1For more than seventy years, schoolbooks all over the world listed nine planets travelling around our Sun.
2The smallest and most distant of these was Pluto, discovered by a young American farmer in the twentieth century.
3Pluto is so far away from the Sun that its light needs more than five hours to reach it.
4In the early twenty first century, astronomers began finding other icy worlds of a similar size.
5One of these newly discovered worlds, which astronomers named Eris, appeared to be even heavier than Pluto.
6Scientists then faced a very difficult question: should the solar system simply keep adding new planets forever?
7After a long and heated debate, they agreed on a new definition of the word planet.
8Under this new definition, Pluto became a dwarf planet, along with Eris and several similar worlds.
9Many people around the world were deeply upset by this decision, and some still refuse to accept it.
10Yet when a spacecraft finally flew past Pluto, it revealed a world of glaciers, plains, and hazy skies.
11Whatever name we finally choose to call it, Pluto remains a truly fascinating member of our cosmic neighbourhood.