Pluto was discovered in 1930 by US astronomer Clyde Tombaugh, who was using the Lowell Observatory in Arizona. 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. For decades it was notable for being our solar system's smallest and farthest planet. Pluto is only about half the width of the United States and lies in a far-out region of the solar system called the Kuiper Belt. We need a telescope to see Pluto.
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. 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.
In August 2006 the International Astronomical Union (IAU) downgraded the status of Pluto to that of the dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet. Pluto meets all the criteria except one i.e. Pluto has not cleared its neighboring region of other objects.
This means that from now on only the rocky worlds of the inner Solar System and the gas giants of the outer system will be designated as planets. The inner Solar System is the region of space that is smaller than the radius of Jupiter’s orbit around the sun. It contains the asteroid belt as well as the terrestrial planets, Mercury, Venus, Earth, and Mars. The gas giants of course are Jupiter, Saturn, Neptune, and Uranus. So, now we have eight planets instead of nine.
The three criteria of the IAU for a full-sized planet are
It is in orbit around the Sun.
It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
It has cleared the neighborhood around its orbit.
A planet has become gravitationally dominant and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence, in its vicinity in space. So any large body that does not meet these criteria is now classed as a dwarf planet, and that includes Pluto, which shares its orbital neighborhood with Kuiper belt objects such as the plutinos.
The IAU reclassified it as a dwarf planet and it is also calling a Trans-Neptunian Object, which prompted outrage from schoolchildren, small planet enthusiasts, and the internet in general.
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.
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.
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 the 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 appeared to be larger than Pluto and giving rise to its informal designation as the Solar System's tenth planet.
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.
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 recognized as a twin planet, and Ceres and Eris granted entry to the exclusive club. But the idea met with opposition.
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.
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.
The IAU's president of planetary systems science Prof Iwan Williams said at that time
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.
Some experts immediately questioned the part of the definition about a planet clearing its orbital neighborhood. 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 IAU's 2006 definition.
Prof Alan Stern, the 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 and stated
I don't have opinions that I require other people to have.
August 24th is celebrated as pluto demoted day. Pluto’s demotion felt like a break from tradition to many of Earth’s people, and it was exactly that a progressive move forward towards a new light, new awareness, and shifting universe perspectives.