PARIS - Pluto was long considered our solar system's ninth planet. Although small, it orbits the sun and has the spherical shape required to be considered a planet.
But today marks 13 years since Pluto received a harsh drop in status -- officially dubbed Pluto Demotion Day.
Pluto was relegated in 2006 when the International Astronomical Union (IAU) created a new definition for planets and decided Pluto did not fit the bill.
But that has not settled the matter for fans of the faraway Pluto.
Pluto planetary days are remembered fondly -- for decades it was notable for being our solar system's smallest and farthest planet. It's only about half the width of the United States and lies in a far out region of the solar system called the Kuiper Belt, which requires a telescope to see.
The dwarf planet was also famous for being the only planet to be discovered in the United States.
It was spotted in 1930 by astronomer Clyde Tombaugh at Arizona's Lowell Observatory (named after the otherwise respected American astronomer Percival Lowell who believed that Martians dug the canals found on that planet's surface).
The story behind Pluto's name is also famous.
It was suggested by an 11-year-old girl in England, who was interested in Roman legends and thought naming the icy planet after the god of the underworld was intriguing. Her grandfather relayed the idea to a member of the UK's Royal Astronomical Society, which then suggested it to their American counterparts at Lowell Observatory. They ended up agreeing on the name Pluto -- possibly because the PL gave homage to Percival Lowell.
The newly discovered planet, orbiting more than 3 billion miles from the sun, would go on to be known as the "King of the Kuiper Belt."
But how the mighty have fallen.
And then there were eight
Things went downhill for Pluto in 2006, when the IAU redefined what it means to be a planet, declaring that a planet must be a celestial body that orbits the sun, is round or nearly round, and "clears the neighborhood" around its orbit. Pluto failed on the third account because its orbit overlaps with Neptune.
The IAU reclassified it as a dwarf planet, also calling it a "Trans-Neptunian Object," which prompted outrage from schoolchildren, small planet enthusiasts, and the internet in general.
For many space lovers, Pluto's demotion felt sudden. But in the academic world of astronomy, it was a process that began just decades after the dwarf planet's discovery.
In 1992, astronomers at the University of Hawaii observatory in Mauna Kea discovered a small, icy celestial body a bit farther away than the orbit of Neptune. Named Kuiper Belt Object 1992 QBI, the object prompted speculation that Pluto was just one of many planet-like objects in the Kuiper Belt.
The final blow came in 2003 when California Institute of Technology professor Mike Brown discovered Eris, a dwarf planet that actually has a bit more mass than Pluto. Astronomers began to suspect that more of these could-be planets were floating around.
Now Brown is dubbed "The Man Who Killed Pluto" because rather than give planet status to Eris and every celestial body larger than Pluto, the IAU decided to knock Pluto down a peg.
New Horizons relaunches old debate
But the debate about Pluto's status rages on.
In 2015, NASA's New Horizons Program flew past Pluto to take close-up photos and measurements of the dwarf planet, ultimately revealing that Pluto is bigger than scientists originally thought.
According to NASA, the data gathered by the New Horizons flyby "clearly indicated that Pluto and its satellites were far more complex than imagined," prompting space enthusiasts to wonder if it would regain planet status.
Even the principal investigator for the New Horizons spacecraft, planetary scientist Alan Stern, didn't agree with the IAU and claimed Pluto was demoted simply because of its distance from the sun.
"In fact, if you put Earth where Pluto is, it would be excluded!" Stern told CNN in 2015.
The year before that, the Harvard-Smithsonian Center for Astrophysics also entered the debate. Following an expert panel discussion on the definition of a planet, they let the audience vote and, of course, the crowd backed planet Pluto.
And new research emerged last year from the University of Central Florida's Space Institute, which argued the IAU's demotion of Pluto was "not valid."
"The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be defined on the basis of a concept that nobody uses in their research," said UNC planetary scientist Philip Metzger in a statement.
Metzger and his team looked at more than 200 years' worth of research and found just one study that employed the orbit-clearing standard the IAU used to downgrade Pluto.
"It's a sloppy definition," Metzger added. "They didn't say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit."
Too cool for school
When Pluto was demoted, it prompted a wave of science textbook reprints to ensure that students of the new millennium would be taught Pluto is a dwarf planet.
But it's still arguably the coolest (non) planet to learn about -- literally speaking.
Pluto has an icy shell, dunes made of solid methane ice, and mountain peaks covered in methane snow (but the snow is red instead of a fluffy white). It's also home to the largest known glacier in the solar system.
In fact, Pluto is so cool that its temperature is around 400 degrees Fahrenheit below zero, and it gets even colder as it orbits farther away from the sun. Typically, Pluto is so far from the sun that sunlight is only as bright as a full moon on Earth. From Pluto's surface, the sun merely looks like a bright star.
Perhaps Pluto's undeniable coolness is why people are still intrigued by its categorization 13 years later.
"The complexity of the Pluto system — from its geology to its satellite system to its atmosphere — has been beyond our wildest imagination," said Stern in a NASA statement. "Everywhere we turn are new mysteries."
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