How many planets on galaxy There are only 8 planets that are known. on the Milky Way Galaxy. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. But why Pluto is not a planet? Pluto is once considered a s a planet but after an analysis done by Astronomical Union (AU). Pluto did not meet the criteria or standards to be called a planet. To be called a planet, it must follow: 1. Orbits the sun 2. Distinct structure 3. Sphere or Similar to sphere 4. Didnt lie on Asteroid Belt or Kuiper Belt. - But Pluto didnt meet the 4th condition because it lies on Kuiper belt. So it was considered as a dwarf planet.