Defining 'planet' - my suggestion

[major_clanger]

I've been interested in space and astronomy all my life, and I'm a lawyer. As such I've been thinking about the question - back in the public eye again with the Pluto encounter - of what the rules should be for defining a planet.

Here is my suggestion on the 'what is a planet' question in light of what we saw at Pluto.

A planet is a celestial body that:

1) Orbits the sun, not any other body.

2) Has achieved hydrostatic equilibrium (i.e. its own gravity has squashed it to be round).

3) Has one of the following properties:

3a) Is a gas giant*; or

3b) Has a surface modified by self-generated geological processes.

(*A more formal definition might be along the lines of 'more than half its mass is not in solid phase')

Point (1) excludes geologically active moons such as Io, Triton or Enceladus.

Point (2) excludes comets.

Point (3a) is probably obvious but ensures that Jupiter etc count despite not having a 'surface' for the purposes of point (3b).

Point (3b) is what distinguishes a planet from a large asteroid and also excludes Io etc as their geological activity is generated externally (tidal forces from their primary body).

On this definition, Jupiter, Saturn, Uranus and Nepture are all planets by virtue of point (3a). Mercury, Venus, Earth, Mars and Pluto (as seen in the last couple of days) are all planets by virtue of point (3b).

Pluto and Charon probably count as a 'double planet' given that Charon seems to meet (3b) too.

Ceres is not a planet because it doesn't meet (3b). Vesta is not a planet because it doesn't meet (2) or (3b).

Until we see their surfaces, we cannot class Eris, Haumea or Makemake as planets because we do not know that they meet (3b).

This definition returns us to the nine-planet solar system. It allows for Kuiper belt objects to be defined as planets if it turns out that they have been geologically active, but until we send probes to them they remain dwarf planets. Finally, in my view it is not an arbitrary criterion: having an active self-generated geology is a significant factor.

Comments

[mishalak]

Why make the definition so complex? Why not just define it as anything bigger than 10^22 kg* that is not of stellar mass? It seems to me that if an object is large enough to be called a planet it ought to remain a planet regardless of orbital status.

*This is an approximation of durable hydrostatic equilibrium.

[watervole]

I like that definition. It gives a definite point to make a decision on. And geology is a good one. (although allow enough time to pass and a plant might change category as it cools internally.)

[drplokta]

Now define "self-generated geological processes" in a way that includes Mercury but excludes Ceres.

Here's a quote from Wikipedia: "The geological evolution of Ceres was dependent on the heat sources available during and after its formation: friction from planetesimal accretion, and decay of various radionuclides (possibly including short-lived isotopes such as the cosmogenic nuclide aluminium-26). These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation."

That certainly sounds like "a surface modified by self-generated geological processes" to me.

[drplokta]

Check also the timeline of the evolution of Vesta on Wikipedia. It's another planet, by your definition.

[major-clanger.livejournal.com]

Except that Vesta is not in hydrostatic equilibrium, so fails to meet point (2).

[major-clanger.livejournal.com]

I may need to clarify the definition, as by 'modified by' I meant subsequent to the initial formation of the crust. Many bodies will have partly or fully internally differentiated, or will have had surface melting through bombardment. The point I was seeking to make is to distinguish those bodies that then remain geologically active enough to modify their surfaces later.

Mercury and Mars have evidence of relatively recent (in solar system context) vulcanism; Mercury in about the last billion years, and Mars within the last ten million years or so (meaning that it is likely that Mars still has sporadic volcanic activity).

In essence, my proposed division is between bodies that may have started off or initially have become hot, but having cooled have only had their surfaces modified by impacts, and bodies that have gone on to have active surface-affecting endogenic geological processes.

[cartesiandaemon.livejournal.com]

That seems fairly good to me, it seems to encapsulate roughly what many people intuitively mean.

Although I would be ok with leaving it nebulous and letting common usage sort it out, (or omitting #1).

[davidwake.livejournal.com]

Why not simply 'spherical' and orbits a star?

[drdoug.livejournal.com]

I salute your noble efforts here, even if I am entirely unmoved by their implicit aim.

