Not a Fact Anymore

Not a fact anymore

Pluto is the ninth planet of the Solar System.

What we know now

Under the International Astronomical Union's 2006 definition, Pluto is a dwarf planet because it has not cleared the neighbourhood around its orbit. The Solar System has eight objects formally classified as planets.

Why it changed

The discovery of Pluto-like bodies in the Kuiper Belt, especially Eris, forced astronomers to define more explicitly what qualifies as a planet. The IAU adopted a formal definition in 2006 and placed Pluto in the dwarf-planet category.

Status
Reclassified
Category
Astronomy
Accepted approximately
1930–2006
Changed approximately
2006

This is a classification change, not a discovery that Pluto disappeared or that its physical properties suddenly changed.

The IAU definition is the formal astronomical standard, but some planetary scientists continue to advocate broader geophysical definitions that would classify Pluto as a planet. The entry therefore describes the official classification rather than claiming that all scientists agree with the terminology.

Evidence

Sources and what they establish

Historical context

  • Pluto Facts NASA

    States that Pluto was long considered the ninth planet and was reclassified as a dwarf planet in 2006.

Current evidence

Primary research

  • List of IAU Resolutions International Astronomical Union

    Provides the official 2006 Resolutions B5 and B6 on the definition of a planet and Pluto.