A NASA researcher wrote an op-ed in the Washington Post calling for the reclassification of Pluto as a planet. Pluto was designated a dwarf planet in 2006 but some scientists have been fighting that classification.
These scientists now want the planet classification system revamped and for Pluto to become a planet once again.
Back To Nine Planets?
The op-ed for the Washington Post was written by Planetary Science Institute astrobiologist David Grinspoon and NASA New Horizons principal investigator Alan Stern. Grinspoon and Stern make the case that Pluto should be reclassified as a planet. They cite Pluto's characteristics such as its icy mountains, glaciers of nitrogen, blue sky, and clouds as proof that Pluto should be called a planet again.
Grinspoon and Stern attack the definition of a planet that was set in 2006 by the International Astronomical Union. In the definition, the IAU used three criteria for a planet.
The first was that a planet would have to be large enough for its gravity to pull itself into hydrostatic equilibrium, the shape would be round if it didn't rotate and spheroidal if it did rotate. The second criteria were that it would have to orbit the Sun. And the last criteria was that it had to have a clear orbit with nothing in its way.
They also detest that the IAU added that a dwarf planet could not be a planet.
Redefining The Meaning Of A Planet
In the op-ed, Grinspoon and Stern add reasons as to why the classification of planets needs to be redefined. They cite the fact some moons in the solar system are larger than some of the planets, they note that Saturn's moon Titan is larger than the planet Mercury and emphasize the planet's physical characteristics.
They do agree with one of the three criteria in the IAU definition. They believe that planets should be rounded by their own gravity, objects in space that are too small to be rounded by their own gravity should have a different classification.
One of the rules they don't agree with is that planets have to orbit the Sun. This complicates the discovery of exoplanets all over the universe, planets that are rotating their own stars, and even those without stars called rogue planets.
They also take exception to the criteria that a planet must have cleared its own zone. Adding that it is absurd to think that if Earth was transported to the asteroid belt it would no longer be considered a planet.
Grinspoon and Stern add a more practical definition for a planet although they didn't come up with it. They cite the presentation called "A Geophysical Planet Definition" from the 2017 Lunar and Planetary Science Conference which defines a planet as a round object in space that is smaller than stars.