Friday Night Science: Why's Pluto Not A Planet?

How many planets are there in our solar system? Come on. Quickly. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and that's it. 

Pluto's not really a planet. It was, though. For seventy-six years since its discovery by astronomer Clyde Tombaugh in the early 1930s, Pluto retailed its place as a planet up to 2006. The trouble began in the final decade of the last century when more and more Pluto-like planetary worlds, in succession, were observed in the previously unseen depths beyond the ninth sphere. With the myriad of these planetary bodies coming to light, astronomers had a hard time reaching a consensus on the definition of a planet. On August 24, 2006, the International Astronomical Union (IAU) passed a resolution where Pluto got voted out of the elite club.

Backlit
With the distant Sun shining behind, its light dimmed by the sheer distance that places the ninth sphere on the edge overlooking interstellar space, Pluto poses for one final shot for NASA's New Horizons. As the sunlight scatters through, Pluto's tenuous atmosphere lights up in a brilliant blue ring.
Credit: Public Domain, NASA 

The IAU considers a celestial body as a planet if it 
(1) is in orbit around the Sun, 
(2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round shape) and 
(3) has cleared the neighborhood around its orbit. 

At first blush, you realize Pluto satisfies two of the three conditions. Sure, its orbit may be highly elliptical and excessively tilted away from the plane of the solar system, illegally cutting across Neptune's track and taking over the eighth place, but set aside that, Pluto orbits the Sun.  

Having a mean radius of 1,188 km (738), Pluto is almost perfectly round — like Mercury (Earth is an oblate spheroid spotting an equatorial bulge). In astrophysics, roundness translates to hydrostatic equilibrium. From my last article two weeks ago, I can finally say that hydrostatic equilibrium is the reason why planets are round. Planets form from the accretion of nebular gas and dirt. As gravity pulls matter into a clump, pressing inward, a resisting force develops deep inside the proto-planetary lumps (seeds that mature into planets). If not for this counter-resisting force, gravity would keep drawing matter until the very fabric of space starts to sink (like throwing a hockey puck on a piece of stretched fabric), and you get a black hole! But, if a perfect balance strikes out between these two opposing forces, the inward pull of gravity and the outward resistance of pressure, which is the norm, you get planets, moons, and even stars. Near-perfect equilibrium results when gravity molds a planet into a perfect sphere. So if you find a perfectly spherical body out there in space, orbiting the Sun, and shares geophysical similarities with any of the four rocky/terrestrial planets from Mercury to Mars, no matter what its size, even only one-tenth of the Moon, by rights, it can be classified as a planet. 

Now, the final nail. What does it mean to have cleared the neighborhood around its orbit? Forty-eight years after the initial discovery, it turned out Pluto has a moon — Charon. The Pluto-Charon system is unlike anything you'll find in the solar system. Since Charon is almost comparable in size to Pluto, they have descended into an extreme situation of mutual tidal locking where they both maintain the same face towards the other. In our case, the Moon moves around the Earth, as the latter circles the Sun. But Pluto and Charon both orbit each other. Earth and Moon orbit each other, but since the latter is only one-sixth in size compared to its primary, the Moon enjoys being Earth's natural satellite. Our ninth planet doesn't share that relationship, both behaving like the other's moon.   

Animated gif of Pluto-Charon system. 
Credit: Public Domain, via Wikimedia Commons. 

However, its alien dynamics are not the deceitful culprit behind Pluto's demotion from a planet. On July 29, 2005, a team of astronomers announced the discovery of a new planetary candidate beyond Pluto at 68 astronomical units from the Sun (at its farthest, Pluto floats at 49 astronomical units from the Sun). Initially, Eris was believed to be larger than Pluto, which prompted NASA to list it as the tenth planet. 

If Pluto deserves to be a planet, then so does Eris. This ups the planetary count to ten. But two centuries ago, following the discovery of Ceres in 1801 (at that time Ceres was counted as the tenth planet, for Neptune was yet to be found), four more similar objects, Pallas, Juno, Vesta, and Astraea were discovered in what was to yet to be later recognized as the asteroid belt. When Neptune was chanced on for the first time in 1846, it became listed as the thirteenth planet.  

By the 1860s, with the count of asteroids growing by the day, astronomers realized that the definition of a planet needed revision. Thus, the debate began. Although Ceres belonged to the asteroid belt, at the time of the discovery of Eris beyond Pluto, Ceres was still somehow considered a planet because of its geophysical resemblances. The discovery of Eris revealed that the region beyond Pluto, or to be more accurate, beyond Neptune, could be scattered with hundreds of thousands to millions of minor planetary bodies similar to the asteroid belt. 

Pluto belongs to the Kuiper belt, a toroidal debris field encircling the Sun at 30 astronomical units. With the Kuiper Belt taking center stage, the IAU asked astronomers to decide upon a consistent definition of a planet. The 2006 resolution listed Pluto as a Kuiper belt object and a dwarf planet. 

You can also classify Pluto as a minor planet, but IAU explicitly states (here's the original): 

Resolution 6A
The IAU further resolves:
Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

This is how Pluto would look if you and I could see from up close at thirty-five thousand plus kilometers (twenty-two thousand plus miles) — the vantage point from where New Horizons' Multispectral Visible Imaging Camera (MVIC) collected and reproduced this true-color image. 
Credit: Public Domain, NASA.  

*A Trans-Neptunian Object (take a look here) or a TNO is any minor planet that orbit the Sun at a distance greater than that of Neptune — 30 astronomical units from the Sun.