The problem is: there's a big belt of frozen space rocks where Pluto is at, Pluto is the biggest, it was considered a planet until 2006, when Neil deGrasse thought about it and decided that it was literally just a big frozen ball of gas. Which it is. Another problem is its atmospere. Pluto doesn't have an atmosphere most of the time, but when it's closest to the Sun in its orbit, frozen methane melts and so do some other frozen gases, giving it a temporary atmosphere until it moves further out.

Here's Neil's argument: