A planetary system is a collection of objects that are not stars, that orbit around a star or set of stars.
A planetary system consists of a set of non-stellar objects orbiting a star or star system. The non-stellar objects are kept in orbit by gravity. Examples of non-stellar objects include planets, dwarf planets, and asteroids. Our planetary system is known as the solar system, consisting of eight planets orbiting a star, the Sun. The solar system is organized in a way that the smaller, rocky planets are found closer to the Sun and large gas planets exist farther away. This is because farther away from the Sun the temperature is cooler, so gases can condense to form planets.
Planetary systems are formed from protoplanetary disks. Protoplanetary disks are made of dust and ice are formed surrounding new stars. Over time the particles in these disks start to clump together and form small balls of matter known as planetesimals, the first stage in a planet’s evolution. These planetesimals attract more matter by their gravitational pull until they get to the size of planets. This formation process explains why all the planets in our solar system are in a orbital plane.
The first planet outside of our solar system was discovered in 1988. It is known as an exoplanet because it does not belong to our solar system. It was discovered by a team from two Canadian universities and confirmed in 1991. It is unofficially known as Latham’s Planet after its discoverer and officially known as HD 114762 b.
Scientists are interested in finding planets where there could be liquid water. All life on Earth requires liquid water. There are a number of factors that would contribute to the possibility of water, but the most important is the planet's distance from its sun, because liquid water can only exist in certain temperature ranges. So far, Earth is the only planet we know of in the universe that has liquid water, but there are more planetary systems in the universe that have not been discovered yet.