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How did Pluto lose its status as a planet?


When I was in elementary school, I learned about the nine planets of our solar system by memorizing the mnemonic “My Very Excited Mother Just Served Us Nine Pizzas.” Besides just making me hungry, this mnemonic helped me remember the names and order of the planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Pluto. Since 2006, though, that sentence has changed into something much less desirable – “My Very Evil Mother Just Served Us Nothing” – after Pluto was declassified as a planet and instead, categorized as a dwarf planet.


Not that Pluto; it’s still sad though.


Pluto was first discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona (Choi).  Since then, technological advances in astronomy have allowed us to observe space much farther out, far beyond Pluto. As a result, new objects have been discovered that either rival or exceed Pluto in size, forcing astronomers reconsider their definition of the word “planet.”

Pluto was as far as we could observe back in 1930, but continuous advancements in space observation are showing us how much there is that we still have yet to discover and explore.


So what is it about Pluto that made it such an anomaly that it got demoted to a dwarf planet? First off, being the furthest planet from the sun, it was set up perfectly to get cut off. The dwarf planet is almost 6 trillion kilometers away from the sun and 1.5 trillion kilometers away from its closest planetary neighbor, Neptune. Furthermore, the eight planets of our solar system are all aligned roughly along the same plane in their orbit around the Sun, whereas Pluto’s path of orbit is inclined 17° off this plane (Masetti).


Because of its sheer distance and wide, skewed orbit, Pluto can actually come closer to the sun than Neptune and then travel more than 49 times farther from the Sun than Earth. It takes 248 Earth years for Pluto to orbit the sun a  single time, and so between the discovery of Pluto and its recent demotion from planet-hood, Pluto hadn’t even completed a third of one orbit!pluto3

Besides its distance and lopsidedness, another factor going against Pluto was Charon, the first discovered moon of Pluto (discovered in 1978). Charon is extremely close to Pluto, only 19,640 km away, less than the distance between London and Sydney, and Charon is nearly half the size of Pluto. To astronomers, it seemed strange to dub Charon as a “moon” when it was so similar to Pluto.  As a result, Pluto and Charon are sometimes referred to as a double dwarf planet system (Choi).

Most importantly, though, was the discovery of various other dwarf planets. Dwarf planets are now known as the awkward middle children of celestial bodies – they are more developed than asteroids, but weaker than the known planets. According to the International Astronomical Union, the requirements for dwarf planets are:

  • orbits the sun
  • has enough mass to assume a nearly round shape
  • has not cleared the neighborhood around its orbit
  • is not a moon


The third point is the one that marks the clearest distinction between dwarf planets and full planets. Since dwarf planets are much smaller, their gravitational forces are much lower (Pluto’s is only 6% that of Earth’s). These weaker forces are insufficient to sweep up or scatter objects near their orbits, such as other asteroids. They tend to orbit around the sun in uncleared zones of similar objects, such as the asteroid belt between Mars and Jupiter and the Kuiper belt, the region beyond Neptune where Pluto is found.

Besides Pluto, there are four other recognized dwarf planets: Ceres, which is in the asteroid belt, Makemake, Haumea, and Eris, the latter three of which are in the Kuiper belt. The turning point for Pluto was the discovery of Eris, discovered in 2005, the one dwarf planet discovered so far that is larger than Pluto (“List of Dwarf Planets”).


When Pluto was first designated a dwarf planet, NASA scientists embarked on a mission to send a spacecraft to the Kuiper belt – the first probe to study Pluto, its moons, and other bodies in this region. New Horizons was launched in January 2006 and is set to fly by Pluto in July 2015. Considering how much we already know about Pluto, imagine what we can learn from these first-hand observations!

 Written by Constance Kaita

Works Referenced 

Images courtesy of, International Astronomical Union, and

Choi, Charles Q. “Pluto, the Ninth Planet That Was a Dwarf.” 11 July 2012. Web. 18 July 2013.

“List of Dwarf Planets.” n.d. Web. 18 July 2013.

Masetti, Maggie. “The Solar System.” 19 November 2010. Web. 18 July 2013. <>

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