Pluto has been demoted and there’s a good reason for it. It hurts, somehow, that the “ninth planet” is considered as less than a planet. It broke hearts and incited debates and controversy. Most of us grew up with the classic, simple nine planet solar system chart; we felt attached to it, almost inspired by it. Suddenly, the map on our bedroom or classroom walls was no longer valid. However, it’s important to consider that there is a very good reason Pluto it not a planet, so let’s dive into it.
One of the most oddly contentious scientific events to have happened in the early twenty-first century was the demotion of Pluto. This celestial body of the solar system was once considered the ninth planet but is now defined as a dwarf planet, which is a classification that many seem to have a hard time accepting. However, what underlies the tale of Pluto is an important lesson about the nature of science and of change, and how these two concepts should always be linked.
Let’s first briefly examine the character of Pluto (I’m referring to the planet, not the dog, at least this time) and then take a look at the tale of how this distant lump of matter created such a stir.
Pluto was originally discovered in 1930 and was then classified as the ninth planet in our solar system. It has an unusual orbit, moving around at an odd angle in comparison to the larger planets that spin around almost evenly inline with the sun. Pluto is also TINY, relatively speaking. It has an average radius (“average” because it’s not perfectly spherical) of about 1188.3km, which is less than one-fifth of the Earth’s radius. Additionally, its total mass is about one-sixth of Earth’s moon. Overall, this makes Pluto a low density object of a comparably small size.
So, what would it be like to land on Pluto? Well, it’s cold, very cold. The luminous sun that arcs across Earth’s skies is a much greater distance away. The composition of the dwarf planet consists of mostly ice and rock. The rocky core of the planet is surrounded by a thick layer of water ice while the top crust is mostly nitrogen-based ice. The mountains of the terrain are also composed of water ice. The pressure on the surface is 100,000 less than Earth, which means you’re definitely going to need a high-quality space suit to keep the pressure on your own body to prevent you from, well. . . evaporating instantaneously. One of the most visually apparent changes, however, is the drastic difference in gravity compared to Earth. On our home planet, the rate of gravity is 9.8m/s2. On Pluto, the rate of gravity is almost sixteen times less at a rate of 0.620m/s2! To illustrate, say you dropped something from about arm height on Earth, obviously, it falls pretty quickly to the ground. However, drop it from the same height on Pluto and you’d have a leisurely few seconds to watch it float to the ground. For another interesting comparison, remember the footage of astronauts bounding around the moon, hopping in great strides as they bounced slowly along the surface? Well, the gravity on the moon is almost three times greater than that of Pluto. The surface gravity experienced on a planet is related to the mass of that planet, and Pluto just doesn’t have the mass to even come close to being comparable to Earth.
Okay, so we have done some work in condescending to poor Pluto about its small stature, but more important is how these measurements impact its scientific classification. Pluto is a body among our solar system’s Kuiper belt. This belt is a disc-like band (much like the asteroid belt between Mars and Jupiter) that contains many objects of various sizes. In 1992, Pluto was still considered the ninth planet from the Sun, but the discovery of other bodies in the Kuiper belt started sowing the seeds of doubt that would later grow into the rationale for Pluto’s demotion to a dwarf planet. Pluto was no longer a unique, solitary mass, but rather a member of a large group of similar objects that form the Kuiper belt. In 2005, Eris, a Kuiper belt object akin to Pluto was discovered and classified as a dwarf planet. However, even though it was classified as “lesser” than Pluto, by virtue of its size it was in fact greater. Eris is 27% larger than Pluto by mass, which throws the whole system of classification into doubt and confusion. This prompted the world’s astronomers to rethink the defined system that was in place.
In 2006, after much deliberation and argument, the International Astronomical Union (IAU) delineated a series of criteria that would serve to differentiate planets from non-planets. The three criteria are:
- The object must be in orbit around the Sun. (Pluto PASSES!)
- The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by hydrostatic equilibrium.(Pluto PASSES!)
- It must have cleared the neighborhood around its orbit. (Pluto . . . FAILS!)
