The Nature of Planets, Dwarf Planets, and Space Objects

The Nature of Planets, Dwarf Planets, and Space Objects — page 5
chapter WHAT IS A PLANET The relatively large natural bodies that revolve in orbits around the Sun or otherstars are called planets The term does not include small bodies such as comets meteoroids and asteroids many of which are little more than pieces of ice or rock The planets that orbit the Sun are part of the solar system which includes the Sun and all the bodies that circle it The Sun governs the planets orbital motions by gravitational attraction and provides the planets with light and heat Ideas about what makes a planet a planet have changed over time According to the current definition there are eight plan ets in the solar system In order of increasing mean distance from the Sun the planets are Mercury Venus Earth Mars Jupiter Saturn Uranus and Neptune Pluto was considered the solar system s ninth and outermost planet from 1930 until 2006 when it was reclassified as a dwarf planet The concept of what defines a planet changed in another way in the late 20th cen tury when astronomers began discovering distant planets that orbited stars other than IO