Pluto, the icy underdog of the Solar System, was “demoted” from a planet to a dwarf planet by the International Astronomical Union (IAU) in 2006, after they contested poor old Pluto didn't fit their new criteria for a true planet. Ever since the controversial decision, Pluto's legions of loyal fans and many astrophysicists have been fighting back against the decision.
Alan Stern and David Grinspoon – principal investigator of the New Horizons mission to Pluto and astrobiologist, respectively – have written an article for the Washington Post that puts forward their case to "Make Pluto Great Again" (or at least a true planet again).
The 2006 IAU definition requires a celestial body to tick three boxes before it can be called a planet. First up, it must be in orbit around the Sun. Secondly, it must have sufficient self-gravity to make a round (or nearly round) shape. Finally, it has to have cleared the neighborhood around its orbit. Since Pluto is surrounded with a swarm of other icy Kuiper Belt objects similar to itself, it does not fit with the definition and is therefore not a true planet, they argue.
Stern and Grinspoon write that the definition was "hastily drawn" and contains "obvious flaws". After all, it would mean that Earth is not a planet because it has many asteroids in its neighborhood that have not been cleared out. Furthermore, the definition fails to consider exoplanets, the countless planets that live beyond our own Solar System. The more of these exoplanets we discover, the more this definition appears needlessly tight.
Instead, they argue the term "planet" should be used to describe worlds with intrinsic geophysical properties, not merely orbital properties.
“We use 'planet' to describe worlds with certain qualities,” explain Stern and Grinspoon. “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.”
The debate has since fired up on Twitter too, as most arguments tend to do:
Speaking to IFLScience last year, Alan Stern further pushed this point, noting: “[The IAU is] primarily made up of non-experts, astronomers who study black holes and galaxies. Other organizations [with planetary scientists] may be more appropriate.”
Instead, Stern and Grinspoon cite another definition put forward in 2017 at the annual Lunar and Planetary Science Conference: "Round objects in space that are smaller than stars." Of course, not all astronomers are convinced by this argument either, as that would mean most Moons in the Solar System would also be considered planets. Even Cosmos host Neil deGrasse Tyson has previously argued on The Late Show with Stephen Colbert that Pluto is not a planet. After noting that Pluto's orbit occasionally crosses Neptune, he shouted: "That's no kind of behavior for a planet. No!"
One thing is more certain, the debate around Pluto's planethood is far from dead.