planet
A planet is a large world that circles a star, sweeping around it again and again. Earth is a planet, and so are its neighbors — Mars glowing rusty red, giant Jupiter, ringed Saturn. The key picture is a ball big enough for its own gravity to pull it round, looping a steady path around the Sun rather than drifting freely through space.
There's a quiet catch in the modern definition: a real planet has also "cleared its path," meaning it's the gravitational boss of its lane, having swept up or flung away the leftover rubble. This is the criterion that led to Pluto's reclassification in 2006, after astronomers found similar-sized icy bodies sharing its zone — most strikingly Eris, discovered in 2005 and at first thought even larger than Pluto — which forced them to define the word precisely. Pluto failed the test and was relabeled a dwarf planet. Size alone doesn't make a planet; dominance does.
For centuries people assumed planets must trace perfect circles, since the heavens were thought flawless. Then Johannes Kepler, poring over careful observations of Mars, found the truth: planets travel on ellipses — gently stretched ovals — speeding up when near the star and slowing when far. Nature's orbits are elegant, but they are not perfect rings.
The eight planets of our Solar System, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Pluto, once counted as the ninth, is now a dwarf planet.
The word "planet" comes from the Greek planētēs, "wanderer" — ancient skywatchers noticed these few bright points drifting against the fixed stars night after night. The International Astronomical Union pinned down the formal three-part definition (orbits the Sun, round, and clears its path) only in 2006.