I don't understand- why isn't the definition of planet based solely on mass?

[-Velocity-]

There's been a lot of debate over where we should draw the line over planet and dwarf planet/minor planet. The IAU's definition of planet in 2006 left much to be desired. Determining from afar whether an object even fits the parameters set out by the IAU can be difficult, and the parameters themselves are tough to define exactly. Numerous alternative definitions have been proposed, such as any object orbiting the Sun and in hydrostatic equalibrium (round). However, the hydrostatic equilibrium definition runs into problems of its own. For example, how round is round enough? And even more troubling, there's the fact that lower strength materials, like ice, will assume a round shape at a lower mass than higher strength materials like rock and metal. For example, Vesta, the second largest body in the asteroid belt, has a mass of 2.6x10^20 kg, but is very much NOT a round body. Meanwhile, Enceladus, a moon of Saturn, is very much spherical but only has a mass of 1.1x10^20 kg- less than HALF of that of Vesta! Personally, I find the hydrostatic equilibrium proposal very unsatisfying for this reason.

Ever since the IAU's definition came out in 2006 and throughout the subsequent controversy, I have been perplexed- why hasn't someone simply suggested a mass range for planetary bodies? Mass is easily measured through gravitational effects, and its the most basic property of any astronomical body. Some may counter that setting a mass range is "arbitrary", but the whole debate of where to draw the line between planet and non-planet is about nothing more than where to draw an arbitrary line anyway. ANY definition of planet we come up with will be arbitrary. At least if we decide a planet is a planet based on mass, then we reduce the problem down to just a single number that is usually quite easy to measure. That single number is also the most basic of all astronomical properties.

Also, our own solar system neatly allows for a clear line to be drawn between planet and non-planet based on mass. The mass of Mercury, the least massive of the current eight planets, is 3.3x10^23 kg. The next most massive known body orbiting the Sun is Eris, at 1.7X10^22 kg- about 1/20 the mass of Mercury (Pluto weighs in at about 1.3x10^22 kg, just a little less than Eris). (In contrast, Earth weighs in at 6x10^24 kg.) So, we could draw the line at 10^23 kg for the planet-vs-dwarf planet line.

It is important to note that several moons weigh in excess of 10^23 kg (Titan, Ganymede, and Callisto). But since they do not orbit the Sun, they would not be considered planets.

Of course, it doesn't have to be 10^23 kg, that's just one suggestion. You could try to put the dividing mass around the range where objects assume a spherical shape (10^20 kg?), but you run into the "problem" that you'd have non-round bodies that are considered planets, and round bodies that are considered non-planets, so if your goal was to segregate round and non-round bodies, you'd fail.

Anyway, does anyone know what the reasons are that a simple planetary definition based on mass has not gained more traction? It seems far and away the most sensible way to define planet vs. non-planet, and I can't understand why it's not floated around more.

Edited October 2, 2014 by |Velocity|

[Dkmdlb]

You don't think that the origin of the body or its position in the solar system is relevant?

[KerikBalm]

So lets say we base it on mass....

Pluto and Ceres can't be planets without the moon being a planet as well.

Problem?

Does a nebula count as a planet all together they are pretty massive?

Of course they don't orbit the sun, and the moon orbits an object that isn't the sun (although we could easily stray into a debate about what constitutes a double planet).

But double planets... what if both of them are below your mass threshold... but combined they are above it?

What makes mass more important to you than shape?

Of course, I also wonder... how round is round enough? Vesta is sort of right on the borderline

[-Velocity-]

You don't think that the origin of the body or its position in the solar system is relevant?

Why should its position in the solar system be relevant? I assume you are talking about the "clearing its orbit" part of the argument? Even if you want it to be relevant, how do you draw a nice clear line? If there was an Earth-mass planet orbiting out way out, perhaps in an orbit similar to Sedna but with perhaps a higher perihelion, it wouldn't encounter other bodies frequently enough to "clear its orbit". By IAU definition, it would not be a planet.

The origin of the body might be relevant if we could easily determine it. How do we tell if a free-floating 8 Jupiter mass object was born in a star system and ejected, or just formed on its own out of gas? Furthermore, how do you tell if a 8 Jupiter mass object in orbit around another star formed more like a stellar companion or more like a planetary companion?

