Why is Pluto not a Planet Anymore

Why is Pluto not a planet anymore has been a question that bother many of those who have learnt Pluto as a planet when they were children. Pluto, discovered in the year 1930, was considered as the smallest planet of the solar system. Since then, it has been considered as a part of our solar system and one of the nine planets that have Sun at their center and keep revolving around it. However, after a detailed study of this planet by International Astronomical Union in 2006, the exalted status of Pluto as a planet of the solar system was downgraded. It is no longer a planet of our solar system as it does not fulfill the main criterion to justify its position. Let us understand the reason Pluto is no longer a planet.

Facts about Pluto

• Pluto is farthest from the sun, being on an average 5.8 billion kilometers away from it. This makes its distance from the Sun 40 times more than the distance between the earth and the Sun. Pluto revolves around the Sun in an oval orbit that makes it closer to the Sun at times. However, even when Pluto is at its closest to the Sun, it remains billions of kilometers away from it.

• Pluto orbit lies in a region called Kuiper Belt. There are thousands of other objects that lie in this belt along with Pluto.

• Pluto has a width of only 2300 kilometers. It is only about half in size of the US. In fact, it is so small that even the moon is bigger than this planet. Pluto has three of its own moons and they are half in size to this planet.

• Pluto takes 248 years to revolve around the Sun once. A day on Pluto is 6.5 times the duration of a day on Earth.

Why is Pluto not a Planet Anymore

Why is Pluto not a planet anymore – Reasons

It was in 2006 that an astronomer identified another object behind Pluto in the Kuiper Belt. He named it Eris. Eris was larger in size than Pluto. The presence of this object that revolved around the Sun just like Pluto gave rise to a debate why Pluto should be termed a planet as this object also behaved like a planet. Many astronomers were of the view that just revolution around the Sun should not be a criterion to declare a celestial body a planet. This led to Pluto being discarded from the list of the planets in the solar system and it was relegated to the status of a dwarf planet.

The definition of planet, according to resolution 5A of the IAU, is as follows.

• A planet is a celestial body that is in orbit around the Sun,

• Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

• Has cleared the neighborhood around its orbit.

Though Pluto revolves around the Sun, it does not have an exclusive orbit as its orbit crosses the path of Neptune’s orbit. Pluto has not been able to clear the neighborhood of its orbit of other objects. With more than 70000 other objects in the Kuiper Belt revolving around the Sun, astronomers have decided that Pluto is not a planet and just another object in this belt. Thus, Pluto, along with Eris, is just a dwarf planet and not a member of our solar system as a planet.


Images Courtesy:

  1. Pluto impression by 京市 (CC BY-SA 3.0)

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