Điền từ thích hợp vào đoạn văn sau

Pluto was (l) …………………. 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) …………………. 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) …………………. Then they  agreed to remove Pluto from the list of planets. It is now (4) …………………. as a “dwarf planet”. Why Pluto is (5) …………………. from other planets? Pluto is less than half the size of any other planet. It is very different (6) …………………. the other gas giants, the nearest planets to it. The first four planets (Mercury, Venus, Earth, and Mars) are rocky planets (7) …………………. solid surfaces. The next four planets are gas giants. Jupiter, Saturn, Uranus, and Neptune are huge planets with small rocky, metal cores surrounded (8 …………………. 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) …………………. of Earth and about 1/100 the size of Jupiter. It is (10) …………………. of rock and ice. Pluto is like a comet, not a gas giant.