On 24 August 2006, after years of heated debate, the International Astronomical Union downgraded Pluto to a dwarf planet in a decision that remains controversial to this day.
When a new definition of planethood was developed, Pluto failed to meet all of the essential criteria.
The new rules stated that a planet –
(a) is in orbit around the sun
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit
While Pluto met the first two of these conditions, sharing its orbital neighbourhood with other icy Kuiper Belt Objects led to its loss of planet status.
Dr Neil deGrasse Tyson has been famously linked to Pluto’s downfall since the front page of The New York Times reported that the Hayden Planetarium, of which he is director, has omitted the object from their solar system displays.
Tyson argued that Pluto was too small and insignificant to qualify as a planet, saying “I didn’t kill Pluto, but I was an accessory.”
Pluto image taken by New Horizons [Photo credit: NASA]
Thanks to the New Horizons mission, we now know more about Pluto than ever before. In 2006, New Horizons took off on a rocket from Cape Canaveral, Florida, to begin it’s over 4-billion-kilometre voyage.
Hurtling through space faster than any man-made object has ever travelled and propelling free of the Earth’s gravity, New Horizons sped out beyond Mars, past the great gas giants like Jupiter and onward toward its final rendezvous with Pluto.
For more on Pluto and New Horizons, check out Mission Pluto.