All of the planets in Earth's solar system — except for Earth — are named after Roman deities. Ancient Roman astronomers tracked Venus, Mars, Mercury, Jupiter, and Saturn thousands of years ago, because they could be seen with the naked eye. These astronomers named the planets after their gods (and one goddess).
The other planets in our solar system were not discovered until much more recently, after the advent of telescopes (Uranus in 1781, Neptune in 1846, and Pluto in 1930), but astronomers decided to continue the tradition of naming them after Roman gods.
In 1905, American astronomer Percival Lowell suggested the existence of Pluto, despite never having seen it. Lowell was studying some unusual nuances in the orbits of Neptune and Uranus and theorized that only the gravity of an unknown planet would be causing them.
In 1930, after Lowell's death, American astronomer Clyde Tombaugh observed Pluto. By tradition, the discovering astronomer of a new space object gets naming rights. But it was an 11-year-old British girl's idea to name the planet Pluto. Venetia Burney told her grandfather that the name fit the new planet because it stayed hidden for so long, and the Roman god Pluto could disappear at will. Venetia's grandfather wrote to Tombaugh and offered the suggestion, and Tombaugh chose it, happy that the name also honors Percival Lowell — the first two letters of "Pluto" are Lowell's initials.
Whether Pluto is really a planet or just a wayward asteroid that got trapped by the sun's gravity has always been a subject for debate. In 2006, astronomers created the new classification of "dwarf planet" and demoted Pluto as such. A dwarf planet is defined as a celestial object that
- Orbits the sun.
- Has enough gravity to be roughly spherical in shape.
- Does NOT have enough mass to "clear the neighborhood" — which is astronomy-speak for being able to use its weight to push other objects out of its way as it revolves around the sun.
Currently there are five known dwarf planets.
- Ceres (the only dwarf planet within the asteroid belt, between Mars and Jupiter)
- Eris (the largest known dwarf planet, about 25% larger than Pluto)