Sentence:  Their decision will force a rewrite of science textbooks because the solar system is now a place with eight <WSD>planets</WSD> and three newly defined "dwarf <WSD>planets</WSD>"&mdash;a new category of object that includes Pluto. 
Meaning of the word marked with <WSD> tags:  A body which orbits the Sun directly and is massive enough to be in hydrostatic equilibrium (effectively meaning a spheroid) and to dominate its orbit; specifically, the eight major bodies of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (Pluto was considered a planet until 2006 and has now been reclassified as a dwarf planet.) (from 17thc.)