Pluto was (1) **discovered** in 1930. At that time, there wasn’t really a definition for what a planet was. We didn’t know as much about space (2) **as** we do now. In the summer of 2006, scientists of the International Astronomical Union met to discuss the naming of new objects in the solar (3) **system**. Then they agreed to remove Pluto from the list of planets. It is now (4) **classified** as a “dwarf planet”. Why Pluto is (5) **different** from other planets? Pluto is less than half the size of any other planet. It is very different (6) **from** the other gas giants, the nearest planets to it. The first four planets (Mercury, Venus, Earth, and Mars) are rocky planets (7) **with** solid surfaces. The next four planets are gas giants. Jupiter, Saturn, Uranus, and Neptune are huge planets with small rocky, metal cores surrounded (8) **by** huge balls of gas. These four planets do not have a solid surface. After these four planets, it comes to Pluto. It is about one-sixth the (9) **mass** of Earth and about 1/100 the size of Jupiter. It is (10) **composed** of rock and ice. Pluto is like a comet, not a gas giant.