Mercury, Venus, Mars, Jupiter and Saturn are visible in the night sky. They were known by the ancient Babylonians and their current names are derived from Roman gods. Uranus was first sighted in 1690 and recognised as a planet almost a century later, the first discovered since ancient times. Neptune was predicted mathematically, based on its gravitational effects, leading to its observation in the 1840s, along with a whole host of other solar system objects.


It wasn’t until 1930 that Pluto was discovered. Using a blink comparator to scan the night sky for small changes in position, it took 23-year old Clyde Tombaugh ten months to discover it, and made headlines around the world. Initial proposals were to name it after Percival Lowell, the universities founder, or his wife. It was an 11-year old English schoolgirl, Venetia Burnley, who proposed Pluto. She had been learning about the Romans and Greeks, and a classical name was deemed appropriate. Like the king of the underworld, Pluto sits alone in a cold, dark and distant realm. 


Pluto was thought to be the mysterious Planet X, responsible for perturbations in Neptunes orbit, and several times the mass of the Earth. However, this was clearly not the case, as 1950s observations by Gerard Kuipier showed that it had a much smaller radius than the Earth. Later it was found that Pluto was highly reflective, and if it was as big as previously thought, should be incredibly bright. Finally, the discovery of Pluto’s moon Charon in the ‘70s determined Pluto’s size once and for all, at a radius of 1200 km, and 0.2% the mass of the Earth; nowhere near as large as originally proposed. 


In the 1990s, lots of objects similar in size and location to Pluto became known. Eventually, its classification as a planet began to become controversial. The discovery of Eris, with a greater mass than Pluto, led to a desire to formally outline the definition of a planet. In 2006, the International Astronomical Union defined a planet as follows:


  1. It orbits the Sun.
  2. It has formed a spherical shape under its own gravity
  3. It has cleared its neighbourhood of bodies of comparable size, due to its own gravitational dominance. 


Unfortunately, Pluto fails to meet the third criterion, making up only a fraction of the total mass of all the objects in its orbit. Thus, it was stripped of its status as a planet. A new designation, “Dwarf Planet” was created, and Pluto, along with Eris and several other large non-planets. Between its confirmation as a planet, and the point at which it ceased to be one, Pluto had only only completed a fraction of its 250-year orbit around the sun. 


In 2015, the New Horizons spacecraft came within 12,500 km of Pluto, sending back stunning images of its surface.