As a star rotates, its gravitational field attracts dust and objects in its vicinity, and this material eventually forms a huge ring. Since much of the material in the disk is debris from the molecular cloud in which the star itself formed, a complex blend of elements and molecules from that cloud will be absorbed by the various clumps that form over millions of years in the disk. As these clumps collide and form larger bodies or smaller pieces, chemical and physical processes will affect the molecules that ultimately become planets, moons, asteroids and comets. In the case of planets, where conditions are right, atmospheres and perhaps life may ultimately develop. This original presence of complex molecules in the disk of the star is the source of complex matter: everything is stardust.
Planets and moons. Distinguishing planets from moons is not always straightforward. Both objects are spherical and both vary in size. Basically, planets are spherical objects that orbit stars in a regular path. Size and path are important criteria; in Pluto's case the size and path issues led to a demotion to "dwarf" planet. Astronomers think there may be a number of these dwarf planets in the outer reaches of our solar system.
Moons orbit planets, so even a rather large moon like our own can be easily distinguished from a planet. It is just a question of which object has the greater mass. The relative gravity of a planet attracts objects such as moons and asteroids. Giant planets such as Jupiter can have numerous moons, some of which with environments that may support some form of life. Jupiter's moon, Europa, is one such example, but there are several other moons around Jupiter and Saturn that are potential sources of life, despite frigid and nearly dark environments.
Rock vs Gas Makeup.Planets closest to a star tend to be rocky, but distant and large planets may be orbs of gas over a small rock core or they may be made entirely of gas, like a small, failed star. The outer planets of our solar system --Jupiter, Saturn and Neptune-- are the gas type.
Exoplanets. Astronomers have located hundreds of planets elsewhere in the Milky Way and the means to identify planets at such distances is constantly improving. This work is technically difficult, as planets are much smaller than the stars they orbit, and they are not luminous. Most of the exoplanets so far discovered are huge (Earth-size planets are too small to locate). Interestingly, a number of the solar systems so far discovered are associated with dwarf binary stars. Our solar system with its solitary --although otherwise ordinary--star may be a bit unusual.
When a few pioneer astronomers started looking for exoplanets in the 1990s the search was thought to be a kind of "fringe" effort, one that could compromise an astronomer's career. Today this research is no longer bordering science fiction. There are high expectations that eventually --especially with technological improvements-- planets that can support life will certainly be discovered. The real question is whether there is intelligent life somewhere else in our galactic neighborhood.
In addition to planets, many other types of objects may be part of a solar system: objects like meteors, asteroids, quasi- or dwarf planets like Pluto, and comets. Together, the relationship between the star and all the objects and dust that form an orbiting system is highly complex and dynamic. The understanding of this complex system now engages scientists from many fields, and this is reflected in designations such as astrophysics, astrobiology, astrochemistry. It is a very exciting time for students entering these fields.