Why is Pluto no longer a planet?

In the time Pluto was found, classified as a planet and reclassified as a dwarf planet, it has not completed one orbit.

One year on the distant ice planet is 248 Earth years, and one day is 153 Earth hours. 

Now known as a dwarf planet, the complex and mysterious world is abundant with mountains, valleys, plains, craters and glaciers. 

And yet, was stripped of its planetary title. Why? 

When was Pluto discovered?

When Pluto was discovered in 1930, it was long considered as the solar system’s ninth planet.

Each of the planets in the solar system, with the exception of Earth, have been named after Roman and Greek Gods and Goddesses.

When Pluto was discovered, it was named after the God of the underworld in Roman mythology.

The dwarf planet was discovered by US astronomer Clyde Tombaugh, who was using the Lowell Observatory in Arizona.

In the decades after its discovery, however, astronomers began to wonder whether Pluto might be the first object to have been spotted as part of a population of small icy bodies beyond the orbit of Neptune.

What’s the controversy behind Pluto’s planetary status?

So, after the discovery of similar worlds deeper in the Kuiper Belt, which is a doughnut-shaped region of icy objects beyond Neptune, Pluto was reclassified as a dwarf planet in 2006. 

A dwarf planet is a celestial body in direct orbit of the Sun that is massive enough that its shape is not controlled by gravitational forces, but has not ‘cleared its path’ (more on that later).

As more and more Kuiper Belt Objects (KBO) were discovered, Pluto’s planetary status was questioned even further.

Astronomers found other KBOs with a similar mass to Pluto, such as Quaoar (discovered in 2002), Sedna (2003) and Eris (2005).

In fact, Eris in particular, appeared to be larger than Pluto – giving rise to its informal designation as the solar system’s ‘tenth planet.’ 

What does the International Astronomical Union class as a planet?

This led to the International Astronomical Union (IAU) to set up a committee tasked with defining just what constituted a planet.

At first, the idea was to up the number of planets in the solar system to 12, seeing Pluto and its moon Charon recognised as a twin planet. Ceres and Eres would also be granted entry to the planetary club, but this was met with opposition.

On 24 August 2006, the committee outlined a resolution for naming a planet. These are:

• It is in orbit around the Sun
• It has sufficient mass for its gravity to overcome rigid body forces so it assumes a hydrostatic equilibrium (nearly round) shape
• It has cleared the neighbourhood around its orbit

Unfortunately, Pluto only meets two of these criteria, and loses out on the third. But what does it mean to ‘clear out its neighbourhood’? 

This basically means that the planet has ‘vacuumed’ up or ejected other large objects in its space. So, the IAU downgraded the planet to a dwarf as it shares its neighbourhood with other KBOs. 

However, this definition of a planet does not seem to please everyone. Earth, which is designated as a planet, shares its cosmic turf with 12,000 near-Earth asteroids, while 100,000 Trojan asteroids lie within Jupiter’s orbit.

So it could be argued that both Jupiter and Earth also fail the IAU definition of a planet.

Does sunlight reach Pluto?

Pluto is around 39 times farther away from the Sun than Earth. From this distance, it takes sunlight 5.5 hours to travel from the Sun to Pluto. But it isn’t completely dark.

If you were to stand on the surface of Pluto at noon, the Sun would be 1/900 the brightness it is here on Earth. There is a moment each day near sunset here on Earth when the light is the same brightness as midday on Pluto.

Since it is so far from the Sun, the temperature on Pluto can be as cold as -226 to -240 degrees Celsius (-375 to -400 degrees Fahrenheit).