According to NASA, a planet is “a large round object that revolves around a star“. In the case of our solar system, the star is the sun and the planets are 8 and are the following (starting from the closest to the sun):
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
The inner four planets (Mercury, Venus, Earth, and Mars) are called “terrestrial planets”, because, like Earth, their surfaces are all rocky. The next two planets (Jupiter and Saturn) are called gas giants and the last two (Uranus, Neptune) are called ice giants (they are also gas giants). The last four are huge compared to the terrestrial planets.
Pluto, discovered in 1930, was identified as the ninth planet and is located after Neptune. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. Pluto kept its planetary status through the 1980s but later it was proposed that it is more useful to think of Pluto as the biggest Kuiper Belt Object (KBO) instead of a planet.
It is possible that a number of other objects may be classified as planets in the future; however NASA currently identifies only the above 8 as the primary planets of our solar system at the moment.