Pluto’s overall mass only amounts to 0.07 times that of the mass of the other objects in its orbit (its moons). Comparing this to Earth, which has a mass that is 1.7 million times greater than the mass that exists in its orbit, it can be easy to see how Pluto’s small size, weak gravity, and low density make it a poor example of a planet. Due to its failure to meet these criteria, Pluto was then demoted to what it is now: a dwarf planet. No longer is it the ninth planet of the solar system.
To put a more colloquial analogy into play to further play against poor Pluto, imagine you are standing in the center of a large field that represents a tiny version of our solar neighborhood. Next to you is a large, brilliant ball of light that is our sun, rotating luminously at the center. You take a large step out and encounter a fist sized-rock at your feet. This is Mercury, the first planet from the sun. You take another, not quite as large step, and encounter a larger rock. This one is bigger and would take two hands to pick up. This is Venus, the second planet. Another step in and you encounter a rock of similar size, which many would call home. This is the third rock from the sun, Earth. A half-step from Earth there’s another rock that is just a bit bigger than the first you came across. This one is Mars, Earth’s closest relative. You take another couple steps outward from the sun, this time walking across some dense gravel dotted with many small rocks, pebbles, and dust. You are stepping across the asteroid belt on your way over to what looks like a decent sized boulder. This is the largest rock in the field, the largest planet in our solar system, the gas giant, Jupiter. Again, another few steps out and you come across a similar, but smaller boulder with a beautiful band of colored stones spinning around it, Saturn, the sixth planet. Several more large steps out (you are quite far from where you started now) and you discover the seventh planet, Uranus. Again, taking several more large steps out into the distance and you finally come across Neptune, the eighth planet, quite far from the sun. These last two rocks are smaller than their gas-giant brethren but still of a hefty size.
Being the intrepid explorer that you are, you are keen on discovering the ninth planet! You take some more steps out and with a mix of disappointment and enthusiasm, you pluck a small pebble from the ground. “A-ha!” You exclaim, “the ninth planet!” This is Pluto. However, as you examine and roll the icy-pebble around in your hand, something catches your eye. There’s another pebble near to where you found Pluto, and as you stoop down to look, you notice even more pebbles, all of various sizes and some even larger than Pluto. As you take another step outwards to look at these pebbles you realize you are standing among thousands of little objects. While these are definitely a lot less dense and icier than the previous objects you’ve come across, you can’t help but be reminded of the asteroid belt you crossed awhile back. You have just stepped into the Kuiper belt. Pluto is not an unique, solitary entity that exists in stark contrast to a great, empty expanse, but is rather just one small pebble in a great expanse of similar pebbles. For this reason, you decide you may not have come across a true ninth planet after all.
The case of Pluto remains a sticking point for many people across science, government, and public arenas. However, Pluto’s demotion is a great example of the way science needs to be. There is no room for stubbornness in science, at least not in the dogmatic fashion. Many folks these days, scientists included, mistakenly view science as some static body of knowledge. The popular concept is that things are “figured out”, then printed in a text book for all eternity for future generations to learn from. This is not, and should never be, the case. Science is a fluid enterprise that needs to be ever-changing, ever-correcting, and most importantly, the people undertaking scientific endeavors need to recognize this and allow their “facts” to be changed as more accurate facts come to light. Humanity is in a constant state of learning. Often, the things we learn have the power to change the concepts we thought we knew already. Science is full of examples of this nature. The world is not flat, peptic ulcers are not caused by acid, but by bacteria, and Pluto is not a planet.
What we know now, whether it’s about medicine, natural history, astronomy, or any scientific topic, is subject to change the moment a more suitable, accurate, and well-reasoned/tested set of evidence comes about. The way we conceptualize the universe and the beings in it could change at a moment’s notice and will always be subject to potential change. This, at least for me, is a source of wonder and excitement, not argumentative pettiness!
Our solar system has eight planets, not nine. Pluto is a dwarf planet and rightfully joins the pantheon of other wonderful dwarf planets. Embrace the change!