A scientific argument which continues to this day broke out on August 24, 2006, when an international body of astronomers declared that Pluto – known as the ninth planet in the solar system since its discovery in 1930 – was to be stripped of its planet status.
At its General Assembly meeting in Prague, the International Astronomical Union (IAU) – which holds the responsibility for naming stars, asteroids and other celestial bodies – decided that due to its small size and tilted orbit, Pluto should be redesignated as a dwarf planet.
In recent years, improved telescopic technology had revealed many bodies orbiting the sun that were similar to Pluto, leading to the potential need to define around 50 more of these bodies as planets.
These would have included the asteroid Ceres, Pluto’s own largest moon Charon, and a body discovered by the Hubble telescope named 2003 UB313 which was revealed to be even larger than Pluto itself.
The debate was sparked partly due to there having been no strict definition of what constituted a planet; the IAU therefore decreed that to hold the name, a body must orbit the sun, be large enough to take on a roughly spherical shape, and must dominate its own orbit, clearing away other objects.
Pluto failed to meet the last criterion, its elliptical orbit overlapping with that of Neptune; however, a number of dissenting astronomers pointed out that Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would also cease to be planets by this measure.
Opinion on the downgrading has remained divided among the scientific community, with Nasa’s Alan Stern calling the decision “embarrassing” and pointing out that fewer than 5% of the world’s astronomers voted on it.
Nevertheless, until a broader consensus is reached, Pluto seems destined to remain reclassified, and our solar system officially home to just eight planets.