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 for the first "resident" to be discovered.
Confirmation of the first KBO invigorated the existing debate. And in , 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 , Sedna and Eris , 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".
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 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. The discussions in Prague during August 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.
Hide Caption. Pluto's Varied Terrain — Pluto taken by NASA's New Horizons spacecraft just before closest approach on July 14, , are the sharpest images to date of Pluto's varied terrain—revealing details down to scales of meters. In this mile kilometer section, the textured surface of the plain surrounds two isolated ice mountains. 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.
Pluto'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. 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 , 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.
The final blow came in 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.
New Horizons relaunches old debate. But the debate about Pluto's status rages on. In , 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.
Liftoff was on time at 2 p. This was the third launch attempt in as many days after scrubs due to weather concerns. The compact, 1,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. A year later, astronomers were no closer to a resolution, and the dilemma hung like a dark cloud over the IAU General Assembly meeting in Prague in At the conference, researchers endured eight days of contentious arguments, with four different proposals being offered.
One controversial suggestion would have brought the total number of planets in the solar system up to 12 , including Ceres, the largest asteroid, and Pluto's moon Charon.
The suggestion was "a complete mess," astronomer Mike Brown of Caltech, Eris' discoverer, told Space. Near the end of the Prague conference, the astronomers who remained voted to create three new categories for objects in the solar system.
From then on, only the worlds Mercury through Neptune would be considered planets. Pluto and its kin — round objects that shared the neighborhood of their orbit with other entities — were henceforth called dwarf planets. All other objects orbiting the sun would be known as small solar system bodies. A contingent of professionals did not take the decision lightly. Such views are shared by many in the public.
Science historian Owen Gingerich, who chaired the IAU's planet-definition committee, asserted that "planet is a culturally defined word that changes over time. Alternative classification schemes continue to pop up. A proposal defined a planet as "a round object in space that's smaller than a star. A year later, Stern, along with planetary scientist David Grinspoon, wrote an opinion article in The Washington Post arguing that the IAU's definition was "hastily drawn" and "flawed" and that astronomers should reconsider their ideas.
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