All planets orbit a star, but not all stars have planets. Stars and planets can form in a variety of ways, and vary greatly in size, shape, and composition. A planet is a sufficiently large body that has a regular orbit around a star. Earth, is a small terrestrial or, rocky planet. Jupiter, on the other hand, is a gas giant. Some gas giants, many times larger than Jupiter, are known as brown dwarfs. These colossal planets are failed stars, their mass was not quite enough. Stars are gigantic and extremely hot, luminescent bodies. When enough mass accumulates into a single object the internal pressure causes nuclear fusion and a star is born.
Planets can exist indefinitely. Although activity may cease, unless destroyed, a planet witl float through space forever. Stars have a life span determined by their size. After a star has consumed most of its fuel it will explode into a supernova and then collapse in on itself. A sufficiently large star can create a black hole, while smaller stars can create neutron stars. The more massive a star the less time it takes to use its fuel. Smaller stars have much longer life times than a large star.
Stars produce most if not all of the light in the universe. The nuclear reactions of the stars produce a tremendous amount of energy and light. Planets are capable of reflecting this light; most of a planets energy is contained within the core. A planetary core will cool and lose energy after much less time than a star unless outside forces cause it to remain hot. Aside from gravitational energy, a planet produces no energy.
Black holes are the most massive object known, followed by stars, then planets and finally moons. These are just a few of many different objects in space. Stars are typically the center of a system of planets and moons. Planets are much smaller than stars, and produce no energy. Stars can exist alone or in the company of other stars, just as planets can. Life could exist on other planets, but that life would depend entirely on the star for energy. Nuclear fusion powers the stars. This fusion is a result of the large mass of the star, mass that planets lack. Planets and stars are very different, as can easily be seen. A planet is defined by its star, but a star has no such limitations.
