In July 2015, NASA’s New Horizons spacecraft sent home the first close-up images of Pluto.
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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 stranger is that it was actually voted on, and by astronomers, not planetary scientists. The IAU redefined what a planet is without regard to any geophysical features, since Pluto failed not because of its small size (no larger than the continental United States), but because it has not “cleaned up the neighborhood around its orbit”. Debate rages over this, with the latest NASA administrator arguing that the asteroid is approaching every “planet” 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 notably, the demotion of Pluto had consequences for the authority of the IAU.
“World” 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 “ice worlds,” “ocean worlds,” and “volcanic worlds.”
What few people remember is why the definition of a planet had to be revised in 2006. The real reason was a recently discovered object called 2003 UB313, first nicknamed Xena and later renamed Eris.
Although typically three times farther in an eccentric orbit from the Sun, Eris is only slightly smaller than Pluto. In 2006 it was thought to be larger than Pluto and it had been thought that Eris might officially gain planet status at the IAU meeting.
3D representation of the officially recognized dwarf planets of our solar system
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However, with several candidate objects found in the early 21st century that were thought to be roughly the same size as Pluto, named Makemake, Haumea and Sedna, the IAU thought there was a problem. If Pluto was a planet, so was Eris and all these other objects. Can we have 10 or 15 planets? With increasingly advanced technology and new telescopes, how about 50 or 100 planets?
So Pluto was demoted… to cut the numbers? Perhaps, although it is 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 fairly eccentric orbits.
It was not the first time that Pluto was rejected. NASA Voyagers’ “Grand Tour” of the outer planets of the solar system stopped short of visiting Pluto in the 1990s after ticking off Jupiter, Saturn, Uranus and Neptune. The same once-in-175-year alignment that made gravity assists possible would have allowed Voyager 2 to continue on to Pluto after circling Neptune, but NASA scientists prioritized looking at Neptune’s moon Triton.
Nor was 2006 the first time a planet had been degraded. Go back to 1801 and Ceres, the largest object in the asteroid belt, was discovered and described as the “missing planet” between Mars and Jupiter. Soon after, demoted to a mere asteroid, the same 2006 IAU meeting that demoted Pluto upgraded Ceres to dwarf planet status.
It was ironic that while the status of the two solar system objects was being voted on, the missions to both were at an advanced stage. New Horizons had launched to Pluto in January 2006, while the Dawn mission launched to Ceres just over a year later.
The two “new” dwarf planets were revealed to be much more than planetary scientists expected. Both were revealed to be candidates for “ocean worlds” such as 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 intriguing as anywhere else in the solar system.
It showed that Pluto was something beyond the wildest dreams of all planetary scientists: geologically active and possibly also 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 huge faults in its crust. It is a place of astonishing geological complexity with vast plains of nitrogen ice, mountain ranges, dunes and ‘ice volcanoes’.
Such are the riches discovered on Pluto that it seems unlikely that the IAU could have removed its planet status after the New Horizons flyby.
Pluto is an intriguing world that deserves a return mission to see if it has an ocean beneath its ice. Maybe one day we’ll call it the ocean world, though then there’ll probably be a new name for that too.
I wish you clear skies and wide eyes.