More planets in our solar system

[IllyrianTheGreat]

Ceres, Europa and Pluto are planets?

http://www.space.com/36214-planet-definition-includes-pluto-most-moons.html

[Steel]

11 minutes ago, IllyrianTheGreat said:

Ceres, Europa and Pluto are planets?

http://www.space.com/36214-planet-definition-includes-pluto-most-moons.html

As well as every single moon and spherical object in the solar system. To my eyes rather than change the definition of a planet, it would make more sense to change the name "Planetary Scientist" since they freely admit that they don't just study planets

[YNM]

15 minutes ago, Steel said:

As well as every single moon and spherical object in the solar system. To my eyes rather than change the definition of a planet, it would make more sense to change the name "Planetary Scientist" since they freely admit that they don't just study planets

Well, all solar system bodies have something to do with planets, much like learning humans could mean a lot, from molecules to social interactions 

[Gargamel]

That's just what's his name from the New Horizon's project trying to get Pluto back on the 'map' again.  If you include Pluto, you gotta include a 100 or so other bodies, including our moon.

I think the current definition could use a little revision though.  The clearing of the path is kinda dumb, as mentioned in (maybe this one ) an article I read on this, all planets have smaller bodies crossing their paths all the time.   It should reflect the fact a planet has it's system's center of mass (barycenter) is within the central body.  The Earth/moon system's barycenter is within the Earth.  The Plutonian  barycenter lies between Charon and Pluto. 

 

Bun that leaves open the very rare, if even possible, case of binary planets.  What if two clearly planet sized planets were in orbit around each other with a barycenter outside both of them, which of them would be the planet?

[Scotius]

Meh. Another attempt to "revert a river with a stick" - as we say in Poland. Some people just can't let go of "Pluto = planet" mindset. I'd understand it if Pluto really would be something unique - but it isn't. There are dozens of similiar bodies beyond Neptune's orbit, and every now and then someone finds another. Pluto's only distinction is the fact that we found it first and this discovery was announced with much fanfare. The same thing happened in XIX century with asteroids. First there was only Ceres, and everyone said "Sure, OK. It's a new planet". But then astronomers found Pallas, Vesta, Juno and dozens, hundreds, thousands of their sisters - and Ceres lost its distinction and planetary status.

[sevenperforce]

Oh dear, not this argument.

*happily enters this argument*

Any definition that says Titan and Ganymede and our moon are planets is a dumb definition.

Language should be descriptive; we should look for definitions that describe common usage. At the same time, accuracy is important, so if our definitions can improve and align common usage, that's good too.

The common usage of terms like "moon" and "planet" is pretty straightforward: a planet is a world that orbits a star and a moon is something that orbits a planet. That's how people think about it, as a hierarchy. So when we say "no, Pluto is a world that orbits the sun but it's not a planet because of this particular mathematical quirk" then it makes no sense.

It's my opinion that the formal definition should reflect a hierarchy. A star is anything that has nuclear fusion happening in its core. A world is any spherical body which is not a star. A planet is a world which orbits a star. A moon is a world which orbits around a barycentre inside a planet.

So Ceres is a planet. Pluto and Charon are a binary planet. We can break planet down into major/minor or dwarf/giant or anything else we want, but they're still planets.

[Gargamel]

20 minutes ago, sevenperforce said:

 

*happily enters this argument*.

It's my opinion that the formal definition should reflect a hierarchy. A star is anything that has nuclear fusion happening in its core. A world is any spherical body which is not a star. A planet is a world which orbits a star. A moon is a world which orbits around a barycentre inside a planet.

 

Here, have a wrench.

By your definition, a moon also orbits a start, while also orbiting a planet, therefore it is also a planet, since it also is spherical and non fusive (is that a word?  It is now). 

I agree with the sentiment, just not the syntax. 

[sevenperforce]

10 minutes ago, gargamel said:

Here, have a wrench.

By your definition, a moon also orbits a start, while also orbiting a planet, therefore it is also a planet, since it also is spherical and non fusive (is that a word?  It is now). 

I agree with the sentiment, just not the syntax. 

Nonfusing?

(Note that under this definition, a brown dwarf is considered a star. That's fine. We can classify stars further.)

Anyhow, this is an artifact of how we use "orbit". Under the formal definition of an orbit, the moon orbits the Sun alongside Earth. Under the common usage, the moon orbits Earth and we orbit the Sun. We'd have to specify that we are talking about orbiting a barycentre within the planet; the Earth-moon barycentre is inside Earth.

