(CNN)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’s primeRead MorePluto 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. IN SPACE - JULY 14:  In this handout provided by the National Aeronautics and Space Administration (NASA), a close-up image of a region near  Pluto's equator shows a range of mountains rising as high as 11,000 feet (3,500 meters) taken by NASA's New Horizons spacecraft as it passed within 7,800 feet of the dwarf planet on July 14, 2015. The 1,050-pound piano sized probe, which was launched January 19, 2006 aboard an Atlas V rocket from Cape Canaveral, Florida, zipped by the planet yesterday.  (Photo by NASA/APL/SwRI via Getty Images)IN SPACE - JULY 14:  In this handout provided by the National Aeronautics and Space Administration (NASA), a close-up image of a region near  Pluto's equator shows a range of mountains rising as high as 11,000 feet (3,500 meters) taken by NASA's New Horizons spacecraft as it passed within 7,800 feet of the dwarf planet on July 14, 2015. The 1,050-pound piano sized probe, which was launched January 19, 2006 aboard an Atlas V rocket from Cape Canaveral, Florida, zipped by the planet yesterday.  (Photo by NASA/APL/SwRI via Getty Images)IN SPACE – JULY 14: In this handout provided by the National Aeronautics and Space Administration (NASA), a close-up image of a region near Pluto’s equator shows a range of mountains rising as high as 11,000 feet (3,500 meters) taken by NASA’s New Horizons spacecraft as it passed within 7,800 feet of the dwarf planet on July 14, 2015. The 1,050-pound piano sized probe, which was launched January 19, 2006 aboard an Atlas V rocket from Cape Canaveral, Florida, zipped by the planet yesterday. (Photo by NASA/APL/SwRI via Getty Images)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 eightThings 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.This high-resolution image captured by NASA's New Horizons spacecraft shows Pluto's surface has a remarkable range of subtle colors, enhanced in this view to a rainbow of pale blues, yellows, oranges, and deep reds. The bright expanse is the western lobe of the "heart," informally known as Tombaugh Regio. The lobe, informally called Sputnik Planum, has been found to be rich in nitrogen, carbon monoxide, and methane ices.This high-resolution image captured by NASA's New Horizons spacecraft shows Pluto's surface has a remarkable range of subtle colors, enhanced in this view to a rainbow of pale blues, yellows, oranges, and deep reds. The bright expanse is the western lobe of the "heart," informally known as Tombaugh Regio. The lobe, informally called Sputnik Planum, has been found to be rich in nitrogen, carbon monoxide, and methane ices. Photos: Pluto in glorious color imagesPluto's Brilliant 'Heart' – This high-resolution image captured by NASA’s New Horizons spacecraft shows Pluto’s surface has a remarkable range of subtle colors, enhanced in this view to a rainbow of pale blues, yellows, oranges, and deep reds. The bright expanse is the western lobe of the “heart,” informally known as Tombaugh Regio. The lobe, informally called Sputnik Planum, has been found to be rich in nitrogen, carbon monoxide, and methane ices.Hide Caption 1 of 4Pluto taken by NASA's New Horizons spacecraft just before closest approach on July 14, 2015, are the sharpest images to date of Pluto's varied terrain—revealing details down to scales of 270 meters. In this 75-mile (120-kilometer) section, the textured surface of the plain surrounds two isolated ice mountains.Pluto taken by NASA's New Horizons spacecraft just before closest approach on July 14, 2015, are the sharpest images to date of Pluto's varied terrain—revealing details down to scales of 270 meters. In this 75-mile (120-kilometer) section, the textured surface of the plain surrounds two isolated ice mountains. Photos: Pluto in glorious color imagesPluto's Varied Terrain – Pluto taken by NASA’s New Horizons spacecraft just before closest approach on July 14, 2015, are the sharpest images to date of Pluto’s varied terrain—revealing details down to scales of 270 meters. In this 75-mile (120-kilometer) section, the textured surface of the plain surrounds two isolated ice mountains.Hide Caption 2 of 4A detailed color map of Pluto taken during the NASA spacecraft's close flyby of Pluto in July 2015. The mosaic shows how Pluto's large-scale color patterns extend beyond the hemisphere facing New Horizons at closest approach, which were imaged at the highest resolution. North is up; Pluto's equator roughly bisects the band of dark red terrains running across the lower third of the map. Pluto's giant, informally named Sputnik Planitia glacier -- the left half of Pluto's signature "heart" feature -- is at the center of this map.A detailed color map of Pluto taken during the NASA spacecraft's close flyby of Pluto in July 2015. The mosaic shows how Pluto's large-scale color patterns extend beyond the hemisphere facing New Horizons at closest approach, which were imaged at the highest resolution. North is up; Pluto's equator roughly bisects the band of dark red terrains running across the lower third of the map. Pluto's giant, informally named Sputnik Planitia glacier -- the left half of Pluto's signature "heart" feature -- is at the center of this map. Photos: Pluto in glorious color imagesPluto Global Color Map – A detailed color map of Pluto taken during the NASA spacecraft’s close flyby of Pluto in July 2015. The mosaic shows how Pluto’s large-scale color patterns extend beyond the hemisphere facing New Horizons at closest approach, which were imaged at the highest resolution. North is up; Pluto’s equator roughly bisects the band of dark red terrains running across the lower third of the map. Pluto’s giant, informally named Sputnik Planitia glacier — the left half of Pluto’s signature “heart” feature — is at the center of this map.Hide Caption 3 of 4Scientists believe the haze is a photochemical smog resulting from the action of sunlight on methane and other molecules in Pluto's atmosphere.  