I still see comments from people on the net that claim that no matter what anyone else says, Pluto is a planet. To them, anyway. Generally, these people aren’t scientists, and their opinions don’t really affect anything else. But, those opinions remain unshaken, in any event.
This brings me back to language coloring perception. We hear the word “planet” and that brings certain images to mind, such as photos of Earth, Mars or Mercury. But, the classification “planet” was originally kind of a dumping ground for any celestial phenomena that wasn’t initially recognized as being something else. That is, “planet” as a definition for a specific kind of thing hasn’t ever really been all that clear.
In science, you want to know what makes two things similar, and what differentiates them from each other. If I give you two perch, you may look at them and say, “they’re the same kind of fish, they’re both perch”. Looking closer, you may notice that one is male and the other is female. Or, if I give you one perch and one trout, you’ll still say they’re both fish, but they’re not the same kind of fish. Why is it important that they aren’t the same kind? Or rather, why is it important that they should be the same kind? Because having two fish that are capable of breeding with each other gives you more fish. Why is it important that we know that there is a similarity? Because if they’re fish, and you’re holding them in your hands, regardless of whether they’re perch or trout, they’re both either dying as we speak, or already dead. Classifying the two things under the same heading (“fish”) and then further classifying them under separate headings (“perch” and “trout”, “male” and “female”) is relevant to our understanding of them.
Look at the night sky. We have small, dirty snow balls, small iron pellets or rocks, larger rocks with iron or nickel cores, big rocks made up of various materials, gas balls, liquid balls, balls of incandescent plasma, balls of compressed matter, dust, radiation, and photons and other subatomic particles. Some of these things are easier to differentiate than others. Big balls of incandescent plasma are not like small dirty snowballs. So, it makes sense to create a classification “star” for plasmatic kinds of critters. And, within this group you have properties that include age, ingredients, mass and best use by date. So, knowing how young, hot, massive or complex the star is can help you figure out other things like how far away it is, what direction it’s traveling, how long it’s been traveling, and what kind of things are surrounding it.
So, let’s jump to the next point. Stars can have accretion disks, that is, dust and gas that surrounds the star. Over time, the dust in the disk is going to clump and form rocks, or ice and dirt balls. What are the differentiators here? Comets have eccentric orbits, they’re not very dense, and they can have tails formed when the ice or dust melts or is blown off by the force of the stellar wind of that star. Asteroids are large solid rocks or metallic clumps. They can be very small, or up to several miles across. So, yes, comets aren’t asteroids and there’s a reason for identifying them as such.
If the clumping gets serious enough, the object can develop sufficient mass to have its own gravity. As the pull of the object’s gravity increases, it attracts other neighboring clumps that smash into it. So, at some point an asteroid can become a planet. And this is where the current classification “planet” breaks down. Mercury is only slighter larger than the Moon, while Neptune is a gas giant 17 times more massive than Earth. Neither Mercury nor the Moon have atmospheres, while Neptune and the Earth do. Earth can support life, as may Saturn’s moon, Enceladus. While the Earth, Mercury and Neptune orbit around the Sun, the Moon orbits around Earth, and Enceladus orbits around Saturn.
Which of these differences are significant enough to justify separate classifications? Obviously, a rock that orbits around another rock can affect the tides between the combined pair, and the smaller rock, if it orbits fast enough, can act like a broom to sweep out all the other rocks to protect its bigger sibling. So in this sense, the classification “moon” is justifiable. But still, “planet”? Just exactly WHAT is so important about a planet that we don’t just call it a “class XX asteroid”?
Atmosphere isn’t important because not all planets have one. Water isn’t important for the same reason. Mass? Mercury, Pluto, Ceres and the Moon are almost indistinguishable on that count. The potential for harboring life? That just gives us Earth, and possibly Mars or Enceladus (two planets and a moon).
What exactly do we want out of our planets that makes them not moons, not asteroids, and not comets? The International Astronomical Union came up with the following definition:
1) is in orbit around the [its] Sun,
2) has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3) has “cleared the neighborhood” around its orbit.
So, what’s somehow important here is that the planet has to be mostly round, has to orbit its star, and can’t be a part of a “belt”. Which means, you can have things that are round and don’t orbit the sun (moons, rogue planets); that orbit the sun and aren’t round (comets, asteroids); or, that orbit the sun, are round, but are part of a belt (dwarf planets).
Is this classification system justified? Do “planets” do something special that we really care about? Or, is there something that we really do care about that is getting ignored in emotional baggage? Giant planets are good for determining if distant stars have planets, based on gravitational pulls or light dimming (as the planet crosses in front of the star). And yes, we can assume that “goldilocks planets” (which are going to be smaller than giants and less likely to be detectable from a distance) support alien life. But, things like Mercury, Pluto or Ceres don’t provide these kinds of benefits, just because they’re round and orbit a plasma ball. Personally, I don’t see the classifications “planet” and “dwarf planet” performing significant functions as-is. It might be better if we stepped back and tried looking again to see if we have a trout and a perch, or just a trout and one really ugly trout.