For tagging purposes a planet is a large spherical astronomical object that typically orbits a star. The tag should be used when at least one planet is being shown from outer space or from the surface of another nearby planet. This includes real and fictional planets.
Scientifically there is some contention on what is a planet. Historically the first nine objects classified as a planet were Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto that are located in our solar system. In 2006 the International Astronomical Union (IAU) came up with their own definition of what was a planet in our solar system which reclassified Pluto as a "dwarf planet" instead of a regular planet like the other eight. In 2008 the IAU working group Committee on Small Bodies Nomenclature further classified Pluto as a "plutoid," a new subtype of "dwarf planet." The term though was rejected by the IAU working group Planetary System Nomenclature.