getty Poor Pluto. On August 24, 2006 at the General Assembly of the International Astronomical Union (IAU) the ninth planet was cleared just 76 years after its discovery. Even more surprising is that it was actually voted down, and by astronomers, not planetologists. The IAU redefined what a planet is without considering any geophysical features, with Pluto failing not because of its small size (it’s no bigger than the continental US), but because it hasn’t “cleared the neighborhood around its orbit.” Debate rages about it, with the latest NASA administrator claiming that the asteroid is approaching all the “planets” in the solar system. The IAU also explicitly created a new term – “Pluto class object”. The term has never been used by planetary scientists. Most noticeably, Pluto’s demotion had consequences for the authority of the IAU. The term “Cosmos” is now used instead of “planet” to describe places in the solar system. Few talk about dwarf planets or moons, but instead we hear terms like “icy worlds,” “cosmic oceans,” and “volcanic worlds.” What few people remember is why the planet definition had to be revised in 2006. The real reason was a newly discovered object named 2003 UB313, at first nicknamed Xena and then renamed Eris. Although it is typically three times farther in an eccentric orbit than the Sun, Eris is very slightly smaller than Pluto. Back in 2006 it was actually thought to be larger than Pluto and it was thought that Eris could officially receive planet status at the IAU meeting. 3D rendering of the officially recognized dwarf planets in our solar system getty However, with several other candidate objects found in the early 21st century that were thought to be roughly the same size as Pluto—since named Makemake, Haumea, and Sedna—the IAU thought there was a problem. If Pluto was a planet, then so was Eris and all these other objects. Can we have 10 or 15 planets? With ever-evolving technology and new telescopes, how about 50 or 100 planets? So Pluto was demoted … to keep the numbers down? Perhaps, although it’s also true that all of these objects – including Pluto – are in the Kuiper Belt, a ring of icy objects around the Sun that extends beyond the orbit of Neptune. They also have rather eccentric orbits. It wasn’t the first time they snubbed Pluto. NASA Voyagers’ “Grand Tour” of the outer solar system planets stopped to visit Pluto in the 1990s, after Jupiter, Saturn, Uranus and Neptune. The same alignment every 175 years that allowed Voyager 2 to continue on to Pluto after orbiting Neptune, but NASA scientists prioritized taking a look at Neptune’s moon Triton. 2006 wasn’t the first time a planet was demoted either. Go back to 1801 and Demeter – the largest object in the asteroid belt – was discovered and described as the “missing planet” between Mars and Jupiter. Downgraded shortly thereafter to a mere asteroid, the same 2006 IAU meeting that downgraded Pluto upgraded Ceres to dwarf planet status. It was ironic that while votes were being taken on the status of the two solar system objects, missions to both were at an advanced stage. New Horizons was launched to Pluto in January 2006, while the Dawn mission launched to Ceres just over a year later. The two “new” dwarf planets turned out to be far more numerous than planetologists had hoped. Both were revealed as candidate “ocean worlds” like Europa (a moon of Jupiter) and Enceladus and Titan (moons of Saturn). When New Horizons flew past Pluto on July 14, 2015, it revealed a world as interesting as anywhere else in the solar system. It showed that Pluto was beyond any planetary scientist’s wildest dreams—geologically active and possibly volcanically and even tectonically active. Here, 40 times farther from the Sun than Earth, Pluto was shown to have its own complex atmosphere, organic compounds on its surface, and massive cracks in its crust. It is a place of amazing geological complexity with vast nitrogen ice plains, mountain ranges, sand dunes and ‘ice volcanoes’. Such is the wealth discovered on Pluto that it seems unlikely that the IAU would have stripped it of its planet status after the New Horizons flyby. Pluto is a fascinating world that deserves a return mission to see if it has an ocean beneath its ice. Maybe one day we’ll call it ocean world, though by then there’ll probably be a new name for that too. I wish you clear skies and open eyes.
