Today is a ‘Pluto demotion’ day. This day means that Pluto has been excluded from the elite list of planets, bringing the total number down to eight. On August 24, 2006, Pluto’s status was downgraded from the full-sized planet it had held for more than 70 years to a dwarf planet by the International Astronomical Union (IAU), a worldwide society of professional astronomers.
On February 18, 1930, this hidden planet was spotted by astronomer Clyde Tombaugh, an observing assistant working at the Lowell Observatory in the United States. Renowned astronomer Percival Lowell believed that this planet, then called Planet X, had disturbed the orbits of Uranus and Neptune. However, this hypothesis of Lowell’s later proved to be incorrect.
The IAU website describes the discovery as follows: “As he (Tombaugh) was moving from one plate to the other, trying to see if anything moved slightly between them (a sign of the planet he was hunting), he noticed something. In one part of the frame, a small object flickered by a few millimeters as it switched between two plates. Tombaugh has found his new planet! The American Astronomical Society, the Royal Astronomical Society in Great Britain and the IAU recognized Tombaugh’s discovery and officially listed the object as a planet and adopted the name Pluto.
How did the script change?
But after subsequent discoveries and advances in technology, astronomers have found over 1,000 objects orbiting beyond Neptune. This zone is now called the trans-Neptunian region, and these bodies are designated as trans-Neptunian objects. These discoveries have puzzled astronomers into reconsidering whether Pluto is a planet, as it may simply be the first of a population of small, icy bodies outside Neptune’s orbit.
“With so many trans-Neptunian objects being found, it seemed inevitable that one or more would be found to rival Pluto in size,” the IAU states. How to define a planet?
At the IAU General Assembly in Prague in 2006, astronomers established three conditions for celestial objects to become planets of the Solar System. A celestial body which:
a) revolve around the Sun
b) has enough mass for its own gravity to overcome the rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
(c) cleared the vicinity of its orbit
Although the decisions and recommendations are not enforceable by any national or international law, it has been a general convention to accept the IAU decisions for the naming and nomenclature of planetary bodies and their satellites since the early 20th century. But the debate is far from settled. Experts argued that Earth, Jupiter, and other planets also could not meet the IAU’s 2006 definition. They argued that the zone-clearing clause also affects Earth, Mars, Jupiter, and Neptune, none of which have a clear orbital zone. Earth orbits 10,000 near-Earth asteroids, while Jupiter’s orbit is littered with more than 100,000 “Trojan asteroids” (pieces of debris left over from the formation of the Solar System more than four billion years ago).
Professor Owen Gingerich of Harvard, who chaired the planet definition committee, said only 10% of the 2,700 scientists who attended the 10-day meeting were present to vote on Pluto. Professor Alan Stern, chief scientist of the New Horizons mission, also rejected the resolution, calling the result a “terrible decision” and describing the new definition as “intrinsically inconsistent”, the report added.
