This is Why Pluto is no Longer a Planet

Updated on April 8, 2018
Jana Louise Smit profile image

Jana is an amateur everything when it comes to space, nature and science. She loves exploring mysteries, both classic and new.

A close look at Pluto's surface.
A close look at Pluto's surface. | Source

Pluto's Arrival in Astronomy

Astronomers saw Pluto's physical powers before they saw the planet itself. In 1905, Percival Lowell studied Neptune and Uranus and got wind of something strange. Something was bothering their orbits. The way they behaved suggested a third world's gravity was responsible. Despite attempts to find the mystery planet and even calculating its position, Lowell never found it. After his death, in 1930, several astronomers studied the night sky at the aptly named Lowell Observatory. Among them was Clyde Tombaugh who discovered the elusive body while stuyding photographs.

Any planet worth its salt – especially the celebrated ninth addition to the solar system – deserved a name. In this case, a little girl pinned Pluto's name to its chest. Venetia Burney, a mythical minded 11-year-old said 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.

That Pesky Eris

The discovery of Pluto rewrote many books. Many believed that the solar system was now a better understood place, but some scientists began to wonder if the planet was really a planet. Their calculations predicted the Kuiper Belt; large icy bodies clustered in a neighborhood somewhere past Neptune. These Doubting Thomases noticed that Pluto was a comfortable fit (if not for its size), to belong to this cold crowd. Then in 1992, the first Kuiper object was discovered. After the existence of the cluster was proven, the region became more intensely studied. Almost a decade later, two Pluto-sized objects turned up in the Kuiper cloud. But the last straw for scientists 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. | Source

The Demotion

Pluto was not immediately stripped of its place as the ninth planet. The Kuiper objects excited some researchers enough to suggest that the number of planets should actually be twelve. The idea was that one of Pluto's five moons, quite a hefty rock called Charon, would be acknowledged as its twin planet. Also lined up was the trouble-maker 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 our system caused the unusual thumbs up. However, the nomination of twelve planets met with a fierce resistance and some even called it a step backwards for astronomy. As a result, the movement took a new direction – the criteria for what constituted a planet was agreed upon. There were only three but Pluto failed perhaps the most important one.

The first two “laws” state that a true planet orbits the Sun and should be round. The one that got Pluto kicked out of the club was gravitational dominance. In other words, it must have no other bodies in the way of its orbit. Obviously, 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, 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. | Source

A Possible Promotion

Things are far from settled. As soon as Pluto was sent packing, other scientists felt the classification system was too rigid for the complexities of space. They rightly felt that more data was needed to truly understand what it means to be a planet. Their main concern was the requirement that Pluto failed. Earth itself, which is decidedly a planet, shares its orbit with a host of asteroids. Not five or seven, but 12,000 asteroids are considered as “near-Earth” bodies. Yet, Earth is not considered a dwarf world.

This solar double standard is fueling support to return Pluto to the realm of planets. If pro-Pluto researchers have their way (and their numbers include NASA scientists), then the history books will have to be rewritten again. However, the pages won't just describe Pluto's ability to boomerang. The new definition of a planet proposes to look at physical properties and not position. Planetary scientists argue that this is more logical than the conflicting third definition that keeps Earth a planet but not Pluto. It also recognizes 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 known solar system - one searching to reclassify several other bodies. If location and clear orbits no longer matter, it is estimated that hundreds of objects would morph into planets. Among the most amazing of them – Earth's very own Moon.

An Uncertain Future

The standoff between the two groups is a bitter one and is likely to remain so for years. 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 thorny 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, planet status could be returned to Pluto or it could be stubbornly kept as the biggest dwarf planet in the solar system. It remains to be seen.

Fascinating Facts About Pluto

  • Pluto is smaller than Earth's Moon, about two-thirds its size.
  • If you were to 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 and it's 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 (friction from a larger planet or inner radioactive heat) 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 kilometers long.



Questions & Answers

    © 2018 Jana Louise Smit


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      • Jana Louise Smit profile imageAUTHOR

        Jana Louise Smit 

        2 years ago from South Africa

        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 Fish profile image

        Larry W Fish 

        2 years ago from Raleigh

        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.


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