Origin should not be considered because it's too hard to determine. When choosing the definition of a planet, it's important to choose defining characteristics that are easy to determine, otherwise your definition is worthless because you'll never know if an object you're seeing is a planet or not. Nothing is more simple than a single number.

[Dkmdlb]

So lets say we base it on mass....

Pluto and Ceres can't be planets without the moon being a planet as well.

Problem?

Obviously it would have to be something which was orbiting the sun to count. Thus, the definition would be any body over size X orbiting the sun.

[rtxoff]

Obviously it would have to be something which was orbiting the sun to count. Thus, the definition would be any body over size X orbiting the sun.

It's not as easy as that, moon is orbiting the sun too.

[Streetwind]

Obviously it would have to be something which was orbiting the sun to count. Thus, the definition would be any body over size X orbiting the sun.

Ah, but |Velocity| posited here the question why the definition is not solely based on mass. I.e.: no other influencing factors. Not even the parent body. (by the way: even the Sun has a parent body...)

[cantab]

While which physical characteristics to consider and which to not consider is in a sense "arbitrary", it's not the same kind of arbitrary as setting a numeric figure. Why 10^23 kg and not e^115 proton masses, for example?

[Dkmdlb]

Ah, but |Velocity| posited here the question why the definition is not solely based on mass. I.e.: no other influencing factors. Not even the parent body. (by the way: even the Sun has a parent body...)

But in that very post he also said this:

It is important to note that several moons weigh in excess of 10^23 kg (Titan, Ganymede, and Callisto). But since they do not orbit the Sun, they would not be considered planets.

[Beowolf]

Obviously it would have to be something which was orbiting the sun to count. Thus, the definition would be any body over size X orbiting the sun.

In less than an hour you needed to add a second criterion. All you need is to add a third, and you'll be pretty much where IAU ended up.

The definition of "planet" isn't supposed to only apply to our own solar system. Note your definition doesn't allow for double planets, since they orbit one another instead of the sun. Or "rogue planets" drifting through space without being anchored by a sun. Since double planets are obviously not moons, now we need a whole new category with its own rules. How close to the masses need to be for it to be a double planet rather than a planet/moon?

And that's why we don't end up with simple rules. It's a big, complex universe, and exceptions abound.

[ZedNova]

Why isn't a breed of dog defined by it's weight?

[metaphor]

Any classification of physical objects is arbitrary in some way. But the current definition of planet works pretty well, it's pretty clear-cut for all the objects we know about. Whenever we discover something that's not clear-cut, we'll change the definition to better classify things (which is what has happened in the past).

[Camacha]

I feel it should be very simple. Objects that are rounded* and circle a star are planets. Anything that circles a planet is a moon.

And yes, that means we get a boatload of planets all of a sudden, but the current definition seems totally arbitrary and vague. Dwarf planet? Clearing its neighbourhood? Really? So it is a planet, but not a planet? Of course, dwarf planet could still be a classification, but just to indicate a planet of small size or mass.

*under their own gravity, obviously.

Edited October 3, 2014 by Camacha

[ZetaX]

Obviously the definitions will be vague as long as you ever want to distinguish things. That's why one always should leave some freedom to allow to "follow your gut".

For example, yours is bad as now shooting a marble into space, orbiting the sun, adds a full planet.

So it is a planet, but not a planet?

Sorry, but that's not an argument.

[Camacha]

Obviously the definitions will be vague as long as you ever want to distinguish things. That's why one always should leave some freedom to allow to "follow your gut".

For example, yours is bad as now shooting a marble into space, orbiting the sun, adds a full planet.

Obviously I meant objects that are rounded under their own gravity, as is always the case in this discussion, not any old round object. That would be silly.

Sorry, but that's not an argument.

I hardly considered my statement an argument, just an indication of how muddied the current state of things is. Of course, saying something is not an argument is not an argument either.

Edited October 3, 2014 by Camacha

[No one]

Jupiter is 5750.1 times the mass of Mercury. Mercury and Jupiter are both planets. What were you saying about the difference between Mercury and Eris mattering?

