Take a look at these five oblate spheroidal spacey things and tell me which one is the odd one out.
Not sure? I’ll admit, it’s not too easy. They’re all unique in their own ways. One of them has a dense atmosphere. Four of them contain large amounts of subsurface water. One of them has more water than Earth does. One has lakes of liquid on its surface. Two show evidence of cryovolcanism. One has an extended atmospheric haze layer which is much larger than Earth’s.
Would it help to see them all to scale?
As it happens, the odd one out is the one to the lower left. That grey, rocky orb is Mercury, and it’s the only one which is a planet. It isn’t the largest of the group, and it’s arguably the least interesting, yet it holds a privileged place in our Solar System, being one of the Sacred 8.
In discussions about defining planets, a lot of people ask, “Why does it matter?” An important question, and one which I think is a little overlooked. Between people fighting over Pluto, and people fighting to maintain the status quo of 8 planets, the actual reason for definitions gets lost sometimes.
The thing about a good definition of something is that it lets us have some idea of what it is we’re looking at. All definitions are arbitrary and human-made. Natural phenomena don’t care what we call them. They’re supposed to be made by and for us, to help with our view of the Universe and what’s in it.
To me, this is the point of trying to properly define planets. Those five objects up there are all fascinating in their own ways. And it comes down to more than just mass and orbital characteristics. Consider Enceladus and Vesta to the right, here here. They’re both roughly the same size. One orbits a planet, and one orbits the Sun. But just by looking, you can tell that they’re very different objects.
The thing is that the Universe doesn’t play by any set of rules. When new discoveries are made, they’re frequently surprising. Things aren’t always clear cut, and there are always borderline cases. But knowing loosely where to make a distinction helps us as observers.
In a similar way, hand someone a box of crayons and they’ll probably be able to pick out a red and an orange for you. But if you ask someone to point to the exact place on a spectrum where red stops and orange begins, your mileage may vary★.
When defining anything, you need to use its characteristics, and these can be conveniently subdivided into two types. Intrinsic characteristics and extrinsic characteristics. Intrinsic characteristics originate solely from the thing itself. An object’s colour, mass, and shape, for instance, are all intrinsic. Extrinsic characteristics, on the other hand, originate from outside the object in question. An object’s location, and the energy it receives from elsewhere are examples of extrinsic characterstics.
So how does this all apply to planets? Well, the thing is that the current definition for a planet is needlessly convoluted, using both intrinsic and extrinsic characteristics. In case you missed the background, you can catch up here. Long story short, the current definition of “planet”, according to most, meets the recently proposed geophysical definition:
A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameter.
As well as the astronomical definition:
- Orbits its parent star.
- Dominates its orbit in terms of mass and orbital distance.
- Would clear out any debris in their orbit in well under 0.1 billion years.
- Its orbit, barring any outside influences, will be stable as long as its star exists.
The problem in this, as some of us see it, is the use of extrinsic characteristics in the definition. There seem to be 3 spanners in the works here.
A stray planet is not a planet
The galaxy is a big, chaotic mess of dancing stars. As they orbit the galactic centre, they stray and occasionally interact with each other. Sometimes they come close enough to perturb each other gravitationally. Tidal interactions happen. Planetary orbits get tugged and tweaked. If two stars pass close enough to each other, a planet may be ejected into space.
Except it’s not called a planet anymore. Its planet card has been revoked. Rogue planets aren’t really planets at all. I want my money back!
In astronomy, there’s a weasel term sometimes used – planemo or PMO (an abbreviation of “planetary mass object”). Technically, a planet is actually a sub-class of planemo. But this feels like a needless subdivision, and it’s questionable whether this nomenclature would be necessary if we simply referred to these objects as planets.
A planet orbiting a planet is not a planet
This is something which has always bugged me. In astronomy, I started out looking at stars. Stars frequently orbit stars. In fact, there are a huge number of multiple star systems out there. At no point has anyone seen fit to question whether or not these all qualify as stars. Mostly because that would be ludicrous.
Similarly, galaxies orbit galaxies and asteroids orbit asteroids. In all of astronomy, the only class of object which isn’t allowed to orbit another of its kind is a planet. This is not logical.
Planetary requirements vary dramatically
This seems to be the central point which I’m refuting. As I pointed out in part I, this means that the definition of a spheroidal planet sized object actually changes according to where that object is. I’ve spent awhile thinking about this, and I can’t think of a single other physical phenomenon anywhere else in science whose definition is dependent upon its location♣︎. Feel free to leave a comment if you can think of one.
Additionally, subdivide any physical phenomenon into its component parts, and they too are defined by intrinsic characteristics. Clouds on Earth are made of water droplets. An iron nail is made of metal crystals, made of atoms, made of subatomic particles. All of these things are defined intrinsically. An iron atom doesn’t cease to be an iron atom because you remove it from its crystal lattice, and a neutron doesn’t cease to be a neutron if you pluck it from an iron nucleus.
Personally, I’d be much more comfortable with a definition based on intrinsic characteristics, such as the geophysical definition I included above. The criticism of using the geophysical definition alone is, apparently, that it’s too inclusive. We’ve catalogued over 4000 comets, with estimates for the total of up to 100 billion. It’s estimated there are 1.9 million asteroids over 1 km in diameter in the main belt alone. But 110 planets is too many? I find this argument… unconvincing.
However, there’s one line in the proposed geophysical definition which I’m unsure about – “a spheroidal shape adequately described by a triaxial ellipsoid” has obviously been included so that the dwarf planet Haumea would be included. This would also include the asteroid Vesta (shown in that image above). I think it’s fair to say, Vesta’s claim to planethood is a can of worms best saved to open in a later blog post.
Of course, there is one thing the current astronomical definition can tell us about an object, and that’s its formation and history – which is what I intend to discuss in part 3 of this series. Thanks for reading!
In the meantime, for no particular reason, here’s the most hilarious photograph of Alan Stern that I could find❉.
★ Ultimately all photons are very much the same. Minuscule packets of electromagnetic radiation travelling at 2.998 × 10⁸ m/s. But still, we categorise them. Infrared. X-ray. Optical. Radio. Not just for the way we experience them, the way we perceive colours, but the way they interact with the rest of the universe. Infrared photons and ultraviolet photons may be relatively similar in energy, but their effects on matter which they interact with is vastly different.
♣︎ You could, I suppose, argue cause and effect to some extent. For instance, an Auger electron is specifically an electron ejected from the inner shell of an atom due to the Auger effect. But it’s still unquestionably an electron. And it should be called the Meitner effect anyway…
❉ I’d look for a hilarious picture of Ethan Siegel, but to be honest, it seems difficult to find one which isn’t hilarious in one way or another.