By "weather" on another planet a scientist means changing conditions in the atmosphere. so the scientist hs to determine
To answer 1. the scientist can look through telescopes of different kinds and determine if the light, heat, and radio waves that reflect off or pass through the planet are typical of gases or solids. if the answer is that there is gas on the surface of the planet then that is an atmosphere.
To answer 2. if analysis that supports the conclusion that there is an atmosphere is repeated during a longer time shows variations then that is weather.
it would break or if a small planet bumped itnto juipeter, juipeter would have another moon.
The sun is the original source of all weather changes on the planet. Without it the world would be completely frozen and would have no atmosphere ie. weather.
There is no specific word for a scientist who studies tornadoes. Such scientists would fall under the general label of meteorologists; weather scientists.
A seismologist would be one scientist. Also an oceanographer would be another scientist interested in studying tsunamis.
A moon? I don't think a planet orbiting another planet would be called a planet.
We call that path the "orbit" of the orbiting body. Note: You would not ever see a planet orbiting another planet. At least, if you did, you would not call them both planets.
meteorology
Since tornadoes are a form of weather, they would be predicted by a meteorologist.
I think No.
another planet
If we didn't have meteorologists then we would not know what the weather would be like and we would not understand our planet or atmosphere.
I would search the planet. Observe it. Comare it from earth. Try to find aliens. Dance. Jump around. Breathe.