How many planets are there in the solar system?

For many years, the Solar System was the only known example of planets in orbit around a star. The discovery in recent years of many extrasolar planets has led to the term "solar system" being applied generically to all the newly discovered systems. Technically, however, it should strictly refer to Earth's system only, as the word "solar" is derived from the Sun's Latin name, Sol. Other such systems are usually referred to by the names of their parent star: "the Alpha Centauri system" or "the 51 Pegasi system".