Solar System Diagram. Image Credit: Wikimedia

Michelle asks,

Dear Astronomer, Why isn’t Pluto a planet anymore?

Great question Michelle, one that still sparks debate over five years after the controversial decision by the International Astronomical Union (IAU). For the benefit of those who don’t know the full backstory, this will be a pretty in-depth explanation.

Discovered by Clyde Tombaugh in 1930, Pluto is certainly an interesting case. During the early 1900′s there was speculation of another planet lurking past Neptune. After many years of studying Pluto, the determination was made that Pluto was not massive enough to be the cause of perturbations in the orbits of Neptune and Uranus. The determination was also supported by revised estimates of Neptune’s mass.

Pluto, Charon, Nix, Hydra and newly discovered "P4".
Image Credit: NASA/HST

As astronomers discovered more about Pluto, the less of a “fit” with the rest of the solar system became clear. Even when I was in grade school (in the late 70′s/early 80′s), textbooks and even some planetarium presenters described Pluto as more of a “giant comet”, than a planet.

Further adding to the confusion is Pluto’s moon ( for lack of a better term ) Charon. Technically speaking, Pluto and Charon are a “binary” system – the only one known to exist in our solar system. Additionally, three more objects orbiting Pluto have been discovered. Nix, Hydra and an unnamed object now designated “P4″. Given these objects, one could describe the Pluto system as a loosely bound “cloud” of icy objects.

So how does all this fit in with “planetary status” ?

Consider that most people like to organize things in similar groups. We don’t put plates in the silverware drawer, nor do we put shoes in our refrigerator. A quick glance at our solar system shows three readily apparent classes of planets; Terrestrial planets ( Mercury, Venus, Earth, Mars), Gas giants (Jupiter, Saturn), and “Ice” giants ( Uranus and Neptune ). Out past Neptune (out to a distance of almost a light year from our Sun!) there are many icy/rocky objects, few of which have been studied in great detail.

Eris and Dysnomia.
Image Credit: NASA/HST

While studying the region past Neptune, astronomer Mike Brown and his team discovered an object that at its furthest point orbits at nearly twice the distance from our Sun as Pluto. The object, first dubbed 2003UB₃₁₃ (now known as Eris) was estimated to be nearly the same diameter as Pluto and additional studies of this object and its moon revealed a mass roughly 25% more than Pluto’s.

Brown’s discovery raised some interesting questions. If Eris, being much further from the sun was the same diameter and had more mass than Pluto, had a new planet been discovered? If more objects like Eris were to be discovered past Pluto’s orbit, would they also be planets?

A controversial decision by the International Astronomical Union in 1996 set out to provide an “official” set of criteria for planetary status. The IAU states an object is a planet if it:

In order to classify objects like Pluto and Eris (as well as Makemake, Ceres, and Haumea) which meet the first two criteria, the IAU adopted the term “dwarf planet”. Based on studies of the outer solar system, estimates of how many dwarf planets exist range between a few dozen and a few hundred.

On August 24th 2006, the IAU’s proposal passed a vote, and thus, Pluto no longer was considered a full-fledged planet, but instead the first of the dwarf planets. Many people to this day criticize the vote and the decision, however science is not dictated by popular opinion. With the discovery of many additional objects past Neptune’s orbit that share similar characteristics with Pluto, the IAU’s decision appears to make sense, for the time being.

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