Scientists believe the haze is a photochemical smog resulting from the action of sunlight on methane and other molecules in Pluto's atmosphere.  Photos: Pluto in glorious color imagesPluto's Haze in Bands of Blue – Scientists believe the haze is a photochemical smog resulting from the action of sunlight on methane and other molecules in Pluto’s atmosphere. Hide Caption 4 of 4NASA SenegalNASA SenegalNASA SenegalNASA SenegalFor 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 debateBut 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. KENNEDY SPACE CENTER, FLA.  —   Into a cloud-scattered blue sky, NASA's New Horizons spacecraft roars off the launch pad aboard an Atlas V rocket spewing flames and smoke.  Liftoff was on time at 2 p.m. EST from Complex 41 on Cape Canaveral Air Force Station in Florida. This was the third launch attempt in as many days after scrubs due to weather concerns.   The compact, 1,050-pound piano-sized probe will get a boost from a kick-stage solid propellant motor for its journey to Pluto. New Horizons will be the fastest spacecraft ever launched, reaching lunar orbit distance in just nine hours and passing Jupiter 13 months later. The New Horizons science payload, developed under direction of Southwest Research Institute, includes imaging infrared and ultraviolet spectrometers, a multi-color camera, a long-range telescopic camera, two particle spectrometers, a space-dust detector and a radio science experiment. The dust counter was designed and built by students at the University of Colorado, Boulder. The launch at this time allows New Horizons to fly past Jupiter in early 2007 and use the planet's gravity as a slingshot toward Pluto. The Jupiter flyby trims the trip to Pluto by as many as five years and provides opportunities to test the spacecraft's instruments and flyby capabilities on the Jupiter system. New Horizons could reach the Pluto system as early as mid-2015, conducting a five-month-long study possible only from the close-up vantage of a spacecraft.KENNEDY SPACE CENTER, FLA.  —   Into a cloud-scattered blue sky, NASA's New Horizons spacecraft roars off the launch pad aboard an Atlas V rocket spewing flames and smoke.  Liftoff was on time at 2 p.m. EST from Complex 41 on Cape Canaveral Air Force Station in Florida. This was the third launch attempt in as many days after scrubs due to weather concerns.   The compact, 1,050-pound piano-sized probe will get a boost from a kick-stage solid propellant motor for its journey to Pluto. New Horizons will be the fastest spacecraft ever launched, reaching lunar orbit distance in just nine hours and passing Jupiter 13 months later. The New Horizons science payload, developed under direction of Southwest Research Institute, includes imaging infrared and ultraviolet spectrometers, a multi-color camera, a long-range telescopic camera, two particle spectrometers, a space-dust detector and a radio science experiment. The dust counter was designed and built by students at the University of Colorado, Boulder. The launch at this time allows New Horizons to fly past Jupiter in early 2007 and use the planet's gravity as a slingshot toward Pluto. The Jupiter flyby trims the trip to Pluto by as many as five years and provides opportunities to test the spacecraft's instruments and flyby capabilities on the Jupiter system. New Horizons could reach the Pluto system as early as mid-2015, conducting a five-month-long study possible only from the close-up vantage of a spacecraft.KENNEDY SPACE CENTER, FLA. — Into a cloud-scattered blue sky, NASA’s New Horizons spacecraft roars off the launch pad aboard an Atlas V rocket spewing flames and smoke. Liftoff was on time at 2 p.m. EST from Complex 41 on Cape Canaveral Air Force Station in Florida. This was the third launch attempt in as many days after scrubs due to weather concerns. The compact, 1,050-pound piano-sized probe will get a boost from a kick-stage solid propellant motor for its journey to Pluto. New Horizons will be the fastest spacecraft ever launched, reaching lunar orbit distance in just nine hours and passing Jupiter 13 months later. The New Horizons science payload, developed under direction of Southwest Research Institute, includes imaging infrared and ultraviolet spectrometers, a multi-color camera, a long-range telescopic camera, two particle spectrometers, a space-dust detector and a radio science experiment. The dust counter was designed and built by students at the University of Colorado, Boulder. The launch at this time allows New Horizons to fly past Jupiter in early 2007 and use the planet’s gravity as a slingshot toward Pluto. The Jupiter flyby trims the trip to Pluto by as many as five years and provides opportunities to test the spacecraft’s instruments and flyby capabilities on the Jupiter system. New Horizons could reach the Pluto system as early as mid-2015, conducting a five-month-long study possible only from the close-up vantage of a spacecraft.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 schoolWhen 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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