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

[KerikBalm]

NASAfanboy... define an atmosphere?

Triton clearly has one... enough for clouds and percipitation to form... but it is less than 1/70,000th that of Earth's at sea level...

Does it count?

Presuming Triton to be very similar to Pluto, orbiting at a similar distance from the sun, Pluto will likely have an atmosphere like Triton's... does it count?

Do we take into account that the colder something is, the easier it is to hold on to an atmosphere?

[NASAFanboy]

NASAfanboy... define an atmosphere?

Triton clearly has one... enough for clouds and percipitation to form... but it is less than 1/70,000th that of Earth's at sea level...

Does it count?

Presuming Triton to be very similar to Pluto, orbiting at a similar distance from the sun, Pluto will likely have an atmosphere like Triton's... does it count?

Do we take into account that the colder something is, the easier it is to hold on to an atmosphere?

The atmosphere must be of a certain PSI to be even counted as a atmosphere; for this case, Pluto's atmosphere is a little controversial on whether it counts or not. Tritons atmosphere counts, for Triton (Atleast, by my standards, but I'm not in the IAU). A atmosphere must have multiple layers, be able for clouds to form, and be of a certain pressure. The points for a atmosphere are lowered by distanced from sun, so yes, that is taken into account.

Edit: Checked on Triton, it's atmosphere doesn't count. For Pluto, we need more information before we can reclassify as Planet.

[KerikBalm]

So what are you standards?

What is the PSI limit?

If we set it to 0.01 x Earth's atmosphere, then Mars doesn't even count - just the gas giants, Earth, and Venus....

[running]

The short answer:

Everything is relative. So if you use mass as the deciding factor... how would that hold up in another solar system with a much larger or smaller star?

Let's just call everything that orbits something a satellite.

[cantab]

If you want to draw a lower limit for a "proper" atmosphere, I'd say the requirement is that it should be dense enough to have a layer below the exosphere. In other words, a layer where any given gas particle is more likely to collide with another gas particle than to hit the ground or escape.

Triton, by that standard, has a proper atmosphere, while Mercury does not.

[-Velocity-]

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 moon exception was in the original definition I made. However, personally, I'd be perfectly fine saying that ALL objects beyond a certain mass are planets. Even if they orbit a body much more massive than them. Our own solar system proves that moons orbiting gas giants are very interesting worlds of their own; there is far more interesting stuff going on on Titan, for example, than on Mercury, even though Titan is only about 1/3 the mass of Mercury.

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

We don't end up with simple rules because we demand that planets have to fit certain preconceptions. But mass is a single number. And, if we really wanted planetary sized moons to not be planets, then it would be simple to come up with rules about whether an object is a double planet vs. a regular planet + moon. Here's a very simple one- just use the ratio of masses. For example, if a moon was less than 1/20 the mass of the body it orbits, then it's a moon, but if more, and both objects exceed the minimum mass for planets, then you consider the system a double planet. Yes, you could theoretically end up with Neptune-mass moons orbiting Jupiter+ mass planets, but that's the price you would pay for the insistence that moons cannot be planets.

[-Velocity-]

The short answer:

Everything is relative. So if you use mass as the deciding factor... how would that hold up in another solar system with a much larger or smaller star?

Let's just call everything that orbits something a satellite.

There may be some merit in the idea of just admitting that trying to draw a line anywhere is so arbitrary as to be unscientific. Astronomers and anyone educated in astronomy understands that there is no clear line between planet and non-planet. In a very real sense, the term "planet" means more to the general public than to astronomers.

I sometimes do astronomy outreach to elementary school students. Because of the curriculum in this state, I usually end up talking about the planets. When I do, I find that the students have heard more about dead, boring worlds like Mercury, or objects we know very little about (like Pluto), than about living, interesting worlds like Europa, Titan, or Enceladus. What happens is that when astronomers arbitrarily pick which worlds are "planets", then that leads to the general public being educated on those objects to the exclusion of other, more interesting objects. I usually spend most of my time talking about the moons of Saturn and Jupiter, since the kids were never really educated on them.

Maybe it would make things better if we switched from focusing on planets to focusing on worlds, which would include all the planets plus the large moons, Ceres (maybe Vesta too), Pluto, Eris, Makemake, maybe Sedna, etc.

[metaphor]

There may be some merit in the idea of just admitting that trying to draw a line anywhere is so arbitrary as to be unscientific. Astronomers and anyone educated in astronomy understands that there is no clear line between planet and non-planet. In a very real sense, the term "planet" means more to the general public than to astronomers.

I sometimes do astronomy outreach to elementary school students. Because of the curriculum in this state, I usually end up talking about the planets. When I do, I find that the students have heard more about dead, boring worlds like Mercury, or objects we know very little about (like Pluto), than about living, interesting worlds like Europa, Titan, or Enceladus. What happens is that when astronomers arbitrarily pick which worlds are "planets", then that leads to the general public being educated on those objects to the exclusion of other, more interesting objects. I usually spend most of my time talking about the moons of Saturn and Jupiter, since the kids were never really educated on them.

Maybe it would make things better if we switched from focusing on planets to focusing on worlds, which would include all the planets plus the large moons, Ceres (maybe Vesta too), Pluto, Eris, Makemake, maybe Sedna, etc.

The current classification of a planet is based mainly on orbital characteristics. "Clearing the neighborhood" is a pretty good dividing line between planets and non-planets in the solar system, it's very clear-cut (look at something like the Stern-Levinson parameter), it's generalizable to exoplanets, and it has to do with the way planets form in the first place.

