Pluto is Not a Planet!

As an aspiring astronomer/astrophysicist, I am often asked why Pluto is not a planet. I also get many defiant people telling me they know better than we scientists, and that Pluto is without a doubt, a planet. Well, I have to be honest with you all. Pluto is not a planet. Sorry for the length, but now you’ll know why you’re wrong if you’re shaking your head at this first paragraph in dismay. It’s for your own good, trust me.

After reading The Pluto Files by Neil deGrasse Tyson (I had to, he works a few doors down from me), I have become further convinced that when it comes to planets, Pluto is pretty pathetic. Why? Well, before I get into the “official” definition of a planet, let’s just discuss some properties of everyone’s favorite underdog. First, let’s discuss size. NASA’s Jet Propulsion Laboratory has a measured a radius for Pluto of 1137±8 km—to give some perspective, the radius of the moon is roughly 1737 km, 600 kilometers larger! It’s also about 1/5 of our moon’s mass.

Continuing on size, Pluto has a moon (3 actually, but two of them are really wimpy) Charon, whose size is not much smaller than Pluto. You might think, “Well, Pluto has a moon, so clearly that’s planet-like.” Indeed, you might think that, but it’s more of an embarrassment than a triumph for Pluto. Charon is so big compared to Pluto that the two objects are more of a double-system…this means that they orbit around a point in between the two of them, rather than Charon going around Pluto. Additionally, both Charon and Pluto are “tidally locked” to each other. This means that the same side of Charon alway faces Pluto, and vice versa. This effect is seen with our moon—the same face of the Moon always faces Earth (although Earth is not tidally locked to the Moon, so we see it rise and fall in the sky just like the Sun).

Unlike other planets, Pluto has a very eccentric elliptical orbit, and is inclined out of the plane of the solar system; this is something that none of the planets share with Pluto. Pluto is also more like a comet than it is a planet—it is believed to be around 30% water ice, and if it was placed near the Sun, it would have a tail, just like a comet. If it was placed where Mercury is, it would disintegrate!

Convinced yet? Well, controversy started to stew once a slew of large objects similar to Pluto began to surface in a region known as the Kuiper Belt, at similar distances to Pluto. These objects were very similar to Pluto, and it was hard to decide what to call them. Should they also be planets? Should they be super comets? It didn’t help when one of them, Eris, was found to be even larger than Pluto! That’s right, sink this in—Eris is larger than Pluto. It was soon clear that something had to be done.

The final nail in the coffin came in August of 2006, when the International Astronomical Union redefined the word “planet”. Planets now had to meet 3 criteria (direct quotes)…it must be a celestial body that:

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.

Pluto meets the first two criteria—it is orbiting the Sun, and it is nearly round (hydrostatic equilibrium shape means that its gravity is felt roughly the same anywhere on the surface, and so it is nearly spherical. This is why all massive celestial objects with high gravity are round). But Pluto has not cleared its orbital path, which means that if you add up the mass of all of the objects in its vicinity, the mass is significant compared to that of Pluto’s mass. So what does this mean? Like all the recent large Trans-Neptunian Objects (large objects past Neptune) discovered, Pluto is now a dwarf planet.

Ceres, a large, round asteroid in the asteroid belt, was bumped up. Interestingly, according to Tyson’s book, Ceres was once a planet! When it was discovered that many asteroids, of similar properties as Ceres, existed in the same area, Ceres was demoted to being king of the asteroids. Sound familiar? Well, nobody complained about Ceres, but there was quite a uproar from the community over Pluto’s demotion.

Convinced yet? You should be. Pluto is now, so we think, where it belongs, and it shows you that science is ever evolving as we learn more about objects in our Universe. Indeed, progress is good, not bad. We may have “lost” a planet, but we gained a whole bunch of new, awesome objects in our own backyard, and I think it’s something to be celebrated.

Some size comparisons, courtesy of NASA, ESA, JPL, and A. Feild (STScI). I think the resolution of some of these images may be misleading. The key is to note the size comparisons. Also, read Neil’s book—it’s good.