Why is Pluto No Longer a Planet?

Pluto sits within the Kuiper Belt.

Pluto was discovered in the year 1930 by US astronomer Clyde Tombaugh. Although originally classified as a planet, Pluto was reclassified as a dwarf planet in 2006, a controversial decision by the IAU Committee. As such, Pluto is no longer considered to be the 9th planet of our solar system.

Pluto's Small Size

Research has shown that Pluto is an extremely icy planet, colder even than Antarctica. Further, Pluto has less gravity compared to that of Earth. Pluto has five moons: Charon, Hydra, Kerberos, Nix, and Styx. Interestingly, Earth’s moon is larger than Pluto, and the the US is twice as wide as Pluto.

The Start of the Controversy

Over time, astronomers started questioning whether Pluto was really a planet. Some astronomers theorized that Pluto was merely an icy body located beyond Neptune’s orbit in a region that was later called the Kuiper Belt. The first body to be discovered in the Kuiper Belt was discovered by David Jewitt in 1992 using a telescope owned by the University of Hawaii. This discovery only fueled the Pluto debate further.

Further discoveries made in the Kuiper Belt revealed even large objects than Pluto. Eris was announced in 2005, and some astronomers took to calling Eris the tenth planet. Other objects discovered in the belt are Quaoar and Sedna announced 2002 and 2003 respectively. These discoveries led to a setting up of a committee by the International Astronomical Union (IAU). The committee was tasked with coming up with the proper definition of a planet and presenting its findings to the union’s General Assembly in 2006 which was held in Prague.

The IAU Committee Outcome

After heated debates in Prague, a definition was finally agreed upon. On the last day, the committee voted and passed a resolution on the criteria for determining what a planet is. The criteria state that a planet is a celestial body that fulfills three conditions. A Planet must orbit the sun, contain enough gravity for itself such that it overwhelms rigid body forces to assume a nearly round appearance, and have cleared its orbit’s neighborhood.

The last condition means that the celestial body has completely removed other large celestial bodies within its space. For a body to do this, then it must have more gravitational force than the neighboring bodies. However, Pluto is in the Kuiper Belt sharing the space with other objects like Erin. As a result, the IAU immediately declared that Pluto was no longer a planet.

Pluto’s status designation changed to that of a “dwarf planet” alongside Ceres. Ceres is the biggest object in the belt. Other dwarf planets are Eris, Sedna, and Quaoar.

Moving Forward

The resolution passed in 2006 has brought outrage from some astronomers and scholars around the world. The outrage is because of the belief that the criteria has its faults. One argument says that our planet, Earth, and other planets share their space with several objects. As such, these planets, Earth included, do not meet the criteria. The debate continues. For the moment, however, Pluto remains a dwarf planet.


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