Is Pluto A Planet?

Over the past decade or so there has been an ongoing debate on whether Pluto is a planet or not. If you take books from 20 years ago, they would say Pluto is a planet. The latest books, however, say Pluto is not a planet. Why is Pluto not a planet anymore? If it isn't a planet then what is it? These are some of the many questions you probably have. Don't worry, All the answers are below.


In the early 20th century the existence of a ninth planet was suggested by Percival Lowell. He theorized that the disturbances in the orbits of Neptune and Uranus were caused by the gravity of an unknown planet. He, however, couldn't find it. Later, in 1929, using the calculations of Lowell as a guide, the search for the unknown planet was resumed by Clyde Tombaugh at the Lowell Observatory, Arizona. On February 18, 1930, Tombaugh discovered the small, faraway planet.

Pluto is yet to complete a full orbit of the sun since its discovery as one Plutonian year is around 250 Earth years.

What is a planet?

Before coming to Pluto we must first know what exactly a planet is. Over time the definition has kept evolving. The ancient Greeks considered the Earth's Moon and Sun as planets along with Mercury, Venus, Mars, Jupiter, and Saturn. They thought the earth was at the center and all other celestial bodies orbited it. It was only later with the aid of the telescopes that the astronomers realized that the sun was at the center around which all planets, including earth, orbited around. They also realized that the moon wasn't a planet but a satellite of the earth.

The planet debate

Fast forward to 2005, a group of astronomers claimed that they had found a tenth planet, similar in size to Pluto. It was around then that people began to wonder what the term 'planet' actually meant. Suddenly the answer to that question didn't seem so obvious. In 2006, the International Astronomical Union (IAU), a worldwide organization of astronomers passed a resolution that defined a planet.

IAU definition of a planet

A "planet" is a celestial body that: (i) is in orbit around the Sun (ii) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape (iii) has cleared the neighborhood around its orbit

Explanation of IAU definition

A "planet" is a celestial body that:

(i) is moving around the sun in a curved path

(ii) is massive enough so that its gravity makes it almost round

(iii) has no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence in the vicinity of its orbit.

Pluto: A planet or not

As we now know what a planet is, lets come to the all-important question. Is pluto a planet? let us answer this by checking if Pluto satisfies the three conditions to be a planet.

Pluto does move around the sun in a curved path. hence, the first condition is satisfied.

Pluto is also big enough so that its gravity makes it round. So, the second condition is also satisfied.

However, the problem lies with the third condition. Pluto lies in a region called the Kuiper belt. This region lies beyond the orbit of Neptune and consists mainly of small bodies or remnants from when the Solar System formed. There are even bodies similar in size to that of Pluto in this region. Hence Pluto hasn't cleared its neighborhood.

As the third condition wasn't satisfied by Pluto its planetary status was taken away in 2006. So currently it isn't a planet.

What is Pluto?

If Pluto isn't a planet, then what is it? Well, when the IAU demoted Pluto they thought about this. It was in the same year that they introduced a new category of celestial bodies called dwarf planets. Dwarf planets had to satisfy the first two conditions as a normal planet did, but the third condition was different. They didn't have to clear their neighborhood and also they shouldn't be a satellite. Pluto satisfies all three criteria for a dwarf planet. Hence, in 2006 the IAU officially stated that pluto is a dwarf planet. Even today Pluto is officially not called a planet but as a dwarf planet.


Pluto to date has remained a controversial topic. Although it is officially considered as a dwarf planet there are many critics. However, the new conditions for a planet have made it much more difficult to discover one. But if someone truly does discover something that satisfies all the conditions it will be one of the biggest discoveries in the history of mankind.

Hopefully, somebody does find it and maybe even discovers life on it, Who knows. Till then we have just got to wait and study whatever planets we have already discovered as we don't even know everything about our own planet yet.

Interesting Stuff For Interesting People.


© 2023 by Train of Thoughts.