Obligatory Planet Post

I have this vague memory of kindergarten, or elementary school, or sometime, when all the other kids were really into the planets. Me? I really couldn't care less. They seemed so... boring. I simply didn't care which order they came in, or when they were discovered, or what color they were, or whatever other nonsense people could come up with to talk about regarding these spheres with funny names.

It naturally follows that fifteen or so years later I decided to become an astronomer.

Now when I hear about "planets," I don't normally think first of our own Solar System. About 200 planets orbiting stars not our own have been discovered to date. As much has been speculated, but very little is actually known, about planet formation, it is very interesting indeed to think we might someday be able know what distributions of mass, distance from the parent star, brightness of planet star, etc. etc. planets take on. But our own Solar System? It's essentially one data point. Granted, it's the data point we know (and care) the most about, and since it's the one we can study the most about, we certainly should.

And so this week I have been getting a bit of a deja vu: "do you think Pluto should be a planet?" Well, the thing is... I don't really care. Neither, really, does any other astronomer. It's a definition. Nomenclature. Convention. Perhaps if we knew exactly how bodies orbiting stars formed then we could discover there is an obvious way of defining a "planet," but, well, we don't.

Personally, I think I like the current IAU proposal (I've posted the text of the actual proposed definition below). Like most people, my reaction went something like: "Ceres? ... Charon?!" But after reading the proposal, it mostly makes sense now. I think I can see what they were thinking, which is... good.

The reason I like the proposal is two-fold. First of all, I like that it acknowledges the eight "classical" planets. The terrestrials (Mercury, Venus, Earth, and Mars) and the gas giants (Jupiter, Saturn, Uranus, Neptune) just seem like the should be considered separately from the other minor bodies in the solar system. My real attraction to the phrase "classical planets," however, is that these are not the classical-in-the-real-historical-as-in-Greek sense: the word "planet" originally meant "wandering star" and referred, understandably, to the objects that moved across the sky in a detached manner from the background stars. The original seven planets were the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn. Why else would there be seven days in the week?

My second reason for liking the proposed definition is that it appears to be physically motivated. Even if the proposal is not approved, people around the world are not merely aruging over "oh no! they're demoting Pluto!! poor pluto!" but actually being forced to think about the physical differences between different kinds of objects. Why is our Moon not a planet, but Charon is? (Hint: the center of mass for the Earth-Moon system is beneath the Earth's crust, while the center of mass for the Pluto-Charon system is not located inside either rock.) And, of course, it amuses me to no end to think of all the hapless souls in the non-scientific general public who, for whatever reason, have "always been interested in astronomy" and are now actually being forced to think a little bit about the conventions and panoramas they were taught in third grade.

Now, I didn't go through the "dinosaur stage" when I was six years old, either, but at least no one can count the current number of dinosaurs on their fingers. On the other hand, not everyone openly acknowledges that the dinosaurs actually existed ...

Current proposed definition of planet from the IAU:
Draft Resolution 5 for GA-XXVI: Definition of a Planet

Contemporary observations are changing our understanding of the Solar System, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries force us to create a new definition, which we can make using currently available scientific information. (Here we are not concerned with the upper boundary between "planet" and "star".)

The IAU therefore resolves that planets and other Solar System bodies be defined in the following way:

(1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape[1], and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.[2]

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a "dwarf planet."[3]

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian
Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutons".

(4) All non-planet objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".[4]


Notes:

1 This generally applies to objects with mass above 5 x 1020 kg and diameter greater than 800 km. An IAU process will be established to evaluate planet candidates near this boundary.

2 For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet.

3 If Pallas, Vesta, and/or Hygeia are found to be in hydrostatic equilibrium, they are also planets, and may be referred to as "dwarf planets".

4 This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets. In the new nomenclature the concept "minor planet" is not used.