[Hannu]

Many have already argued that nature do not work like hierarchic classes. Any possible classification produces problems. If classification have strict limits, there are always some nasty borderline cases or things that not fit to the classification at all. And if the classification is very wide it loses practical usability and becomes empty words.

But it is not only thing and not even the most significant. Human opinions are. IAU is a group of hundreds of people and everyone have their own opinions where the limits should be. Some opinions are based to facts, but different areas of astronomy research different things and need different classification. Some opinions are just as feeling based as everyone's "Pluto have always been a planet, so it should be forever even hundred other similar bodies are not". Even researchers are human beings. Therefore this kind of classification is also a political compromise. They are seldom very reasonable from any single point of view and anybody can show some stupid details or consequences, but there is not better method to maintain the human society as political decision making. At least not yet found.

[peadar1987]

I feel it should be very simple. Objects that are rounded* and circle a star are planets. Anything that circles a planet is a moon.

And yes, that means we get a boatload of planets all of a sudden, but the current definition seems totally arbitrary and vague. Dwarf planet? Clearing its neighbourhood? Really? So it is a planet, but not a planet? Of course, dwarf planet could still be a classification, but just to indicate a planet of small size or mass.

*under their own gravity, obviously.

Then you get into the question of "how round is round enough". Is it to do with the standard deviation of the surface from an average datum? What if a giant impact or volcanic event changes the topography of a borderline planet? Does it then cease to be a planet, even though it might even have gained mass?

What about bodies that are relatively massive, but rotating quickly, causing them to become oblate, like Haumea?

To be honest, I'm a fan of calling everything a planet, and just having subcategories. Pluto, Orcus, Eris, Sedna, Haumea, possibly Vesta, and Makemake are dwarf planets, Mercury, Venus, Earth and Mars are rocky planets, Jupiter and Saturn gas giants, Uranus and Neptune ice giants.

We didn't need to demote the smaller moons of Jupiter and Saturn to less-than-moons once we discovered there were literally hundreds of them. They're all just moons, divided into subcategories.

[Albert VDS]

I feel it should be very simple. Objects that are rounded* and circle a star are planets. Anything that circles a planet is a moon.

So if I would put some spherical stones in a solar orbit, would they be called planets too?

What about spherical stones around Earth, would it suddenly have X amount of moons?

[Superfluous J]

Honestly I don't see the problem with having a couple dozen planets in our Solar System.

Putting Mercury and the Moon in the exact same class makes more sense to me than putting Mercury and Saturn in the same class.

[Albert VDS]

But technically Mercury and Saturn aren't in the same class.

Mercury is a terrestrial planet and Saturn a gas giant.

[Hannu]

Honestly I don't see the problem with having a couple dozen planets in our Solar System.

Putting Mercury and the Moon in the exact same class makes more sense to me than putting Mercury and Saturn in the same class.

It depends on point of view. If you make orbital simulations of the solar system, Mercury and Saturn belongs in the same class, things which have to be taken into account. Pluto is so lightweight and so far away that it is probably impossible to make simulation which have smaller computational errors than error from neglecting gravity of Pluto. On the other hand, from geological point of view Mercury, Pluto and Moon are much more close to each other than Saturn. Most things in bodies structure and surface processes does not depend on what kind of object they circulate.

[NASAFanboy]

I personally rate the planets by "points". For example, a celestial body needs ten points to become a planet, not clearing out its nearby neighborhood would subtract points while having a atmosphere and a moon would add points. It goes somewhat like this system:

+4 Points if Atmosphere Existant

+5 Points if has cleared out nearby region

+1 Points if has Moon

+1 Points if of sufficient mass

+3 Points if of sufficient size

+1 Points if in a reasonable orbit

Of course they can lose points like this

-2 Point if not cleared out area

-1 Point if no atmosphere

-6 Point if not of sufficient size

-9 Point if not of sufficient mass

-1 Point if no moon

-99 Points if a Moon

Personally, I think a modified version of this would be best for figuring out what is a planet and what isn't. Just my opinion.

[Albert VDS]

So by that system the Sun would be a planet too?

[bonyetty]

Also I have read that the planets don't orbit the sun but the center of mass of our solar system, above the suns surface towards Jupiter .So does this mean that our solar system has no true planets as no objects orbit our star? Nature is so lovely messy!