The names of planets are capitalized since they are proper nouns referring to the planets in our Solar System or other solar systems. Therefore, you should capitalize the following planets and dwarf planets:
What about the word planet?
The word “planet” is generally not capitalized since it is a common noun that refers to any planet. The only time you should capitalize “planet” is if it is used in a title such as the name of the seven-movement orchestral suite called “The Planets” by composer Gustav Holst.
Do you capitalize solar system?
Generally the phrase “solar system” is lowercase except when you are referring to the solar system that we live in. In that case, you should refer to our Solar System in capital letters.