Ah, Pluto. Everyone’s favorite (dwarf) planet. For some reason unbeknownst to the general public, scientists stripped Pluto of its planetary status in 2006. Many Millennials seem to feel personally attacked for this demotion. They dramatically claim that losing Pluto is like losing a member of our interplanetary family.
Unfortunately, scientists are right. Pluto is not a planet.
Discovering Pluto
In the 1840s, scientists noticed that Uranus’ orbit was inconsistent with predictions from the physics they knew of at that time. Using math, they concluded that another planet must be out there. Scientists pointed their telescopes to the sky and found Neptune. Even after finding Neptune, many felt that another planet must be out there because Neptune didn’t seem to solve all the problems with Uranus’ orbit. Since it had worked once before, they once again pointed their telescopes to the stars. And they found something.
Scientists noticed something that was not a far away star—something close. Something orbiting the Sun. They assumed they had found a new planet because astronomers didn’t yet understand the structure of our solar system. They thought that Pluto was much larger and much farther away. However, as time went by, they calculated an increasing small mass for Pluto.
Eventually, astronomers realized the calculations that led to Pluto’s discovery were wrong. There were no problems with Uranus’ orbit. And even if there was, Pluto’s tiny mass wouldn’t account for this difference.
As time went by, astronomers found more Pluto-like objects in the outer solar system, including Eris. A dwarf planet discovered in 2003, Eris is more massive than Pluto (although a little smaller). This forced astronomers to re-evaluate their definition of a planet.
Defining a Planet
In 2006, scientists gathered at The International Astronomy Union’s General Assembly and tackled the question plaguing astronomy at the time: what defines a planet? They decided that a planet must do three things:
1. Orbit a star.
Pluto does this. Check.
2. Be massive enough to hold itself together in a round shape.
Pluto does this too. Check.
3. Dominate its neighborhood.
Pluto does NOT do this. Pluto lies in a portion of the solar system known as the Kuiper Belt, a region that contains small icy bodies, like a second asteroid belt.
To be fair, Pluto is a fairly large object for it part of the Solar System; it’s the second most massive non-planet orbiting the Sun. That being said, some moons are larger than Pluto. Looking at the image above, Pluto definitively does not dominate its neighborhood. Scientists ultimately demoted Pluto (and Eris) to a “dwarf planet.”
More evidence
Tilt with the ecliptic
All eight of our planets lie within the same plane of the solar system called the ecliptic plane. However, Pluto is 17 degrees off of this plane. The only other planet to be off the ecliptic is Mercury, but this difference is easily explained by General Relativity. So the question remains: how did Pluto get off the ecliptic?
One theory is that Pluto may have collided with another object, knocking it out of the plane. Another theory is that Pluto may be a captured satellite from a different solar system. Either way, this orbital tilt is very weird for a planet, but quite normal for a Kuiper Belt object.
Charon and Pluto
Another damning piece of evidence comes from Pluto’s moon, Charon. Charon’s mass is about one-eighth of Pluto’s mass, which is relatively large for a moon. For comparison, our moon is only 1.2 percent of the Earth’s mass. This giant moon doesn’t actually orbit Pluto; instead, Pluto and Charon orbit a spot in between them, outside of either body.
Some feel that Charon and Pluto should be considered a binary system. However, Charon’s official classification is one of Pluto’s Satellites.
It is important to note that Pluto’s moons don’t make it a planet. 87 Pluto-like objects are also known to have moons. Also, not all planets have moons; Mercury and Venus don’t.
Sorry folks, Pluto is not a planet
When you look at all the evidence, it’s clear that Pluto isn’t a planet. Pluto was only considered a planet because we didn’t know what our solar system looked like. Once we learned more, we needed to reclassify certain objects. That’s what science is all about—learning and adapting.
Pluto didn’t change, it’s official classification did. This, of course, doesn’t mean that we should forget about Pluto. There is still much Pluto can tell us about our solar system; that’s why scientists continue to study the dwarf planet. However, to say that Pluto is as important in the solar system as the planets is completely false. Regardless of what we call it, Pluto is undoubtedly one of the most loved objects in the solar system.
madison brewer 