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This Is why Pluto Is no Longer a Planet

A close look at Pluto's surface

A close look at Pluto's surface

Pluto's Arrival in Astronomy

In 1905, an astronomer named Percival Lowell studied Neptune and Uranus. He noticed that their behaviour was not normal. The only thing that could explain their orbits was the influence of an unknown planet. Despite his best attempts, Lowell never found it.

After his death in 1930, another astronomer was employed at the aptly named Lowell Observatory. Clyde Tombaugh was combing through photographs of the night sky when he found the elusive world.

The celebrated ninth addition to the solar system needed a name. A little girl did the honours. Venetia Burney, who was a mythical-minded 11-year-old said that the new discovery should be called after the Roman god of the underworld. It suited the secretive nature of the new world, hidden in the dark for so long. The first two letters also matched Lowell's initials.

Hello, Pluto.

That Pesky Eris

The discovery of Pluto rewrote many books. It also satisfied scholars who felt that they now understood the solar system better. But other scientists began to wonder. Was this planet really a planet?

What cast doubt on Pluto was something called the Kuiper Belt. Yet unproven to exist, astronomers predicted that past Neptune the Kuiper Belt was a zone filled with frozen, rocky bodies. Pluto's position placed it among this hypothetical crowd. But even those who doubted its planet status thought that Pluto was too big for a Kuiper rock.

In 1992, the first Kuiper object was discovered and confirmed the existence of the Belt. After a decade of intense study, two Pluto-sized objects turned up in the Kuiper cloud. The last straw came when a Kuiper object, which was not a planet, was discovered in 2005. Called Eris, it was bigger than Pluto. This meant that size was no longer a factor that safeguarded Pluto's status as a planet.

The Current Arrangement

The true planets and dwarf planets

The true planets and dwarf planets

The Demotion

The discovery of the massive Kuiper objects excited some researchers enough to suggest that the number of planets should be twelve. The idea was to keep Pluto as a planet and classify its moon, a hefty rock called Charon, as its twin planet. Also lined up was the troublemaker Eris and for some reason, Ceres. The latter is an acknowledged asteroid. Perhaps its spherical shape and the fact that it remains the largest asteroid in the solar system caused the unusual thumbs-up.

The twelve planets met with fierce resistance. Some even called it a step backwards for astronomy. Both sides agreed that the best thing to do would be to create a planet "checklist". After much discussion, everyone agreed on three laws.

Pluto failed perhaps the most important one.

The first two laws state that a true planet orbits the Sun and has a spherical shape. The one that kicked Pluto to the curb was gravitational dominance. In other words, it must have no other bodies in the way of its orbit. Unfortunately, Pluto shares its space with the Kuiper crowd.

The final decision was made in 2006 and after almost eighty years as the most distant planet in the solar system, Pluto was downsized to a dwarf planet. The asteroid Ceres and the Kuiper object that killed Pluto, Eris, were both given dwarf planet status as well.


The four largest asteroids in the solar system

The four largest asteroids in the solar system

A Possible Promotion

As soon as Pluto was demoted, some scientists said that the classification system was too rigid for the complexities of space. They felt that more data was needed to truly understand what it means to be a planet. Their main concern was the requirement that Pluto had failed. Earth shares its orbit with 12,000 asteroids that are so close that they are called “near-Earth” bodies. Yet, Earth is not classified as a dwarf planet.

This double standard is fueling support to make Pluto a planet again. If pro-Pluto researchers get their way, the definition of a planet could change again. Planetary scientists feel that physical properties are more important than the object's position in space. They also recognize that some objects, no matter their breed, size or location, can never clear their orbits of other objects. Earth will never be rid of her asteroid swarm, for example.

A decision to reinstate Pluto will have a ripple effect throughout the solar system. If location and clear orbits no longer matter, hundreds of objects could morph into planets. Among the most amazing of them – Earth's own Moon.

An Uncertain Future

The standoff between the two groups remains a bitter one. The 2006 decision was taken by a single scientific organization, the International Astronomical Union. The choice was not given to all experts in the field, most notably planetary scientists, who are mostly pro-Pluto. The latter correctly pointed out that the IAU's third definition not only kills Earth as a planet but also Mars, Jupiter and Neptune. All three are regularly buzzed by asteroids. In the end, Pluto might become a planet again or it might stay the biggest dwarf planet in the solar system. Watch this space.

Fascinating Facts About Pluto

  • Pluto is smaller than Earth's Moon.
  • If you make a snowman on Pluto, the jolly guy would be made of red snow.
  • There are impressive mountains, valleys, glaciers and plains. There is a vast plain called Sputnik Planum made entirely of frozen nitrogen.
  • One day on Pluto equals six days on Earth.
  • One year on Pluto takes several human generations – 248 Earth years.
  • A mysterious heat source exists within the planet but nobody knows where it comes from. To generate heat requires some kind of geological activity but all known processes are absent.
  • Pluto has a tail, just like a comet. The world ejects about 500 tonnes of nitrogen every hour, forming a tail 109,000 kilometres long.


This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2018 Jana Louise Smit


Jana Louise Smit (author) from South Africa on April 08, 2018:

I agree, Larry. The things they do in space these days are simply amazing and that goes double for all the new and strange discoveries.

Larry W Fish from Raleigh on April 08, 2018:

A very interesting article, Jana. I love reading about the mysteries of space. I remember being a little boy back in the 1950s, lying on the grass in our yard and looking up at the moon. I never imagined that a little over 10 years later men would land on the moon. Now with the International Space Station and the Hubble Telescope many mysteries of space are becoming more fact than fiction.