The planet best known to humans is the Earth; however there are many other planets. There are seven other major planets in the solar system of which the Earth is a part; there are also thousands of other minor planets in the solar system, of which the best-known are probably Pluto (technically a dwarf planet) and Ceres, which is also known as the largest asteroid. There have been in recent decades with improved telescopes been important discoveries of planets in solar systems other than are own; one of the first was a planet found to be orbiting Barnard's star.
Planetary science is in some ways a part of astronomy; in other ways it is similar to geology and is sometimes referred to as planetary geology, however a common objection to this term is that the Greek words from which the term "geology" is made mean "earth study" and hence are not appropriately applied to other planetary bodies.
Planetary science in a broad sense is the determination of how long it takes a planet to orbit its star and at what distance, the mineral makeup of the planet, and an attempt to learn its age. It may also have a more narrow focus, such as the mapping of the planet's surface features and the makeup of its atmosphere.