It was almost 12 years ago when Pluto was demoted from its planet status, leaving our solar system with only eight planets. However, there are still some scientists who believe that Pluto should be classified as an actual planet, and not as a dwarf planet, due to factors such as its geographical features.
In an op-ed penned for the Washington Post, Planetary Science Institute astrobiologist David Grinspoon and NASA New Horizons principal investigator Alan Stern justified their stand on whether Pluto is a dwarf planet or the ninth planet in our solar system, first by comparing it to Ultima Thule, the provisional name given to an object New Horizons is expected to explore around New Year’s Eve. While the scientists are confident that Ultima Thule is not a planet, they maintained that Pluto can be considered one, as it has the geographical qualities that scientists normally associate with planets.
“When we see one like Pluto, with its many familiar features — mountains of ice, glaciers of nitrogen, a blue sky with layers of smog — we and our colleagues quite naturally find ourselves using the word ‘planet’ to describe it and compare it to other planets that we know and love,” wrote Grinspoon and Stern.
As explained by BBC News in a 2015 article, Pluto was first discovered by American astronomer Clyde Tombaugh in 1930 and quickly classified as the ninth planet in our solar system. In the decades that followed, astronomers debated on its planetary status, particularly when the Kuiper Belt — a region beyond Neptune’s orbit with various “small, icy bodies” — was discovered. The years of debate came to a head in 2006 when the International Astronomical Union voted to demote Pluto as a planet, as it failed the last of three requirements decided on by the IAU — planets need to orbit around the sun, have enough mass to maintain a “nearly round” shape, and to “clear the neighborhood,” or remove other large objects, around its orbit.
Grinspoon and Stern, whose new book, Chasing New Horizons: Inside the Epic First Mission to Pluto, was published on May 1, disagreed with this decision in their Washington Post op-ed, calling it a “flawed” move, adding that some of the largest moons in our solar system are oftentimes referred to as planets due to the presence of geographical features like lakes, rivers, mountains, and clouds. But with that said, the two authors and scientists agreed with the IAU’s second of three planetary requirements, emphasizing that Ultima Thule cannot be called a planet because of its “lumpy,” non-spherical shape.
Further justifying why they believe Pluto should be called a planet, and why they feel the term should be redefined, Grinspoon and Stern cited the discoveries of numerous exoplanets beyond our solar system and the fact that there are many icy objects in Pluto’s vicinity that orbit the sun. But that brought them to their explanation of why the IAU’s decision to reclassify Pluto was “deeply flawed,” as its requirement that planets should orbit around our sun automatically disqualifies all exoplanets, or any other planet in the universe, from being a planet.
Talking about the criterion that an orbiting planet has to have “cleared [its] neighborhood,” which Pluto had failed to meet, Grinspoon and Stern referred to the requirement as being “imprecise,” and one that doesn’t take into account the intrinsic properties of a planet candidate.
“This leads to many bizarre and absurd conclusions. For example, it would mean that Earth was not a planet for its first 500 million years of history, because it orbited among a swarm of debris until that time, and also that if you took Earth today and moved it somewhere else, say out to the asteroid belt, it would cease being a planet.”
In conclusion, Grinspoon and Stern expressed hope that the IAU would reassess its classification of Pluto and its definition of the word “planet,” as people continue to refer to any round object with distinctive geographical features as such, regardless whether they may be in our solar system or not.