Today, we all know that our solar system consists of the sun and the eight planets revolving around it. But this wasn’t always the case. In the late 90s, our solar system had nine planets. Then what happened to the ninth planet? Was it destroyed or did it just vanish into space?


Pluto the 9th Planet
Pluto the 9th Planet

The answer is none of these. The planet was discarded from the planet list as it didn’t meet the qualifications to be a planet. So the planet wasn’t destroyed (well at least not physically). Let’s take a look at the story of our beloved and once upon a time – the ninth, smallest, and outermost planet, Pluto.

The discovery of Pluto

Pluto was first discovered in 1930 by an American astronomer Clyde Tombaugh. He took photos of the night sky every two weeks and when he compared those photographs he found a tiny dot moving back and forth. After the verification of the photographic plates, that dot was confirmed to be a new member of our solar system and was deemed to be a planet.

Pluto has an eccentric and inclined orbit that takes it between 4.4 to 7.3 billion km from the Sun. This actually means that at some points it is actually closer to the sun than Neptune. When Pluto comes closer to the sun, the surface ice thaws and results in the formation of a thin atmosphere of nitrogen, methane, and carbon monoxide.

The newly found planet made headlines all over the world and was named “Pluto” after the Roman God of the underworld, the Roman alternative for the Greek God Hades. Pluto is slightly smaller than our moon. In 2015 when the New Horizons spacecraft visited it, it revealed a variety of surface features including the Tombaugh Regio (Image credits: NASA)- a smooth region of Pluto that looks like a giant heart. So although Pluto does bear the name of the god of the underworld, it is, frankly, adorable. 

What caused Pluto’s identity crisis?

Pluto was considered as a planet and no one had anything to say about it until the discovery of Eris in 2005, which was somewhat larger than Pluto. As there was no cut-off line for classifying a body as a planet, people started arguing whether to consider Eris a planet along with other icy bodies in the Kuiper belt just like we consider Pluto to be.

While some argued that Eris should be named the tenth solar system planet, others saw it as the strongest case for Pluto’s reclassification. Thus, after an intense debate over the definition of a planet, on Aug 24, 2006, researchers at the International Astronomical Union (IAU) voted to create three new categories for objects in the solar system and updated the definition of the planet.

What exactly classifies as a “planet”?

The English term planet derives from the Greek word “planets“, which means “wandering star“. The planets- Mercury, Venus, Mars, Jupiter, and Saturn are visible with the unaided eye and can be seen shifting in strange pathways across the sky compared to the more distant background stars; thus the name wandering stars.

8 Planets of Solar System
8 Planets of Solar System

As per the IAU, the formal definition of a planet 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, and has cleared all the neighborhood around its orbit.  

Pluto, unfortunately, could never fulfill the third criterion. Instead, it lives in the Kuiper belt just beyond Neptune, surrounded by objects of a similar size, and hence joins a new category along with Ceres, Eris, Haumea, and Makemake: the Dwarf Planets.

For many people around the world, this demotion of Pluto caused much sadness. But if we’re going to personify Pluto, as a planet Pluto was the smallest, coldest, slowest, and most irregular one of them all. However, as a dwarf planet, not only is it a member of its own new group, on equal footing with the rest, but also it was the first member of that group to be discovered. So our adored Pluto is no longer the 9th, but the 1st!

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