Around 20 years ago, if you were asked in school, “How many planets are there in the Solar System,” the answer was unanimous: “NINE!” A familiar memory of class on the universe throws up this learning that the planets of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, make up the nine planets that orbit the Sun. But here is a fact! There are not nine planets in the Solar System, but EIGHT!
On August 24, 2006, the International Astronomical Union (IAU) downgraded Pluto from its position as the ninth planet from the Sun, to one of five ‘dwarf planets’. The decision was taken at the then General Assembly of the IAU, emerging as a defining moment for Astronomy. The whole debate and subsequent decision emanated from the definition of the term ‘planet’.
A small, mysterious world
The celestial body formerly known as the planet Pluto was discovered on February 18, 1930, at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W Tombaugh. Named by 11-year-old Venetia Burney of Oxford, England, Pluto emerged as a favourite among astronomy enthusiasts and stargazers, owing to the awe permeated from its sheer distance from the Sun (5.9 billion km) and its substantially small size compared to its larger neighbours in the Solar System.
With a diameter of just 2,376.6 km, Pluto is even smaller than our Moon, with its 3,474.8-km diameter. Pluto’s landscape is characterised by mountains, valleys, plains, and craters, with temperatures ranging from -226°C to -240°C. This inhospitable climate is attributed to Pluto’s thin atmosphere, and its icy surface composed of frozen methane and nitrogen.
Pluto’s unusual orbit around the Sun is the longest compared to any planet. Its 248-year-long, oval-shaped orbit can take it as far as 49.3 AU from the Sun, and as close as 30 AU. Pluto is currently in the middle of a revolution, which will be completed on March 23, 2178.
Planet no more
The term ‘planet’ once loosely described any large object within the Solar System, orbiting the Sun. Around 2006, planets came to be defined as “celestial objects large enough to be made rounded by their gravitational orbit around the Sun and to have shooed away neighbouring planetary objects and debris”.
Accordingly, the IAU classified Pluto as a “dwarf planet” because, while it is large enough to have become spherical (hydrostatic equilibrium), it is not big enough to exert its orbital dominance and clear the neighbourhood surrounding its orbit. Throughout its existence, it has not managed to clear its neighbourhood. This means that a planet must become gravitationally dominant, like the Earth is in relation to the Moon. Hence, any large body that does not meet these criteria is now classed as a dwarf planet, and that includes Pluto, which shares its orbital neighbourhood with the Kuiper Belt.
This means that now only the rocky worlds of the inner Solar System and the gas giants of the outer system are planets. And like this, Pluto lost its planetary position in the Solar System, but forever remains the favourite ‘smallest, ninth planet’ for many.