4 WHAT IS A DWARF PLANET In 1930 an American astronomer named Clyde Tombaugh noticed something unusual while looking through a tele scope He thought it was a new planet and other scientists agreed The scientists named it Pluto It was considered the smallest planet in the solar system until 2006 In 2006 a group of astrono mers decided that the definition of planet did not apply to Pluto It is now considered a dwarf planet Like planets dwarf planets are large roundish objects that orbit or travel Clyde Tombaugh took this photograph of Pluto shown with an arrow in 1930
5 Gravity is a pulling force between any two objects that works across space The greater the mass of an object the greater is its force of gravity For example the sun which has a very large mass has a great force of gravity Even a speck of dust has a force of gravity But its gravity is very very small VOCABULARY around the sun or other stars Both planets and dwarf planets have gravity The gravity of planets is strong enough to clear their orbits of smaller objects by attracting the objects to them The gravity of dwarf planets is weaker Pluto was one of the names ancient Romans used for their god of the underworld
6 Dwarf planets can have satellites such as moons like planets do They can spin around an axis like planets do too Dwarf planets are also similar to asteroids These are small rocky bodies that orbit the sun Asteroids are considered minor planets but they are not always big or round enough to be dwarf planets There are five known dwarf planets in our solar system Pluto Eris Ceres Makemake and Haumea Scientists believe there are many more although they do not always Charon left is one of the five known moons of the dwarf planet Pluto right
7 VOCABULARY Satellites are small objects that orbit larger objects in space They can be natural or made by humans agree on what counts as a dwarf planet In 2008 scientists came up with a new subcat egory of dwarf planets plu toids A plutoid is a dwarf planet whose orbit takes it farther from the sun than the planet Neptune Pluto Makemake Haumea and Eris are dwarf planets and also plutoids Ceres which orbits much closer to the sun is a dwarf planet but not a plutoid Pluto seen here might be the most famous dwarf planet but there are at least four others in our solar system
