain mass. We call those objects “stars”. However, the line between a star and a planet is not clear. There are small star-like objects known as brown dwarfs which are either too small to undergo fusion or, if they do, are too cold to emit strongly in visible light. However, if they don’t orbit another star, they’re not planets by definition. Astronomers tend to call them “brown dwarfs” because, well, they’re small, brownish balls of hydrogen.
Just like Jupiter.
Edit: For those of you complaining that Jupiter isn’t a star, you’re missing the point. Yes, stars fuse hydrogen and Jupiter doesn’t. And, yes, I agree that defining “star” based on nuclear fusion is the best possible definition.
However, that doesn’t affect my answer.
Jupiter is a ball of hydrogen.
If it was about 13 to 15 times more massive, it would be able to fuse deuterium and we’d call it a brown dwarf. However, it would STILL be a ball of hydrogen.
If it was about 80 times more massive, it would fuse hydrogen and we’d call it a star. However, it would STILL be a ball of hydrogen.
In other words: if Jupiter was bigger, it would have enough mass to produce a fusion reaction and we’d call it a star.
The mass of the object is the only thing that makes a difference, so please stop being pedantic and saying I don’t understand the difference between planets and stars.