But you could also make another classification based on physical characteristics. For example, a "world" could be anything that's large enough to be rounded by gravity, and isn't a star. So Pluto and Ceres would be worlds (but not planets), and so would Europa and other large moons.

I do think kids in school should learn, along with the planets, at least the 7 major moons in the solar system (Moon, Io, Europa, Ganymede, Callisto, Titan, Triton), since they're relatively unique objects that each have a different story. Also the Kuiper belt along with the asteroid belt. Pluto could be given as a representative KBO once we image it close up next year.

[Camacha]

The current classification of a planet is based mainly on orbital characteristics. "Clearing the neighborhood" is a pretty good dividing line between planets and non-planets in the solar system, it's very clear-cut (look at something like the Stern-Levinson parameter), it's generalizable to exoplanets, and it has to do with the way planets form in the first place.

You keep on saying that, but it is not accurate. This classification is written for our solar system in its current state and that almost certainly makes it a hampered definition. Also, when you look at the numbers, the spread is rather big. The difference between Mars and Jupiter is about as big as that between Mars and Eris, depending on what type of scale you use. Not as clear-cut as it is purported to be by a long shot.

Edited October 4, 2014 by Camacha

[NERVAfan]

I agree that origin shouldn't count for planet vs brown dwarf since it can't necessarily be determined. I think the 13 Jupiter masses thing is good enough.

I don't really mind Pluto being "demoted", but clearing the neighborhood is IMO problematic since (as I understand it) really far away bodies wouldn't have had time to clear their neighborhood - a Mars-size body in a Sedna-like orbit wouldn't be a planet.

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?

IMO, if the barycenter is outside the more massive body it's a double planet, if it's inside the more massive body it's a planet/moon system.

[metaphor]

You keep on saying that, but it is not accurate. This classification is written for our solar system in its current state and that almost certainly makes it a hampered definition. Also, when you look at the numbers, the spread is rather big. The difference between Mars and Jupiter is about as big as that between Mars and Eris, depending on what type of scale you use. Not as clear-cut as it is purported to be by a long shot.

It is written for our solar system since that's the only one we have any data on. We haven't discovered any small bodies around other stars so it doesn't make sense to have a definition based on a supposed distribution of bodies that might or might not exist. If we get a lot more data on other stellar systems and find that our current definition isn't generalizable, we'll revise it to take that data into account.

Planets form by clearing their orbit. They are also generally able to clear their orbit after migrating to another orbit. The Stern-Levison parameter measures the approximate time in which a body would be expected to clear its orbit (by scattering other planetesimals).

There is a big difference between Mars and Jupiter (something explained by the Grand Tack model), but that range contains the other 6 planets. The gap between Mars and Eris is just as big but doesn't have anything in it, that's why it's clear-cut. It's possible that that gap might not just exist in our solar system, but also in other star systems, assuming exoplanets form in a similar way to the solar system planets. We don't really know yet though. If we find that the gap is just a big local coincidence and doesn't exist in other star systems, we'll have to revise the definition. But first we have to get a lot more data.

[Camacha]

It is written for our solar system since that's the only one we have any data on. We haven't discovered any small bodies around other stars so it doesn't make sense to have a definition based on a supposed distribution of bodies that might or might not exist. If we get a lot more data on other stellar systems and find that our current definition isn't generalizable, we'll revise it to take that data into account.

We do have data on other systems, though that is incomplete at best. No matter, it lacks sense to tailor a model to one situation, especially if you have data that suggests that other systems look quite different. Why not opt for a definition that is bound to work - or at the very least is not almost guaranteed to break down? That brings me to the other potential issues, such as planets switching to dwarf planet status and back without ever physically changing, and early solar systems not containing planets by defintion, while they obviously do by common sense.

[NERVAfan]

dead, boring worlds like Mercury

Mercury (and Venus) are so underappreciated....

(I do agree with your point, though. Europa and Enceladus are probably the best hopes for extraterrestrial life in the solar system, and Titan is awesome.)

[Albert VDS]

To clarify the Clearing the neighbourhood criteria is not about completing a whole orbit around a star.

It's about it being gravitational dominant in it's region, meaning there will not be similar objects "near" this object.

Crossing orbits and Trojan objects are excluded.

[metaphor]

We do have data on other systems, though that is incomplete at best. No matter, it lacks sense to tailor a model to one situation, especially if you have data that suggests that other systems look quite different. Why not opt for a definition that is bound to work - or at the very least is not almost guaranteed to break down? That brings me to the other potential issues, such as planets switching to dwarf planet status and back without ever physically changing, and early solar systems not containing planets by defintion, while they obviously do by common sense.

And the data on other systems that we have so far says that all the objects we discovered so far are planets. You can look at any exoplanet and calculate its Stern-Levison parameter if you know its mass and semi-major axis (the k parameter is mostly constant, but can vary a little bit based on the star's mass). They're all above Mars's, so definitely planets. What data suggest other systems look quite different from ours in terms of what objects are planets or not?

How do planets change to dwarf planet status and back without physically changing? Early solar systems do not in fact contain planets, since there's only a disk of gas and dust. Once the planets start coalescing, there's a point at which their Stern-Levison parameter increases to much greater than 1, at which point you could say they are planets. This happens within ~100,000 years for gas giants and about 1 million years for rocky planets. Most likely there are many more bodies with Stern-Levison parameter greater than 1 (called oligarchs), but they collide with each other forming larger planets within a few million years.