Pluto demoted to ‘dwarf planet’ (24 August 2006)

In a historic decision, astronomers have voted to adopt a new way of defining what is a planet which relegates Pluto to a secondary category of ‘dwarf planet’.

According to the IAU decision, the solar system will now have a total of 8 planets and 3 'dwarf planets': Pluto, Ceres and the unnamed 2003 UB313. Image: The International Astronomical Union/Martin Kornmesser

Over 2500 astronomers participated in the vote at the General Assembly of the International Astronomical Union (IAU) in Prague, The Czech Republic.

A celestial body can now be classed as a planet if:

• it is in orbit around the Sun
• it has enough mass to take on a nearly round shape
• it has cleared the neighbourhood around its orbit.

Pluto no longer qualifies under these criteria because its highly eccentric orbit overlaps with that of Neptune. It will instead join a new category of dwarf planets which also includes Ceres and the unnamed 2003 UB313. Pluto is also recognised as an important prototype of a new class of trans-Neptunian objects.

The IAU’s decision means that the Solar System now consists of just eight major planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.