I’m sure this is not interesting to most people, but I wanted to get it onto the Internet for those who become interested in Pluto’s planethood during the flyby and want more details.
Determining the definition of a planet is really a scientific question. The first step in science is not writing down a hypothesis but rather observing nature, which means describing the things we see, naming them, and breaking them into a classification system. This clarifies our thinking and enhances discussion about unsolved problems. If we have muddled thinking then we will do muddled science.
In defining “planets”, we are faced with a continuum of bodies from dust specks up to giant stars. We have decided that planets and stars are essentially different categories because stars emit light, but drawing the boundary isn’t so simple. Even planets make some of the light that they emit (mostly in the infrared) because of their warmth. That warmth comes from many sources including gravitational collapse and radioactive decay. We have decided that a star is an object that has nuclear fusion as the source of its warmth. Planets are bodies that do not have nuclear fusion. Still, this is not an adequate boundary because there are objects that formerly supported nuclear fusion but don’t any more, and there are objects that will eventually support nuclear fusion but haven’t started yet. Are they planets? A more precise boundary is this: a planet is a body that never has supported nuclear fusion and never will. Those that do not meet this requirement are either protostars (on their way to nuclear fusion), brown dwarfs (which supported fusion of deuterium for a brief period), stars, or stellar remnants (like black holes or neutron stars).
We started with a general feeling that stars and planets are different, then we found a boundary in nature that translates this feeling into something specific. To define the lower end of the planet size-scale we must do the same thing: we have a general feeling that meteoroids (mere rocks) are different than planets, but how do we translate this feeling into something specific? The continuum of sizes includes dust specks, meteoroids, asteroids, and planets. Is there a natural break in the continuum that captures our intuition in a scientifically useful way?
Gibral Basri and Mike Brown, in their paper,”Planetesimals to Brown Dwarfs: What is a Planet?”, discuss several natural boundaries we might choose. They ask, is it a planet when it is large enough so that:
- gravity holds it together as a rubble pile?
- gravity overcomes material strength and pulls it into a round shape?
- internal convection begins in its mantle?
- gravitational compression is enough to drive chemical reactions in the interior?
- gravity causes significant volumetric compression?
- gravity captures and holds enough volatiles that it become an ice giant or gas giant?
- free electron degeneracy pressure becomes important, so the planetary radius begin to decrease with added mass rather than increase?
- fusion begins? (but that makes it a star)
This list is in order of increasing size, so item 1 happens with smaller bodies, but item 8 happens only with larger bodies. Where in this list do we draw the line? Clearly it must be something smaller than item 6, because Earth and Mars don’t meet item 6 and yet they are planets. Clearly, also, it must be something larger than item 1, because a mere rubble pile — a sandbar in space — doesn’t meet our intuition of a planet, which is a single body rather than a loose agglomeration of bodies.
We might also consider these boundaries: is it a planet if it is large enough for
- differentiation to occur, so heavier materials sink to form a core, mantle, and crust?
- gravity to hold an atmosphere?
We can rule those out. As for the first, differentiation can occur for even very small bodies (small asteroids) if they got hot enough through impacts or tidal forces or radioactive decay, and it depends on too many factors so it isn’t measurable for planets in other solar systems. As for the second, planets like Mercury don’t hold an atmosphere and yet our intuition says Mercury is a planet.
So that leaves us with the numbered list between items 2 and 5. Items 3, 4, and 5 don’t seem related to our intuition about the nature of planets. They occur in the interior of bodies and are therefore “invisible” to whatever formed our intuition about it. This leaves only item 2: a planet must be gravitationally rounded.
The International Astronomical Union (IAU) included this as part of definition of a planet. According to many scientists (myself included), this gravitational roundness and the lack of nuclear fusion should be the only requirements in the definition of a planet. Ironically, the gravitational rounding requirement is mere window dressing in the IAU definition, because there are no bodies orbiting the sun that cleared their orbital neighborhoods and are too small to be round.
Eventually we will do the right thing and eliminate the dynamical requirements (a) and (c) from the IAU definition and add the requirement that a planet never has fusion so it applies to other solar systems. The dynamical requirements produce absurd results when we try to compare planets around other stars. This frustrates the comparison of planets, and thus it does the exact opposite of what a scientific classification system is supposed to do. Gravitational rounding and lack of nuclear fusion really are the right requirements for defining a planet.
Even saying a planet must be gravitationally rounded is not sufficiently precise. Gravity changes the shapes of some bodies very slowly, and it might take a billion years to overcome material strength and pull a body into a rounded shape. Stern and Levison, in their paper, “Regarding the criteria for planethood and proposed planetary classification schemes,” recommend a specific timescale in nature to make it even more precise. They suggest that we use the Hubble Time, which is the inverse of the expansion rate of the universe, or about 14.4 billion years. That’s approximately the age of the universe, so basically it says that if a body is big enough to ever round itself out by gravity then it is a planet. This is a slightly more inclusive boundary than saying the body must already be round. Vesta for example used to be round, but it was knocked slightly out of roundness by a giant collision, yet it is still large enough that over a Hubble time it will relax to roundness again according to some estimates. Therefore, Vesta would still be a planet according to Stern and Levison’s refined definition. This detail, like all scientific questions, is open for debate and should be continually reassessed as we learn more. Classification systems are frequently refined in other branches of science, and it should be the same in planetary science, too.
So how many bodies in our solar system meet the size requirements to be a planet? Several hundred that we know of, so far! This includes the eight classical planets, a few of the asteroids (depending on the precise roundness requirement), over a dozen of the larger planets’ moons, and hundreds of bodies beyond Neptune. We live in an exciting time when the Kuiper Belt is overthrowing and rewriting our understanding of the solar system and the nature of planets. Observing nature is cool, because it does that for us.