Pluto has been the center of astronomical debate for quite a while. Its claim to planethood has forced astronomers to rethink what qualifies as a planet, and would eventually lead to the “demotion” of Pluto to the classification of dwarf planet. But where did all these planetary politics begin? What caused Pluto’s change in status? What other strange categories of celestial satellites are there? The world of space taxonomy is strange and constantly changing. As new discoveries are being made, it may be time to rethink the classifications of our universe.
The Wandering Stars
People have been looking to the stars for millennia. The ancient Greeks were able to identify and differentiate the fixed stars from the planets. The “fixed stars” being the relatively stationary stars seen at night by the simple observer. And “planet” is derived from the ancient Greek word “planētēs”, which translates to “wanderer”, so thus planets were “wandering stars”. But the term planet didn’t have the same meaning it has today until around the 17th century. The known planets, the sun, and the moon were all considered celestial bodies orbiting the Earth. This was the status quo, until the theory of a heliocentric solar system was popularized by Copernicus and Galileo. From then on, a planet was any object orbiting the sun.
In 1781, Uranus became the first planet to be discovered with a telescope. Then in 1801, Ceres was discovered. Then Pallas, Juno, and Vesta, and soon there were hundreds. These planets were eventually rebranded as asteroids in the asteroid belt. Finally, in 1846, Neptune would be next, and the last planet in the solar system. There were eight planets, and it remained that way for nearly a hundred years. That is, until Pluto.
[Most accurate natural color image of Pluto. The article’s cover image is an enhanced rich color variation.]
The Problem with Pluto
Pluto was discovered on February 18, 1930. Originally, it was thought to be much larger than it actually was, and for many decades students would memorize the nine planets. There would be harmony for a while until - much like Ceres - the spotlight would be stolen. In 1992, the first asteroids of the Kuiper Belt were found, yet Pluto kept its planetary title for a little while longer. This all changed in 2005 with the discovery of Eris. Another large object in the Kuiper Belt, only slightly smaller than Pluto.
Finally, in 2006, the International Astronomical Union (IAU) decided to meet and revise the definition of a planet. Now, in order for a celestial body to be considered a planet, it must possess the following three requirements:
a) is in orbit around the Sun,
b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
c) has cleared the neighborhood around its orbit.
Source: IAU Definition of a Planet in the Solar System, Resolution B5, XXVI General Assembly. (2006). http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf
Since Pluto is located in the Kuiper Belt, it doesn’t pass the third requirement, and in the same document, is further resolved as a “dwarf planet”. Another new category was also drafted, these were trans-Neptunian objects, objects in the solar system beyond Neptune. There were also plutoids, which are trans-Neptunian dwarf planets.
Unlike the IAU would’ve liked, the resolution wasn’t met with much enthusiasm and applause. One complaint is that it doesn’t include rogue planets, planets that don’t orbit a star, but may have previously. Another criticism is the exception of a brown dwarf, a star that cannot ignite (I won’t go any further into star classification) with a very similar makeup to the gas giants in our solar system. There are some star systems where brown dwarfs orbit larger stars, which raises the question, should these be considered planets?
At the moment, there are nine dwarf planets, but the exact number in the solar system is yet to be known. There could be hundreds. Interestingly, Ceres is the only dwarf planet outside the Kuiper Belt. This is because Ceres is so much bigger and different from the other planetoids (essentially a fancy way of saying asteroid) in the asteroid belt. But should Pluto even be considered a dwarf planet?
[Pluto and its largest moon Charon]
Pluto is an exceptionally strange dwarf planet. It has an overly elliptical and tilted orbit due to its interaction with Neptune. Pluto also has a moon, Charon. In fact, Pluto has five moons. However, Charon is the largest, and is nearly the size of Pluto. Charon is so large that the barycenter (center of mass) of the two objects is outside Pluto’s main body. This is not like any other planet, and they’re actually tidally locked. The Pluto-Charon system should be classified as a double planet, more specifically, a double dwarf planet.
Not all dwarf planets have moons. The only known dwarf planets with moons are Pluto, Eris, and Haumea. Neither Mercury nor Venus have any moons at all, and Mercury is even smaller than some moons, like Jupiter’s Ganymede, the largest moon in the solar system.
[Photo of Pluto’s icy mountains and ice plains]
Conclusion
“Space taxonomy” isn’t an actual scientific term to categorize space jargon (but it should be). The universe is filled with uniqueness yet to be discovered, and with new discoveries there always comes classification.
In 2015, when the New Horizons space probe flew by Pluto after its nine year journey, the results were beautiful. It reminded people why astronomy has captivated the world for thousands of years. Though you can’t see Pluto from Earth, maybe someday it will be restored back to planethood, because everyone loves a comeback story.
All images in this article are from the NASA Pluto Gallery linked below. Each image was shot by the New Horizons spacecraft in 2015…
https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/galleries/?page=1&per_page=25&order=created_at+desc&search=&href_query_params=category%3Dplanets%2Fdwarf-planets_pluto&button_class=big_more_button&tags=pluto&condition_1=1%3Ais_in_resource_list&category=51
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