Planets and Things

I’m a guy who likes to make lists and put things into categories. Give me a bucket of wheat cents and I’ll be occupied for hours. Give me a solar system with millions of objects, in a wide variety of shapes and sizes, and I’m ready to get to work. Give me a universe with strange and unexpected things being discovered or theorized every day, and I almost don’t know where to begin!

Classification is important because we need a common language. How can we have a meaningful conversation about planets if your definition of a planet is different from mine? The same goes for stars, dwarf planets, brown dwarfs, black holes, and so on.

The IAU has made attempts recently to clarify this situation. The most newsworthy outcome has been the reclassification of Pluto from planet to dwarf planet. Ceres has also been promoted from largest asteroid to dwarf planet. A few of the larger Kuiper Belt objects have been designated as dwarf planets as well. However, this classification only applies to our solar system. With 453 extrasolar planets discovered, some of them not much bigger than Earth, we are very soon going to need a classification system that applies to any solar system, or more generally, to any objects in the universe.

Many astronomers want to classify things according to the way they formed. An object that forms by accretion of a dust cloud would be different than one formed by collision of two large objects. A stellar remnant (what’s left of a star after its thermonuclear reactions have stopped) would be different from a brown dwarf of the same mass. The trouble with this is that it may be difficult to determine how an object formed. Our classification system should make clear distinctions based on observable properties.

Others would like to classify things in terms of their orbits. A planet, for example, must be orbiting a star. Under this definition, a planet that is gravitationally ejected from a solar system is not a planet any more. But it’s still the same object!

There is even a requirement that a planet must have cleared the neighborhood of its orbit of any debris. This would be really hard to verify without a very thorough survey.

Some rely on the shape of an object to determine its classification. Anything bigger than about 400 km in diameter gets pulled into a roughly spherical shape by its own gravity. These are sometimes called worlds. Objects less than 200 km tend to be much more irregular. Between 200 and 400 km we find a wide variety; some very spherical, some very irregular. The shape is generally hard to determine, until you get close.

What observable properties are easiest to detect? First would be thermonuclear reactions. That’s one point most astronomers agree on. A star is an object that is emitting radiation due to thermonuclear reactions. It’s thought that an object with more than about 13 times Jupiter’s mass will have enough pressure at its core to sustain the reaction that fuses hydrogen into deuterium. That factor of 13 depends on a lot of things, like chemical composition. In practice, it’s usually pretty easy to tell a star from a non-star. Although there are borderline cases!

Another property that is usually one of the first to be observed is mass. We can determine the mass of an object by the orbital characteristics of anything near it. Properties like size, shape, chemical composition and solidity of surface are harder to determine. You have to get close.

Also, the word dwarf is overused. A dwarf planet is smaller than a planet, but a brown dwarf is a big planet. Red dwarfs and white dwarfs are small stars. There are even dwarf galaxies!

Here’s what I humbly suggest. We classify objects and orbits separately. Any object can be in any orbit. Never mind how it got that way. Objects should be classified by thermonuclear reaction, then mass. A planet is just a non-thermonuclear object with mass greater than, let’s say, one-half Mercury. Wouldn’t that clarify the discussion?
This doesn’t apply to exotic objects like black holes. We can cross that wormhole when we come to it. And we will probably need something besides mass when we want to look at interplanetary or interstellar dust particles of various sizes. We can expect to find all imaginable shapes and sizes of things out there, and even some unimaginable!