English Across the Curriculum

The Solar System

Dwarf Planets

Dwarf planets are like the solar system's eight planets, but smaller. They orbit the sun but are not moons. Today, scientists have classified 5 dwarf planets including Pluto, which was originally the ninth planet until 2006.

Pluto was discovered in 1930 by an American astronomer. It is so far away that astronomers can hardly see it, even with the most powerful telescopes.

Pluto is only 2,200 km in diameter, even smaller than our moon. It revolves around the sun once every 248 years. Sometimes Pluto crosses the path of Neptune and for about 20 years Neptune is farther from the sun than Pluto.

When it gets nearer to the sun, Pluto has more of an atmosphere. The frozen gas melts a bit and even clouds sometimes form. When it moves away from the sun again the atmosphere freezes and falls back to the surface.

The gravity on Pluto is only about 8 % of the gravity on Earth. If you weigh 70 kg on earth, you would weigh only about 5 kg on Pluto. It is very, very cold and the sun can only be seen as a very tiny spot. Temperatures can go down to - 240 ° C. Pluto's only moon , Charon, is almost the size of Pluto itself.

Pluto and its moon Charon

Size comparison
Image (modified) : NASA, Public Domain
via Wikimedia Commons


Ceres is a dwarf planet between Mars and Jupiter, in the asteroid belt. It has a rocky surface with no atmosphere. Ceres was the first asteroid that was discovered.

Eris is even larger than Pluto. It orbits the sun at a distance of almost 15 billion kilometers. Like Pluto Eris has a surface made up of methane ice.