Pluto was erroneously rejected as a planet: Scientists

Pluto's controversial demotion from planetary status may have been based on an incorrect scientific criterion, according to a team of researchers who have challenged the reasoning behind the International Astronomical Union's (IAU) landmark 2006 decision.

A team led by Philip Metzger, a planetary scientist at the University of Central Florida in Orlando, said a review of scientific literature found little historical support for the requirement that a planet must “clear” its orbit to qualify as a planet.

In 2006, the IAU defined a planet as a celestial body that orbits the Sun, is massive enough for its gravity to make it nearly spherical and has cleared the neighbourhood around its orbit of other objects.

Under that definition, Pluto was reclassified as a dwarf planet because it shares its orbital region with numerous objects in the Kuiper Belt and is also influenced by Neptune's gravity.

However, Metzger and his colleagues argued that orbital clearing has not traditionally been used by planetary scientists as the primary way to distinguish planets from other celestial bodies.

The researchers reviewed scientific publications covering roughly two centuries and found only one paper, published in 1802, that used the concept of orbital clearing in the classification of planets. They said the reasoning behind that publication had subsequently been disproved.

“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be defined on the basis of a concept that nobody uses in their research,” Metzger said.

He also argued that the definition was insufficient because no planet completely clears its orbital neighbourhood when the criterion is interpreted literally. Every planet shares its orbital region with smaller objects to some extent.

Co-author Kirby Runyon of Johns Hopkins University said the historical evidence contradicted the IAU's claim that orbital clearing had traditionally been used to distinguish planets from asteroids.

“We showed that this is a false historical claim,” Runyon said, arguing that applying that reasoning specifically to Pluto was therefore flawed.

The researchers suggested that a more useful definition of a planet should focus on its intrinsic physical characteristics rather than the changing dynamics of its orbit.

Metzger said planetary classification should instead consider whether an object is sufficiently massive for its own gravity to shape it into a roughly spherical body.

The debate over Pluto reflects a broader question in planetary science: whether planets should be classified primarily according to their physical properties or their position and behaviour within a planetary system.

Pluto was discovered in 1930 and remained the ninth planet in the Solar System for more than seven decades before the IAU's 2006 decision. The latest research does not change Pluto's official classification, but it adds fresh scientific arguments to the long-running debate over what should qualify as a planet.