Leading astronomers around the world are debating the criteria for classifying a celestial body as a planet, comparing Pluto with a newly discovered asteroid, Sedna.
Some astronomers suggest that biases related to the positions of celestial bodies within the Solar System should be disregarded.
The International Astronomical Union (IAU), responsible for establishing these standards, has convened special working groups to consider whether Sedna should be classified as a planet in the Solar System and whether Pluto should still be considered a planet.
The asteroid Sedna was discovered in July 2005 within the Kuiper Belt, beyond Neptune. The discoverers labeled it as the tenth planet, citing its larger size compared to the ninth planet, Pluto.
| The Solar System is traditionally considered to have nine planets, ordered from the innermost to the outermost as follows: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
Among these, Jupiter is the largest, with a mass 2.5 times that of all the other planets combined (318 times the mass of Earth) and a diameter of 142,800 km. Pluto is the smallest and farthest planet, with a diameter of 4,000 km and located 6 billion km from the Sun. Between Mars and Jupiter lies a belt of asteroids, which is the source of wandering comets in the Solar System. Beyond Neptune, in the region of Pluto, is the Kuiper Belt, which contains asteroids with flat orbits located at the edge of the Solar System. |
The debate has intensified as groups have failed to reach a consensus. Two opposing views have emerged, based on entirely different criteria.
The proposal from the Southwest Research Institute in Boulder, USA, is straightforward and relies solely on the size of the object. This means that Sedna would be classified as a planet, and the number of planets in the Solar System could increase if similar-sized objects to Pluto are found in the Kuiper Belt.
However, the second proposal, from the IAU’s Minor Planet Center in Cambridge, argues that setting a size threshold to determine whether something is a planet is entirely arbitrary.
“There is no scientific basis for asserting there are nine planets, including Pluto. There are many small celestial bodies similar to it; can they also be classified as planets?” questioned group leader Brian Marsden.
Thus, according to the second proposal, the number of planets in the Solar System would decrease, implying that Pluto would not qualify as a planet either. From this perspective, a celestial body is classified as a planet if it is a standout individual compared to neighboring bodies, both in size and orbital dynamics.
Other smaller bodies in the region beyond Mars are not classified as planets due to the abundance of similar objects in that limited space. Moreover, Pluto is not considered a planet because it is near the larger planet Neptune.
Another, more distant proposal was presented by a group from London, which deviated significantly from the original topic. This perspective categorizes planets based on their characteristics such as location, composition, or the level of civilization present on those planets. According to this classification, Earth would be termed a “living terrestrial planet,” while Pluto would be labeled a “historic planet.”
Brian Marsden appreciates this third idea as it allows for explaining Pluto to the public as something unique. However, Alan Stern, a member of the Southwest Research Institute, vehemently opposes this notion, arguing that it strays from the core issue. “People will wonder why a PhD cannot define when a celestial body is a planet while ordinary individuals can.”
Groups are expected to vote on these proposals in the near future, but the debate may continue indefinitely…
