Why Is Pluto Not a Planet Any More?

Step 1 of 5

Pluto was found in 1930 and looked unique.

For decades it appeared to be a lone small world beyond Neptune, so calling it the ninth planet seemed entirely reasonable.

Step 2 of 5

Then telescopes found many more like it.

From the 1990s astronomers spotted a growing crowd of icy bodies beyond Neptune, several of them comparable in size to Pluto.

Step 3 of 5

One of them turned out to be heavier.

Eris was found to be more massive than Pluto, which forced an awkward question: either it was a tenth planet, or Pluto was not a planet.

Step 4 of 5

So astronomers wrote a definition.

A planet must orbit the Sun, be round from its own gravity, and have cleared its orbital neighbourhood of other similar bodies.

Step 5 of 5

Pluto fails only the third test.

It shares its region with countless icy bodies, so it became a dwarf planet — a category, not an insult, and one it now leads.

Pluto is smaller than our Moon, and its orbit is tilted so steeply that it sometimes comes closer to the Sun than Neptune.