When my daughter was in grade four, she was instructed to write about a planet of her choice. She chose Pluto. Alas, about fifteen years later, the International Astronomical Union (IAU) decided Pluto was no longer a full-fledged planet—partly because it’s just one of a number of similarly sized objects in an area of our solar system called the Kuiper Belt.

Pluto: A Dwarf Planet in the Kuiper Belt | NASA Planetary Sciences | PBS LearningMedia

Learn the three criteria for classifying planets and discover why Pluto is now considered part of the Kuiper belt. When Pluto was discovered in 1930 it was classified as the ninth planet in the solar system. For decades Pluto was believed to be much larger than it actually is, based on incorrect interpretations of observations. Advances in technology brought about new knowledge of Pluto and it is now recognized to be a member of the Kuiper reclassified as a dwarf planet. In 2006, the New Horizons mission was launched; it will be the first spacecraft to visit Pluto and the Kuiper belt.

NASA

A newly drafted definition of the word “planet” limited the category to celestial bodies that (a) orbit around the Sun, (b) have sufficient mass so as to pull themselves into a nearly round shape, and (c) have “cleared the neighbourhood” around their orbits. Pluto failed the third criterion, and so became a mere “dwarf planet.”

The word “moon” may now be in for similar revisionism: It was announced this week that the International Astronomical Union (IAU) has recognised no fewer than 128 new official moons of Saturn, bringing that planet’s total to 274 known moons. At some point, surely, the word “moon” begins to lose its currency.

Saturn has a whopping 274 moons ― scientists want to know why

A huge haul of 128 newfound satellites might be a hint of past collisions in the planet’s orbit, or something else.

NatureJonathan O’Callaghan

When I was growing up, life was simpler. The National Hockey League had six teams, and all the planets had a reasonable number of moons. Jupiter led the way with thirteen (which increased to sixteen in 1979 when the Voyager spacecraft passed by and took a look). At the time, Saturn came close with nine moons, though the Voyager Science Team then added a few more, and so the two gas giants were, for a time, neck and neck.