Alas, I think your criteria suffer somewhat from looking a lot like they're drawn specifically to include the nine that were planets when we learned about them as kids, and exclude any later interlopers, which doesn't seem right.

I take your point that having a self-generated geology is a significant factor, but unless you're going to say it has to have a currently-active geology (which I fear would rule out Mercury, and possibly even Mars), you have to have some sort of arbitrary cut-off date that is going to be hard to police.

Also, your ruling that an object can't count as a planet until we have sufficient data to rule on whether it has (ever?) had self-generated geology means that Pluto only became a planet this week, which I don't think can be right.

Of course, the current official rules also suffer from this sort of problem - 'cleared its orbit' is a fairly nebulous concept when you stare hard at it, and requires lots of exceptions and fudgings to get the 'right' answer. Sure, there's a sharp difference between the undisputed eight and Pluto when you measure the ratio of the object's mass to the mass of other known objects within in its orbit. But that is quite possibly happenstance in our particular system. And also leaves things that are definitely planets (like Earth!) vulnerable to being downgraded in the (admittedly very unlikely) event that we find a previously-unknown very massive object in a massively elliptical orbit.

I really don't see what's so wrong with just sticking with criteria (1) and (2) and being done with it. A planet is something that goes round the Sun and is big enough to make itself round. This is nice and simple, clear to apply, and matches most people's naive idea about what a planet is.

Sure, that means there's now a lot more planets than there used to be. And the Kuiper Belt planets don't orbit the same way the inner planets do. But that's a sign of scientific progress! We're also learning a lot more about planetary formation from exoplanets, and our idea that the C20th conception of the Solar System is "normal" is looking increasingly parochial. Which is great and really cool!

"Goes round the star" and "is big enough to make itself round" are nice clear criteria we could apply to planets in other systems as we learn more about them. The other stuff looks parochial, arbitrary, and un-necessary.

[doubtingmichael.livejournal.com]

I think I agree with what Dr Doug says. Despite your statement that it's a "significant factor", (3b) feels rather arbitrary, and it implies that two objects of the same size, following similar orbits, might be one planet and one non-planet. It also means that we can't talk about "exoplanets" until we have investigated their geology, which I think is a long way off.

[davidwake.livejournal.com]

Could we possible modify the round clause to be some arbitrary value of g, thus:-

1. A planet goes around a star.
2. Either it's a gas giant or you can walk on it.

[drdoug.livejournal.com]

You can walk on Pluto! Assuming you can get there and have a suitable spacesuit on, of course.

You'd have a pretty spectacular bounding gait. But you don't have to worry about accidentally jumping off it. You're not going to be able to reach escape velocity under your own power. See e.g. http://space.stackexchange.com/questions/2741/could-a-human-reach-escape-velocity-by-jumping-from-the-surface-of-ceres-a-dwar Note that Ceres' gravity is about 0.3 ms^-2 and Pluto's is a beefier 0.7 ms^-2, or about 1/12 of Earth's 9.8, or a bit under half of the Moon's 1.6, which is about 1/6 of the Earth's.

For something small enough to leave by jumping or even walking, you want an asteroid or comet.

I haven't done the sums but I can imagine it being possible to have 'gas giants' in other systems that are actually quite small. And determining whether or not they meet our esteemed host's original "more than half its mass is not in solid phase" definition of 'gas giant' is going to be impossible for many exoplanets until we get probes there. (And I wouldn't go to the wall for our current understanding of the innards of the four Solar System gas giants being correct. We're still finding surprises about the insides of the Earth, and that's waaay more accessible for science.)

So if we're going for some arbitrary value of g as a cutoff, why not go for one that doesn't measure it by human capabilities, or by metrics that require detailed close-up scientific investigation? We could pick one that measures against a feature readily observable about the object itself. That way we don't have to send people or probes there to decide. It's essentially a proxy measurement for mass. Hey, here's an idea: we could decide based on whether or not it's big enough to squish the planetary body in to a round ball? :-)

[doubtingmichael.livejournal.com]

It occurs to me that if we lived on (say) Saturn - which I appreciate is a big if - then we wouldn't be having any of this debate. We'd just say "The solar system has four planets, plus some chaff".