For those still distraught about Pluto's demotion from full-fledged planet to dwarf, the battle is not over. The former planet has made some powerful allies who believe their discoveries will convince astronomers to bring Pluto back into the planetary fold.
Many of us who grew up learning about the nine planets took Pluto's reclassification hard, championing it as a celestial underdog. Meanwhile, astronomers were left to grapple with the question of what defines a planet as such. Mark Sykes of the Planetary Science Institute in Tucson, Arizona, suggests that Pluto's demotion by the International Astronomical Union (IAU) stems from a misconception that full-fledged planets are somehow unusual:
"We are [now] in the midst of a conceptual revolution," he says. "We are shaking off the last vestiges of the mythological view of planets as special objects in the sky - and the idea that there has to be a small number of them because they're special."
As we learn more about Pluto and about objects outside our solar system, astronomers may well learn that the other eight solar planets have much more in common with Pluto than with other celestial bodies that exceed it in size:
Sykes believes that missions currently en route to Pluto and the asteroid Ceres, which orbits the sun between Mars and Jupiter, will reveal these dwarf planets as active and intricate worlds. Meanwhile, astronomers may find distant objects as large as Earth which the IAU would not define as planets.
This leaves many astronomers clamoring for the view that any planet large enough to be pulled into a sphere by its own gravity should be considered a planet. By this definition, not only would Pluto be a planet, so would Ceres, Haumea, Makemake, and Eris, all currently considered dwarfs.
Although the International Astronomical Union, which classifies celestial bodies, convenes this year for the first time since Pluto's demotion, its chief does not expect any challenges to Pluto's status. But in 2015, NASA's New Horizons missions will reach Pluto, giving us our first up-close look at the sphere, and perhaps making Pluto the little planet that could.
Is Pluto a planet after all? [New Scientist]