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What Are Some Problems With the Current Definition of a Planet and What Would an Alternate Approach Be?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.


In August of 2006, the International Astronomical Union (IAU) attempted to do what science had not done before: vote on a definition of planethood. This was sparked by the then-recent findings of the Kuiper Belt and the questions it raised about Pluto. Astronomers decided to hash out the details on what makes good qualification for planethood. Out of the 2,412 scientists that were present for the IAU of that year, only 424 were participants in the actual vote. The definition they passed had three criteria: 1. the object has to have hydrostatic equilibrium (be round), 2. it must orbit a star, and 3. it must dominate its area of space by being the prevalent massive object in its zone. It was this last qualification that did Pluto in as a planet and resulted in its distinction as a dwarf planet.

Now, let me be clear about something: two of those guidelines are great and help classify a group of objects. By making a planet something that must be round and orbit a star helps remove moons, asteroids, and comets from the equation. It is the last qualification that earns a lot of flak, and rightly so. What does it mean exactly to clear out the orbital zone? The IAU was attempting to show that the object had to be distinguishable from its surroundings, otherwise it was really a class of objects that orbited the sun. In the past, this happened to Ceres, Vesta, and other asteroids when scientists found more and now the same has happened to Pluto and company. But the IAU needed to make this clear. Because Neptune technically hasn't cleared its zone. After all, Pluto's orbit crosses past Neptune's for decades at a time. Has Neptune therefore cleared its zone? And what about Trojan and Centaur objects caught in Lagrange points? The IAU classification needed a better execution.

Now, to be fair there were plenty of other ideas spread out at the time and since then that are not good at all. Some have suggested making any primary object orbiting a star a planet, but then the Asteroid Belt, Kuiper Belt, and Oort Cloud would have to be included. Others feel anything that has a moon is therefore a planet but Mercury and Venus lack those. What is needed instead is a system that shows trends and helps enlighten us, not get lost in ambiguity or a non-empirical feeling of right vs. wrong.

One would think that turning to exoplanet systems would offer us better insights into general trends to perhaps guide us, but this hasn't been the case. There are simply a wide variety of systems out there, so the IAU tried to keep it simple in 2003 by cutting off anything greater than 13 Jupiter masses (because once past this, star-like fusion can begin to take hold) but anything less than the smallest planet in our solar system (which isn't a clear benchmark). 400 astronomers in 2018 tried to address the wide selection of systems that defy these parameters but updating the definition to make the orbiting object less than 4% the mass of the host star(s). This was established to demonstrate that the relation between how a star forms with its planets needs to be correlated. Also, like the 2006 vote, an exoplanet needs to dominate its area of space. But how can we actually detect this from Earth? What if the exoplanet orbits a small star?

How can the problems of orbiting objects be solved?

An Alternative

So let's get to work. What do we like about current ideas? Orbiting a star is great because it provides a frame of reference, but what about rogue planets, drifting in interstellar space without a host star? Instead of viewing a star as a mandatory component of planet hood, use it instead for naming purposes and in the cases of rogue planets maybe assign name based on stars it was closest to.

We like static equilibrium for sure, for it makes a planet a round object and differentiates it from asteroids, comets and KBO's. But what about objects like Ceres, which seem to push the boundary? What about Vesta, which seems to be layered? And what to do with moons, who not only have these features but in the case of Pluto cause the barycenter to fall out of the central object?



It may be time to ditch the concept of planethood and instead look at orbiting objects as a layered, nestled structure. We could label objects based on the level they are from a star and also the density of the object. Jupiter would be a orbital level 1 object that is 5th from the sun and has several orbital level 2 objects orbiting it. Unlike the Kepler Space Telescope convention of naming based on order of discovery, this alternate system of mine lets the name tell you much about the place of the object and also potential compositional details. If it seems like an area around a star is filled with objects (like an Asteroid Belt or a Kuiper Belt) then that too could be classified by orbital levels. Of course this is an over-simplified version of what would be done and problems would surely arise from it just like ours, but this has the advantage of the classification possibly showing new trends in data just by rearranging the information. Who knows. Like all sciences, it too will evolve.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2015 Leonard Kelley