(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.
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.
Why is Pluto no longer a planet?
By Paul Rincon Science editor, BBC News website
Image copyright IAU Image caption In 2006, Pluto was voted out of the planetary club by members of the International Astronomical Union
Nasa's New Horizons mission made a close pass of Pluto this week. For more than 70 years, Pluto was one of nine planets recognised in our Solar System.
But in 2006, it was relegated to the status of dwarf planet by the International Astronomical Union (IAU). So why was Pluto demoted?
Where did the controversy start?
Pluto was discovered in 1930 by US astronomer Clyde Tombaugh, who was using 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”.
Image caption The discovery of other icy objects similar in size to Pluto forced a re-think by the IAU
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.
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Media captionDr Marek Kukula: “Reclassification changes the way we think about our corner of the galaxy”
What happened next?
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'.”
Image caption Astronomer Jocelyn Bell-Burnell holds a toy of Disney's Pluto the dog during a vote on the planet's demotion in 2006
Was that the end of the matter?
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.”
Pluto and the Developing Landscape of Our Solar System
The discovery of Pluto
Nearly eighty years ago an astronomer working at the Lowell Observatory in the United States made a discovery that would ultimately initiate a dramatic change in the way we look at our Solar System.
The young astronomer was Clyde Tombaugh, an observing assistant working at the observatory made famous by the great astronomer Percival Lowell.
Tombaugh was continuing the search for an elusive planet – planet X – that Lowell had believed (incorrectly) to be responsible for perturbing the orbits of Uranus and Neptune.
Within a year, after spending numerous nights at the telescope exposing photographic plates and months tediously scanning them for signs of a planet, Tombaugh saw what he was looking for.
At around 4pm on the afternoon of 18 February 1930 Tombaugh began comparing two plates taken in January that year showing a region in the constellation of Gemini. As he flicked from one plate to the other, trying to see if something moved slightly between the two (the tell-tale sign of the planet he was hunting), he spotted something.
In one part of the frame a small object flitted a few millimetres as he switched between the two plates. Tombaugh had found his new planet! (Stern & Mitton, 2005)
The changing landscape of the Solar System
The object Tombaugh had discovered was named Pluto, a name officially adopted by the American Astronomical Society, the Royal Astronomical Society in the UK and the IAU. It is a frigid world, billions of kilometres from Earth, and 30 times less massive than the then-smallest known planet, Mercury.
But Pluto was not alone. It was found to have five satellites. The largest, Charon, was discovered in 1978. The smaller four were discovered using the Hubble Space Telescope in 2005, 2011 and 2012 and officially named Nix, Hydra, in early 2006 (read more) , Kerberos and Styx in 2013 (read more) by the IAU.
The view of our Solar System's landscape began to change on August 30, 1992 with the discovery by David Jewitt and Jane Luu from the University of Hawaii of the first of more than 1000 now known objects orbiting beyond Neptune in what is often referred to as the transneptunian region. More generally these bodies are often simply labelled as Trans-Neptunian Objects (TNOs).
With so many Trans-Neptunian Objects being found, it seemed inevitable that one or more might be found to rival Pluto in size.
On the night of the 21 October 2003, Mike Brown from Caltech, Chad Trujillo from the Gemini Observatory and David Rabinowitz from Yale University were using a telescope and camera at the Palomar Observatory in the US to search the edge of the Solar System.
That night they imaged a region of sky showing an object moving relative to the background stars. Later analysis showed that they had discovered another cold world, around 2500 km across, orbiting the Sun.
Subsequent observations showed that the new object, initially named 2003 UB313 according to the International Astronomical Union's protocol on the initial designation of such objects, was more massive than Pluto and that it too had a satellite (read more). With an object larger and more massive than Pluto now beyond Neptune and ever more of these Trans-Neptunian Objects being discovered, astronomers were beginning to ask: “Just what constitutes a planet?”
A new class of objects and how to define a planet
The IAU has been responsible for the naming and nomenclature of planetary bodies and their satellites since the early 1900s. As Professor Ron Ekers, past president of the IAU, explains:
Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.(read the full article on page 16 of the IAU GA Newspaper)
July 14, 2017: On July 14, 2015, NASA’s New Horizons spacecraft made its historic flight through the Pluto system – providing the first close-up images of Pluto and its moons and collecting other data that has transformed our understanding of these mysterious worlds on the solar system’s outer frontier.
Perspective view of Pluto's highest mountains, Tenzing Montes. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/ Lunar and Planetary Institute/Paul Schenk
Scientists are still analyzing and uncovering data that New Horizons recorded and sent home after the encounter. On the two-year anniversary of the flyby, the team unveiled a set of detailed, high-quality global maps of Pluto and its largest moon, Charon.
Pluto—which is smaller than Earth’s Moon—has a heart-shaped glacier that’s the size of Texas and Oklahoma. This fascinating world has blue skies, spinning moons, mountains as high as the Rockies, and it snows—but the snow is red.
“The complexity of the Pluto system — from its geology to its satellite system to its atmosphere— has been beyond our wildest imagination,” said Alan Stern, New Horizons principal investigator from the Southwest Research Institute in Boulder, Colorado. “Everywhere we turn are new mysteries.”
- Go farther. Explore Pluto In Depth ›
- 10 Things to Know About Pluto
Pluto is about 1,400 miles (2,380 km) wide. That's about half the width of the United States, or 2/3 the width of Earth's moon.
Pluto orbits the Sun about 3.6 billion miles (5.8 billion km) away on average, about 40 times as far as Earth, in a region called the Kuiper Belt.
A year on Pluto is 248 Earth years. A day on Pluto lasts 153 hours, or about 6 Earth days.
Pluto is officially classified as a dwarf planet.
Pluto has a thin atmosphere of nitrogen, methane and carbon monoxide. The atmosphere has a blue tint and distinct layers of haze.
Is Pluto a Planet?
Named after the Roman King of the underworld, Pluto has served as a surprising source of conflict in the astronomical community and for lovers of space everywhere. Growing up, I was taught in school that our Sun hosted nine planets. And as a professional astronomer, I went on to teach this nine-planet solar system model in elementary and middle schools around the globe.
But in a contentious decision in 2006, Pluto was officially stripped of its planetary status leaving our solar system with only eight planets and making me a liar to school children. Now that debate is being reignited with some astronomers calling for the reinstatement of Pluto as a planet, arguing that the icy world should never have been demoted in the first place.
So what is the deal? Is Pluto a planet? Can we rest assured that the Pluto we grew up with is the real Pluto?
Discovery of Pluto
In his studies of the gas giants Uranus and Neptune, Percival Lowell suggested there must be an as-of-yet undiscovered ninth planet to explain the wobbles their orbits.
Lowell never found the mystery planet despite extensive searches, but astronomer Clyde Tombaugh did finally detect Pluto using the Lowell Observatory, so-named in honor of Lowell’s contributions, in 1930.
The mystery of the wobbles was not entirely solved, however, since Pluto still didn’t appear large enough to cause them, until Pluto’s companion Charon—which is about half the size of Pluto in diameter—was discovered almost 50 years later.
Pluto was publicly announced on Lowell’s birthday as the planet he had searched for and children across the U.S. learned the acronym “My Very Excellent Mother Just Served Us Nine Pizzas” to remember the order of the nine planets in our solar system.
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