A group of astronomers still upset at Pluto's demotion are arguing for its reinstatement as a planet - and say dozens of other worlds should be too, including the moon.
The International Astronomical Union in 2006 changed its definition of a planet, after the discovery of several similarly-sized objects orbiting the sun - if they weren't planets, then Pluto couldn't be either.
The new definition not only required a body to be large enough for gravity to have pulled it into a spherical shape, it also had to have cleared its orbit of other objects. Pluto failed the latter test, so was demoted, leaving just eight planets.
But a new paper published in space journal Icarus says the current definition has more to do with astrology than astronomy. Its authors say every decent-sized object orbiting the sun should be considered a planet, based on how the word was used by astronomers dating back to the 16th century.
"We think there’s probably over 150 planets in our solar system," Philip Metzger, study lead author and physicist at University of Central Florida, told NBC News.
He said most planetary astronomers disagree with the IAU's definition, which limits the list of 'planets' mostly to those known to ancient sky-watchers, so excluding Pluto, which wasn't discovered until the 20th century.
The study, which looked at centuries of how the word 'planet' has been used, said their "limited" research had already uncovered 149 planets in our solar system - far more than the eight the IAU acknowledges: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Among the 149 'planets' are Jupiter's moon Ganymede, which is actually bigger than Mercury; Saturn's Titan, which is believed to have a subsurface ocean; Jupiter's Europa, slightly smaller than the Earth's moon with thermal plumes; trans-neptunian objects such as Charon, Ixion, and Quaoar; and our very own moon, which experiences 'moonquakes' so is probably geologically active.
"It is important for every branch of science to reject folk taxonomies and to embrace taxonomies that have the most insightful alignment with explanatory theory, not only to aid scientific practice and to bring consistency to the lexicon, but to communicate deeper scientific insight to the public," the study concludes.
"Therefore, we should embrace the Geophysical Planet Definition and teach it to students and to the public at large as a correction to the folk idea that what or where an object currently orbits is the definition of what it essentially is."
One of the astronomers behind the 2006 definition change said there should only be eight planets, however.
"I think the IAU fixed an embarrassing mistake that had been perpetuated for generations," Caltech's Michael Brown told NBC News. "The solar system is now sensible."
