Pluto and its moons. (Image property of NASA)
Pluto was considered for a long time the ninth planet of our Solar System. However, that changed on 2006, when the International Astronomical Union (IAU) decided to reclassify Pluto in the new category named “dwarf planets” changing the landscape of or Solar System as we knew it. But, what were the reasons that made astronomers reconsider our Solar System distribution? And why was just Pluto affected?
In January 1930, after spending almost one year observing and analyzing the region beyond Neptune, Clyde Tombaugh, an observing assistant working at the Lowell Observatory, discovered the Solar System’s ninth planet. The new member was named Pluto in honor to the Roman god of the underworld. Later studies shown that Pluto was the smaller planet in the Solar System (with a mass of 0.0125x1024 kg, Pluto is even smaller than the Moon that has a mass of 0.07349x1024 kg). Based on this fact, for years many scientists didn’t consider Pluto a planet.
In the late 80’s, the astronomers David Jewitt and Jane Luu started observing the outer region of the Solar System beyond Neptune and Pluto’s orbits. For many years astronomers believed in existence of a region similar to the asteroid belt in the border of the Solar System, that was known as Kuiper Belt or Edgeworth-Kuiper belt. On August 30, 1992 Jewitt and Luu announced the discovery of the first Kuiper Belt Object (also known as transneptunian objects or TNO). Since then, more than 1,000 Kuiper Belt Objects (KBO) have been discovered. The Kuiper Belt was defined as a doughnut-shaped region that extends from around 30AU1 to 55 AU. And, Because this region covers Pluto’s orbit (Pluto’s aphelion and perihelion are 48.871 AU and 29.657 AU), astronomers started questioning whether Pluto could be really considered a Planet.
In October 2003, Mike Brown, Chad Trujillo, and David Robinowitz using the telescope and Camera at the Palomar Observatory discovered a new object in the Kuiper Belt named temporarily 2003UB313 (later called Eris). Subsequent studies and observations led to the conclusion that this object was more massive than Pluto, and many thought that this was the discovery of the tenth planet. This fact was the trigger of many questions; could this new object be considered a planet like Pluto? Or like this object should Pluto be considered a Kuiper Belt Object instead of a Planet?
The IAU responsible for the naming and nomenclature of planetary bodies and satellites decided to create a committee to define the term “planet” to avoiding this kind of dilemma . The committee was conformed by astronomers, planetary scientists, historians, science publishers, writers and educators. It was called the Planet definition committee and on August 24, 2006 on the closed ceremony of their annual General Assembly the final definition was approved. According to IAU a planet is:
(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 neighbourhood around its orbit.Due to Pluto doesn’t meet the third condition that states that a planet has to have a clear neighborhood which means don’t share the orbit with other objects, it was taken out from the Solar System planets, and added in the new “dwarf planets” category. According to the IAU a dwarf planet is:
(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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.So, Pluto, Eris, and Ceres became the first dwarf planets. In 2008, two more were added to the list of dwarf planets: MakeMake and Haumea.
Today, there are still members of the scientific community who disagrees with the IAU resolution and think that Pluto deserves its position as planet. But, for now we just have no more than accept it. Maybe the New Horizon Spacecraft will give us more information about Pluto and tell us whether Pluto should recover its position as Planet (and Eris the tenth Planet) or or it'll confirm the IAU decision.
Sources:
1 AU = Astronomical Units.The mean distance from the Sun to the Earth = 149,597,900 km.
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