But in 2006, it was relegated to the status of dwarf planet by the International Astronomical Union (IAU). So why was Pluto demoted?
Pluto was discovered in 1930 by US astronomer Clyde Tombaugh, who was using one of the most powerful telescopes of the time: the Lowell Observatory in Arizona.
Textbooks were swiftly updated to list this ninth member in the club. But over subsequent decades, astronomers began to wonder whether Pluto might simply be the first of a population of small, icy bodies beyond the orbit of Neptune.
This region would become known as the Kuiper Belt, but it took until 1992 for the first “resident” to be discovered. The candidate Kuiper Belt Object (KBO) 1992 QBI was detected by David Jewitt and colleagues using the University of Hawaii’s 2.24m telescope at Mauna Kea.
Confirmation of the first KBO invigorated the existing debate. And in 2000, the Hayden Planetarium in New York became a focus for controversy when it unveiled an exhibit featuring only eight planets. The planetarium’s director Neil deGrasse Tyson would later become a vocal figure in public discussions of Pluto’s status.
But it was discoveries of Kuiper Belt Objects with masses roughly comparable to Pluto, such as Quaoar (announced in 2002), Sedna (2003) and Eris (2005), that pushed the issue to a tipping point.
Eris, in particular, appeared to be larger than Pluto – giving rise to its informal designation as the Solar System’s “tenth planet”.
Prof Mike Brown of the California Institute of Technology (Caltech), who led the team that found Eris, would later style himself as the “man who killed Pluto”, while deGrasse Tyson would later jokingly quip that he had “driven the getaway car”.
The finds spurred the International Astronomical Union to set up a committee tasked with defining just what constituted a planet, with the aim of putting a final draft proposal before members at the IAU’s 2006 General Assembly in Prague.
Under a radical early plan, the number of planets would have increased from nine to 12, seeing Pluto and its moon Charon recognised as a twin planet, and Ceres and Eris granted entry to the exclusive club. But the idea met with opposition.
The discussions in Prague during August 2006 were intense, but a new version of a planetary definition gradually took shape. On 24 August, the last day of the assembly, members voted to adopt a new resolution outlining criteria for naming a planet:
Pluto met the first two of the these criteria, but the last one proved pivotal. “Clearing the neighbourhood” means that the planet has either “vacuumed up” or ejected other large objects in its vicinity of space. In other words, it has achieved gravitational dominance.
Because Pluto shares its orbital neighbourhood with other icy Kuiper Belt Objects, the resolution effectively stripped the distant world of a planetary designation it had held for some 76 years.
It was immediately relegated it to the distinct category of “dwarf planet”, alongside the biggest body in the asteroid belt, Ceres, and other large Kuiper Belt Objects such as Eris, Quaoar and Sedna.
Commenting at the time, the IAU’s president of planetary systems science Prof Iwan Williams said: “By the end of the decade, we would have had 100 planets, and I think people would have said ‘my goodness, what a mess they made back in 2006‘.”
In a word, no. Some experts immediately questioned the part of the definition about a planet clearing its orbital neighbourhood.
This is because Earth shares its cosmic turf with more than 12,000 near-Earth asteroids. Thus, some have argued that Earth, Jupiter and other planets also fail to meet the IAU’s 2006 definition.
Speaking just after the vote, Prof Alan Stern, chief scientist for the New Horizons mission, called the outcome “an awful decision” and described the new definition as “internally inconsistent”.
Prof Owen Gingerich of Harvard, who chaired the planet definition committee, revealed that only 10% of the 2,700 scientists who had attended the 10-day meeting were present at the Pluto vote. The low turn-out has been blamed on timing; the vote was held on the last day of the General Assembly when many participants had left or were preparing to fly out from Prague.
The debate has rumbled on ever since, on television, in the pages of books and in public talks.
Most recently, Alan Stern challenged Neil deGrasse Tyson to a debate on the matter in 2014. But the latter expert turned down the offer, stating: “I don’t have opinions that I require other people to have.”
The flyby of Pluto is unlikely to provide any information relevant to a change in Pluto’s status. But it will bring into clear focus once more what is, and what isn’t, meant by the term “planet”.