BIG QUESTION: Why is pluto not a planet?

Like continents planets are defined more by how we think of them than by some ones after the fact pronouncement.

Michael Brown

Maybe this world is another planets hell.

Aldous Huxley

How many planets are there in the solar system?  A few years ago the answer was universally accepted to be nine now, however, the issue is more contentious. The word planet itself derives from the Greek word for wanderer, a word that describes there migration across the seemingly static background of the stars. 

We have documentary evidence of the major planets going back many thousands of years. Mercury, for example,  was first recorded by Asian astronomers as early as 1480bc. In the 17^th century we invented the telescope and literally opened the cosmos up. Now we were able to see the majesty of the solar system in its entirety: from  the raging red storm on Jupiter, the splendid rings around Saturn and the coterie of moons that orbit around Jupiter not to mention the canal like channels on mars.

Uranus was discovered in 1781 by William Herschel originally believing it to be a rouge star however thanks to his diligent tracking of the object he was able to prove that it orbited the sun thereby securing its position as a planet.  Herschel originally wanted to call his discovery Georgium Sidrum literally translated as George’s star in honour of his patron king George III who had appointed him astronomer royal.  The name was not popular outside of Britain however and ultimately the name Uranus was put forward by Johann Kale

There are slight imperfections in Uranus orbit that led to predictions that it was disturbed by another celestial body that lay beyond it.  Astronomers scoured the expected location looking for the wandering interloper and in 1840 Neptune was discovered by Frenchman Urbain Jean Joseph Le Verrier narrowly beating British astronomer John Coach Adams to establish the find. Finally in 1930 Pluto was confirmed. The American astronomer Clyde Tombaugh discovered it however he did not name it. A contest was ran and British schoolgirl Venetia Burney won with her classics inspired suggestion of Pluto after the god of the underworld.

Pluto Dethroned

The solar system stood at nine planets for 75 years until Michael Brown and his team discovered that Pluto was not alone in the chill depths of the solar system having found a handful of sizeable objects not far from Pluto’s orbit at the cold edge of the solar system. They even found one larger than Pluto itself, they called it Eris after the goddess of strife in Greek mythology. The astronomical community had a conundrum. Should Eris be recognised as the tenth planet? The outer reaches of the solar system are littered with ice smothered objects of which Pluto and Eris are simply the largest, moreover, rocky asteroids of similar size were known elsewhere such as Ceres (named for the roman goddess of agriculture) a 950km  diameter asteroid that was found in 1801 by Giuseppe Piazzi between mars and Jupiter while searching for Neptune

In 2005 a committee of the international astronomical union (the IAU), the professional body of astronomer, met to decide Pluto’s fate.  Brown wanted to protect the status of Pluto as culturally defined also that Eris should be considered a planet as well.  Others considered that all the bodies beyond Neptune are not ‘real’ planets. In 2006 a vote was held and a new definition of a planet was formulated until this point there was no ‘hard and fast’ scientific definition.  A planet is defined as a celestial body that orbits the sun, is massive enough that its own gravity makes it round and has cleared the region around its orbit. According to these rules Pluto is not a planet  as it hasn’t cleared other bodies from its orbit, so Pluto Eris and Ceres were named dwarf planets smaller bodies apart from moons remained unspecified.

A planet is defined as a celestial body that orbits the sun, is massive enough that its own gravity makes it round and has cleared the region around its orbit

 Beyond our sun.

The definition was made for our own solar system but works just as well for systems beyond it. There are serval hundred planets known to orbit alien suns, identified by their subtle pull they impart on their host stars.  Mostly the planets discovered are massive gas giants like Jupiter. Smaller earth like planets are being detected by spacecraft such as Kepler launched in 2009.  Similarly the definition of a star has been called into question lately, stars are balls of gas that are big enough to have ignited nuclear fusion in cores thus making them shine. It’s not obvious however where the division between Jupiter like planets and the smallest dimmest stars like brown dwarves.

350BC	Aristotle determines that the earth is round
1543	Copernicus publishes his heliocentric theory
1610	Galileo discovers Jupiter’s moons through a telescope
1781	Uranus discovered by Herschel
1843-6	Neptune predicted and found by Le Verrier
1930	Clyde Tombaugh discovers Pluto
1962	First mariner 2 images of Venus
1992	First extrasolar planet discovered
2005	Brown discovers Eris