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Arizona names Pluto as its official state planet — except it's technically not a planet

Spacecraft survives historic trip
New Horizons spacecraft survives trip 1 billion miles beyond Pluto 02:03

Arizona has a new state emblem — and it's one that has been a source of controversy among scientists for years. Gov. Katie Hobbs signed a bill on Friday designating Pluto — once considered the ninth planet of our solar system and since downgraded to a lesser status — the "official state planet" of Arizona. 

The only thing is, Pluto technically isn't a planet. 

Though long considered to be the small, lonely outlier of the solar system, the International Astronomical Union, a nongovernmental organization, downgraded that categorization in 2006. Pluto is now classified one of five "dwarf planets" in our solar system. 

To be considered a planet, objects must meet certain criteria: It must orbit its host star, be large enough to be mostly round and "must have an important influence on the orbital stability" of other objects around it. A dwarf planet is an object that meets those first two rules, but "has not been able to clear its orbit of debris," the IAU says. 

"Pluto now falls into the dwarf planet category because it resides within a zone of other objects that might cross its orbital path, known as the Trans-Neptunian region," the group says. "Pluto is additionally recognised as an important prototype of a new class of Trans-Neptunian Objects: plutoids."

The other four dwarf planets in the solar system are Ceres, Haumea, Makemake and Eris. 

But for Arizona, the downgrade of classification didn't mean a downgrade of importance. 

In 1894, Percival Lowell founded the Lowell Observatory in Flagstaff as part of his search for a potential ninth planet, which at the time he deemed as "Planet X." According to the Library of Congress, he and his astronomer colleague William H. Pickering found several potential ninth planets, which they investigated until Lowell's death in 1916. 

After a years-long hiatus, the search for Planet X resumed in 1929, this time with 23-year-old Clyde Tombaugh at the helm. He discovered Pluto a year later, with an 11-year-old girl from Oxford, England, suggesting the newly-recognized object's name. 

Pluto is officially a planet! A state planet that is 🤩 As of yesterday, a bill was passed to make Pluto Arizona’s...

Posted by Lowell Observatory on Saturday, March 30, 2024

That history was of significant importance to State Rep. Justin Wilmeth, who introduced the bill.

"We in Arizona haven't forgotten about you, Pluto," he wrote last month on social media, adding in a graphic, "we still love you." 

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