It is always hard for a small fish to live in a big pond. Pluto's recent departure from the classical planet family clearly proves that size really matters. Astronomers voted on August 24 to create the first scientific definition of the word planet, and Pluto obviously didn't make the cut. It turned out to be only a "dwarf planet"  . After weeks of heated debate, over 2,500 astronomers from 73 countries voted on the definition of a "planet" at a conference of the international Astronomical Union (IAU). According to the definition, a planet must have a clear neighborhood around its orbit. Pluto has widely been considered a planet since its discovery in 1930. Unfortunately, it has a special orbit which overlaps with Neptune's. It is all because, compared with Neptune, Pluto is very small. It is attracted by Neptune's gravity when the two planets get closer. According to IAU, a "dwarf planet" should have an orbit around the Sun. It will not have a clear neighborhood around its orbit, and must not be a satellite. The new classification means that the science textbooks will have to be updated. The solar system is now made up of the eight "classical planets," together with a number of dwarf planets. The classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The debate over Pluto's status started in 1992. After an advanced telescope was sent into space, astronomers started find a belt of objects, which lies beyond Neptune. The belt contains nearly 1,000 objects, including Pluto. As they continued the exploration, scientists recently discovered at least 41 dwarf planets at the edges of the solar system. There are at least two objects of a similar size to Pluto. It seems that even if the IAU had kept Pluto's status as a planet, the classical theory of nine planets in the solar system would have had to be changed, but the other way around. More dwarf, planets of a similar size to Pluto would join the family of planets. Scientists will probably find many more dwarf planets. According to the passage, Pluto is  _  .
a planet which has a clear neighborhood around its orbit a satellite of Neptune which was discovered in 1930 the smallest planet in the solar system a planet which has an orbit around the sun