Planets are large, spherical bodies which orbit a star.  Unlike stars, they do not have sufficiently high temperatures in their cores to initiate thermonuclear reactions, In 2006, the International Astronomical Union established a new definition of planet, which resulted in the ‘demotion’ of Pluto as a planet. One interesting thing about the sizes of planetary orbits is that they seem to follow a fairly basic mathematical series known as Bode’s Law.
Carpe Caelum Planetary Astronomy
Carpe Caelum Planetary Astronomy