In 2006 astronomers decided what a planet is, and Pluto did not make the cut. Twenty years later, the debate is still alive.
For 76 years after its discovery in 1930, Pluto was the ninth planet of the Solar System. Then, in August 2006, the International Astronomical Union adopted the first official definition of a planet, and Pluto became a dwarf planet.
Three conditions
According to the definition, a planet must:
- orbit the Sun,
- be massive enough for its own gravity to make it nearly round,
- have cleared the neighbourhood around its orbit.
Pluto meets the first two conditions, but not the third. It shares its region of space with thousands of other icy bodies of the Kuiper Belt, some of them, such as Eris, almost as large as Pluto itself.
A world worth visiting
Planet or not, Pluto turned out to be a fascinating place. When NASA's New Horizons flew past in July 2015, it revealed mountains of water ice, glaciers of frozen nitrogen and a thin, hazy atmosphere.
Image: New Horizons Very Best View of Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Source, NASA media guidelines.
Comments
Pluto will always be a planet in my heart.
In reply to by comet_leo
Then you should also count Eris, Haumea, Makemake and maybe dozens of others. Would you like to learn 50 planets at school?
In reply to by quasar_mia
Fair point, but rules can have exceptions. Pluto was here first!
In reply to by comet_leo
Science works best without exceptions for sentimental reasons. But I agree it is the most charming dwarf planet.
In reply to by parsec_sam
Okay, I can live with "most charming dwarf planet". 😄
Some planetary scientists still argue for a geophysical definition: anything round is a planet, whatever its orbit. Then even our Moon would be a planet.
I use this exact question with my students every year. It is a great way to explain that definitions in science can change.