The first letter of each word is the first letter of each planet. Since 1994, evidence has been found that planets also exist beyond our solar system. At least 10 planets existing in other solar systems have been discovered. As long as our own Moon was the only moon known, there was no need for a general term for the moons of planets. It comes from the Latin orbis, which means circle.
Some orbits are nearly circular, but the orbits of most planets are ellipses - shaped like ovals.
Asteroids, also known as the minor planets, are small bodies orbiting the Sun that resemble planets. More than 5,000 asteroids have been discovered, and most are found between Mars and Jupiter. Comets are made up of frozen dust and gases, and have been described as large, dirty snowballs with icy centers. They often travel on extremely elongated orbits around the Sun. Some comets have orbits that take just 10 years to circumnavigate while other comets have orbits that take hundreds of thousands of years to circulate.