Pluto that is now considered as a dwarf planet was once considered a planet. Since its discovery in 1930, this planet is always a bit of a puzzle.
• It is smaller than any other terrestrial planet, even smaller than moon.
• Unlike its neighbors (Saturn, Uranus , Neptune) it is mostly dense and rocky like Earth, Mars, Venus.
• Pluto orbits around the sun at an angle of 17 degree to its plane thus its orbit is erratic.
• Some astronomers consider Pluto and its moons as a binary system rather than than a planet and satellite.
These facts are the cause of the debate continuing from long ago whether to consider Pluto a planet or not. On Aug. 24, 2006, the International Astronomical Union (IAU), passed two resolutions namely resolution 5A and resolution 6A that revoke the status of Pluto as a planet.
Revolution 5A that defines a planet is stated as:
A planet is a celestial body that (1) is in orbit around the Sun, (2) has sufficient mass for its self-gravity to overcome rigid body forces so that it has nearly round shape, and (3) has cleared the neighbourhood around its orbit
Pluto is nearly round and orbits around the sun but its orbit crosses Neptune’s orbit. Critics of resolution argue that earth orbit’s also has asteroids crossing over to oppose the decision.
Resolution 5A also signify Dwarf planet and small solar system bodies and according to resolution 5A a dwarf planet is:
A celestial body that (1) is in orbit around the Sun, (2) has sufficient mass for its self-gravity to overcome rigid body forces so that it has nearly round shape, (3) has not cleared the neighbourhood around its orbit, and (4) is not a satellite.
Small solar system bodies are objects that are neither planets nor Dwarf planet. According to resolution 6A, Pluto is considered as a Dwarf planet.
Some astronomers believe that considering Pluto a Dwarf planet is misleading and confusing. They also questioning the validity of the resolutions as very few astronomers were included in framing the resolutions and to vote for it.
Hence, by resolution 5A and 6A, terrestrial objects are classified as:
Planets: Mercury, Venus,Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Dwarf Planets: Pluto, 2003 UB313 (an object far from Pluto), Ceres (an object between Mars and Jupiter in asteroid belt).
Small solar-system bodies: Everything else left including comets and asteroids.