Sometimes our scientific lens on the world is not just about empirical facts, but also about definitions. Consider the case of poor Pluto, kicked out of the planetary club in 2006.
Poor, poor Pluto. Once regarded as the smallest and most distant but nonetheless one of the most distinctive planets, this body of rock and ice was demoted from full planetary status in 2006 by the International Astronomical Union (IAU). That’s largely because of the discovery in 2005 of Eris, another rocky body similar to Pluto, which also orbits the sun along with other small “rocks” now known to inhabit a region beyond the orbit of Neptune called the Kuiper Belt. Not only are most of the bodies that occupy this region of the solar system far too small to be considered planets, but also tiny Eris itself was initially thought to be larger than Pluto. So, it made sense that if none of these other rocks deserved planetary status, including one even one larger than Pluto, planet status should be denied Pluto as well.
A few reluctant astronomers still recognize Pluto as a planet, despite its official defrocking. To add fuel to the debate, recent findings suggest that Eris, the Kuiper object most responsible for Pluto’s demise, is not actually larger than Pluto as originally thought. In fact, more recent and accurate measurements show it to be just ever so slightly smaller than Pluto. So once again, there is some debate (albeit a very muted debate among a very few) about whether Pluto should regain its former status as a planet.
In an article written by Mike Wall for Space.com, the scientist who led the team that discovered Eris, Mike Brown of Caltech, says it’s very unlikely that Pluto’s present status will be reconsidered. He makes the point in his new book How I Killed Pluto and Why It Had It Coming [Amazon-US | Amazon-UK](?) that had Pluto been discovered today — considering all we know about planetary structure and composition — it would have never been deemed a planet in the first place. That’s why he and many other astronomers seem ill-inclined to consider re-elevating the status of the the lowly “dwarf.”
The debate about the planetary status of Pluto, as well as the very definition of a planet is definitely not completely settled science. Some critics complain that a strict interpretation of the IAU’s definition of a planet (especially the part of the definition that involves the clearing of a zone around the planet from other objects) would necessitate removing planetary status from each of the eight recognized planets in the solar system. But a planetary definition that did not include this requirement would open the door to the inclusion of numerous objects — including bodies in the asteroid belt — in the family of planets. So, for now it seems that the majority view will prevail and poor Pluto will remain an orbital rock of sub-planetary status.
Even among the recognized planets, there is so little that some of them have in common that the question arises about whether or not the term “planet” has any distinctive meaning anymore. Suffice it to say, however, that a bit of nostalgia has developed for the long-recognized members of our solar system, and their status as planets is nowhere near as perilous as that of poor Pluto.
The winter months bring some crisp, clear celestial viewing nights to the northern hemisphere. Even with just a pair of binoculars, many of the night sky wonders can be viewed with a fair degree of clarity, especially if you’re fortunate to be located away from light pollution. The largest, gaseous planet, Jupiter, is particularly viewable this time of year, as are some of its moons, and will be so for a while longer. So, the next chance you get, perhaps you can treat yourself to a little stargazing. And while you’re at it, you might reflect a bit on the pitiful plight of previously planet Pluto. It’s far too remote and too small to be seen. But even so, and despite it’s fall from planetary grace, it’s still not forgotten.
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