Pluto is a dwarf planet, but not because it’s small

Influences of Science Fiction on Science and Pluto
(First Aired on KLRNRadio, Monday, October 3, 2016)

Last month was the 50th anniversary of Star Trek. (Warning: I'm about to go full geek mode for a moment.)

Regular readers of my blog will already know that I'm a BIG Star Trek fan. I even reminisced in a previous post about some of my fondest Star Trek memories, including a rare piece of Star Trek memorabilia given to me for my 21st. So when the 50th anniversary came around, I was actively watching all the videos and reading all the posts.

I came across a video from NASA that had me on serious geek mode. In the video, William Shatner, Nichelle Nichols and George Takei gave their thoughts about the work that NASA is doing. There were a few things that were said that struck a chord with me as the scientist and writer.

There's all that space, all that knowledge, all that challenge, and the human animal is an adventurous animal.
— George Takei

It's phenomenal what NASA is doing with science that, when you look at it, is the equal of science fiction.
— William Shatner

(Okay... Geek mode being toned back now and we can get into the crux of what was covered in the latest episode of Conversations in Science.)

In that video (link above), William Shatner also spoke about NASA's New Horizons mission to Pluto and the Kuiper belt.

New Horizons was launched on January 19, 2006 and conducted a six-month-long reconnaissance flyby study of Pluto and its moons during the summer of 2015, culminating with Pluto closest approach on July 14, 2015. (That's my birthday, people. Seeing photos of Pluto that close up was such an awesome birthday present. Thanks, NASA!)

But there is something about Pluto that many people get confused on. There is a common misconception behind why Pluto is no longer a planet.

My children are among the first generation to grow up with the idea that Pluto is not a planet. They have been told about the change in classification, but even they were among those to make the assumption that Pluto was demoted to dwarf planet status because of its size — at least that was the case until I corrected them. In a recent conversation that I have with one of my nephews, I was heartbroken to have to correct a 5-year-old on this too. And the number of students in my son's high school class who also believe that this classification is because of size is beyond saddening. In truth, I believe it's because of the word dwarf. Let's face it, the word does imply a small size (either that or short men with beards).

Sorry, guys... Pluto may be a dwarf planet, but it's not because of its size.

It comes down to the definition of a planet as defined by the International Astronomical Union (IAU) in August 2006. For a celestial body within our solar system to be classified a planet, it must satisfy three criteria:

1. it must orbit the Sun (i.e. it can't be a moon, orbiting some other body),
2. it must have sufficient mass to assume hydrostatic equilibrium (i.e. it's round), and
3. it has cleared the neighborhood around its orbit.

Notice, nowhere in that definition does size come into it.

The first two criteria are reasonably easy to understand. As long as the celestial body is spherical in shape and orbits the Sun, then it has a shot at being classified a planet. It's the third criterion that puts the kink in Pluto's armor.

This composite of enhanced color images of Pluto (lower right) and Charon (upper left), was taken by NASA’s New Horizons spacecraft as it passed through the Pluto system on July 14, 2015. This image highlights the striking differences between Pluto and Charon. The color and brightness of both Pluto and Charon have been processed identically to allow direct comparison of their surface properties, and to highlight the similarity between Charon’s polar red terrain and Pluto’s equatorial red terrain. Pluto and Charon are shown with approximately correct relative sizes, but their true separation is not to scale. The image combines blue, red and infrared images taken by the spacecraft’s Ralph/Multispectral Visual Imaging Camera (MVIC). (Credits: NASA/JHUAPL/SwRI)

So, what does cleared the neighborhood actually mean? First, let's use the technical terms: a celestial body has cleared its neighborhood if it has become gravitationally dominant, and there are no other bodies of comparable size other than its satellites or those otherwise under its gravitational influence. Translation: there are no other objects that share its orbit about the Sun. The only other objects with it are moons or other objects that orbit the planet.

Here's the problem with Pluto: the orbit of Pluto is... well... hinky. All of the planets orbit the Sun in a near circular orbit within the same plane, known as the ecliptic. Pluto orbits the Sun with an elliptical orbit that is at an angle of approximately 17 degrees to the ecliptic.

Now some readers will see the above image and notice that Pluto's orbit is close to Neptune's. In addition, some might have seen 2D projection images of the orbits and will have noticed that Pluto's orbit crosses Neptune's on the 2D-plane. Before anyone asks... No, Pluto will not collide with Neptune. The closest Pluto ever comes to Neptune (at the points where Pluto's orbit crosses the ecliptic) is 18 AU (2,700,000,000 km) of each other. That's the closest that the Earth ever gets to Uranus and you don't see the Earth colliding into that planet. And for those worried about the 2D projection of the orbits crossing one another... Remember Pluto's orbit is at ~17 degrees to the ecliptic.

(BTW, 1 AU, astronomical unit, is the distance between the Earth and the Sun (149,600,000 km).)

Let's add to this hinky Pluto orbit the fact that there are a few objects in the Kuiper belt that share their orbits with Pluto. We didn't really know much about these objects until recent years, thanks to NASA and astronomers who focus their observations on the Kuiper belt.

While the IAU's definition doesn't specify numbers or equations, the eight planets have all cleared their neighborhood to a much greater extent than Pluto.

To address these orbit issues of Pluto, the IAU devised a new classification for celestial bodies that satisfy the first two criteria of a planet, but not the last: dwarf planet. In fact, the IAU had gone that one extra step and added a fourth criterion to the dwarf planet classification. A dwarf planet is a celestial body that

1. orbits the Sun,
2. has sufficient mass for hydrostatic equilibrium (it's round),
3. had not cleared its neighborhood, and
4. is not a satellite.

Now, I should point out that the term satellite is not referring to the man-made machines that we've send out into space. No... Satellite, in this case, is just another word for moon. (Why scientists can't stick to common everyday laymen's terms is beyond me.)

So, that smallest planet that we all knew and loved as the ninth planet from the Sun instantly became a dwarf planet. Ironically, while the size of Pluto has nothing to do with its dwarf demotion, if Pluto had been just that little bit bigger, it would have likely cleared its neighborhood and remained a planet.

There are currently only five objects recognized as dwarf planets by the IAU: Ceres, found in the Asteroid belt between Mars and Jupiter; Pluto; Haumea, found in the Kuiper belt; Makemake, also found in the Kuiper belt; and Eris, found at a distance from the Sun that is nearly twice that of Pluto's orbit.

There are many questions that we have about the objects found in farthest reaches of our solar system, questions that the NASA's New Horizons mission aims to answer. Exactly what we'll discover is still largely unknown, but one thing is clear: Pluto is challenging our definitions of planet and dwarf planet.

You want to find out more about the New Horizons mission on NASA's website.

(Featured Image: Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this global view of Pluto. The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away from Pluto, show features as small as 1.4 miles (2.2 kilometers). (Credits: NASA/JHUAPL/SwRI) )