Supernovae and novae are key forces in a grand cycle of stellar rebirth and renewal. Heavier elements cooked up in the fusion furnaces of stars are ejected back into space, serving as raw material for building new generations of stars and planets. The Solar System provides a fundamental model for what we might expect to find around other stars, but it's difficult to form generalities from just one example. It may turn out that nature is more varied and imaginative when it comes to building and distributing planets throughout the Galaxy. In it simplest definition, a planet is a nonluminous body that orbits a star, and is typically a small fraction of the parent star's mass.
Planets form out of a disk of dust and gas that encircles a newborn star.
These embryonic disks have been observed around young stars, both in infrared and visible light. The planets' orbits in our solar system trace out the skeleton of just such a disk that encircled the newborn Sun. Planets agglomerate from the collision of dust particles in the disk, and then snowball in size to solid bodies that continue gobbling up debris like cosmic Pac-Men. In the case of our solar system this led to eight major bodies, thousands to tens of thousands of miles across. A planet's mass and composition are determined by where it formed in the disk.