16 years ago today, Pluto ceased to be a planet. Nine years later, it’s become a world

Poor Pluto. On August 24, 2006, at the General Assembly of the International Astronomical Union (IAU), the ninth planet was cleared only 76 years after its discovery.

Even stranger is that it was actually rejected, and by astronomers, not planetary scientists. The IAU redefined what a planet is regardless of geophysical characteristics, Pluto not failing on its small size (it’s no bigger than the continental United States), but because it’s not has not “cleaned up the neighborhood around its orbit”. Debate rages over it, with the latest NASA administrator claiming the asteroid is approaching every “planet” in the solar system.

The IAU also explicitly created a new term – “Pluto-class object”. The term has never been used by planetary scientists.

More significantly, Plutoo’s demotion had consequences for the IAU’s authority.

“World” is now used instead of “planet” to describe places in the solar system. Few people talk about dwarf planets or moons, but instead we hear terms like “icy worlds”, “ocean worlds”, and “volcanic worlds”.

What few people remember is why the definition of a planet had to be revisited in 2006. The real reason was a recently discovered object called 2003 UB313, first dubbed Xena and later renamed Eris.

Although typically three times farther in an eccentric orbit from the Sun, Eris is only marginally smaller than Pluto. In 2006, it was actually thought to be larger than Pluto, and it was thought that Eris might officially gain planetary status at the IAU meeting.

However, with several other candidate objects found in the early 21st century believed to be roughly the same size as Pluto – since named Makemake, Haumea and Sedna – the IAU thought there was a problem. If Pluto was a planet, then Eris and all those other objects were too. Can we have 10 or 15 planets? With ever-changing technology and new telescopes, how about 50 or 100 planets?

Pluto has therefore been demoted… to limit the numbers? Perhaps, though it’s also true that all of these objects, including Pluto, lie within the Kuiper Belt, a ring of icy objects around the Sun extending beyond Neptune’s orbit. They also have rather eccentric orbits.

It wasn’t the first time Pluto had been snubbed. NASA’s Travelers “Grand Tour” of the outer planets of the solar system stopped short of visiting Pluto in the 1990s after checking off Jupiter, Saturn, Uranus and Neptune. The same once-in-175-year alignment that made gravitational assists possible would have allowed Voyager 2 to continue to Pluto after passing Neptune, but NASA scientists prioritized Neptune’s moon Triton.

2006 was also not the first time a planet had been retrograded. Go back to 1801 and Ceres, the largest object in the asteroid belt, was discovered and described as the “missing planet” between Mars and Jupiter. Downgraded shortly thereafter to a mere asteroid, the same IAU meeting in 2006 that downgraded Pluto elevated Ceres to dwarf planet status.

It was ironic that while votes were held on the status of the two solar system object missions, both were at an advanced stage. New Horizons was launched on Pluto in January 2006 while the Dawn mission was launched on Ceres just over a year later.

The two “new” dwarf planets turned out to be much more than planetary scientists had hoped. Both turned out to be candidate “ocean worlds” like Europa (a moon of Jupiter) and Enceladus and Titan (the moons of Saturn).

When New Horizons flew by Pluto on July 14, 2015, it revealed a world as fascinating as anywhere else in the solar system.

This showed that Pluto was something beyond the wildest dreams of any planetary scientist – geologically active and perhaps also volcanically and even tectonically active.

Here, 40 times farther from the Sun than Earth, Pluto showed off its own complex atmosphere, organic compounds on its surface and huge faults in its crust. It is a place of surprising geological complexity with vast plains of nitrogenous ice, mountain ranges, dunes and “ice volcanoes”.

The riches discovered on Pluto are such that it seems unlikely that the IAU could have withdrawn its status as a planet after the New Horizons flyby.

Pluto is an intriguing world that deserves a return mission to see if there’s an ocean beneath its ice. Maybe one day we’ll call it an ocean world, but by then there’ll probably be a new name for that too.

I wish you clear skies and big eyes.

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