[Gargamel]

I'm tired, and slightly drunk from low sugar, I'm sticking with non fusive.  

Toss out the star altogether, cause there's rogue planets out there. 

So what if we simplify it to:  Something that is large enough to be spherical, non fusive (ha!), and any barycenters involved result in them being inside the body.  And large enough that if another body of the minimum radius of the same density were to interact with this planet, the barycenter would remain within the planet. 

This would allow for rogue planets, and remove all those pesky dwarf planets that keep messing it up. 

 

EDIT:   The more I'm thinking about this, the more I'm liking it. 

 

Edited March 28, 2017 by gargamel

[sevenperforce]

"Any barycenters involved" is problematic. The barycentre of the Earth-Sun system is inside the Sun. Trojans are totally bound to their primary, but the barycentre is outside the primary.

The interaction thing is also tricky to parse. Are you trying to eliminate small planets according to the hierarchy approach: e.g., if it's too small to have a round moon, it shouldn't be grouped with the "real" planets?

There are a lot of things that can change. For example, the moon is receding from Earth. Once the moon recedes far enough, the Earth-moon barycentre will be outside the Earth's surface. At this point, does this mean we're in a binary planet system? I think so. Some might disagree.

 

 

 

[insert_name]

Any definition that makes Methone a planet is garbage. While the current definition excludes some edge cases, this one is way too inclusive.

[HebaruSan]

Maybe let people use the phrase "traditional planets" for the original 9. No new definitions needed, and they can use the word they so desperately want to use, with a qualifier.

[IllyrianTheGreat]

3 hours ago, sevenperforce said:

A star is anything that has nuclear fusion happening in its core.

Until we discover first rocky body with such core

34 minutes ago, HebaruSan said:

Maybe let people use the phrase "traditional planets" for the original 9. No new definitions needed, and they can use the word they so desperately want to use, with a qualifier.

Then how it would be different than "dwarf planet" and "planet" categories?

Do we really need categories like that? Can we just use "Celestial body [insert_name]"?

Edited March 28, 2017 by IllyrianTheGreat

[kerbiloid]

7 minutes ago, IllyrianTheGreat said:

Do we really need categories like that? Can we just use "Celestial body [insert_name]"?

Even better. Just use UUID or blockchain to generate valid id for celestial; bodies.

39 minutes ago, HebaruSan said:

Maybe let people use the phrase "traditional planets" for the original 9

Original 5. Sun and Moon are not planets, while Uranus and beyond have been discovered later.

[sevenperforce]

1 hour ago, insert_name said:

Any definition that makes Methone a planet is garbage. While the current definition excludes some edge cases, this one is way too inclusive.

That's why I like the hierarchy approach. Methone is orbiting a barycentre inside Saturn; therefore, Methone is a moon.

If Methone was orbiting the Sun all by its lonesome, would it be considered a planet?

We can say that stars are all objects with fusion in their cores, worlds are all non-star objects in hydrostatic equilibrium, planets are any worlds not orbiting a barycentre inside another world, and moons are any worlds orbiting a barycentre inside another world. Major planets are planets large enough to have a moon; minor planets are planets not large enough to have a moon. Natural satellites are objects too small to be in hydrostatic equilibrium which orbit a barycentre inside a world.

Then you can have various modifiers, like a trojan planet or a trojan satellite, etc.

37 minutes ago, IllyrianTheGreat said:

Until we discover first rocky body with such core

Won't happen. No object large enough to fuse hydrogen will be small enough to have lost its atmosphere.

A brown dwarf is still a star under this definition.

[mikegarrison]

I say that we go back to the original list of planets.

Mercury, Venus, Mars, Jupiter, Saturn. That's it.

If it was good enough for the ancient Greeks, why isn't it good enough for us?

[insert_name]

6 hours ago, IllyrianTheGreat said:

Until we discover first rocky body with such core

Then how it would be different than "dwarf planet" and "planet" categories?

Do we really need categories like that? Can we just use "Celestial body [insert_name]"?

Spoiler

Also I am not a celestial body.

Edited March 29, 2017 by insert_name

[Bill Phil]

5 hours ago, sevenperforce said:

We can say that stars are all objects with fusion in their cores, worlds are all non-star objects in hydrostatic equilibrium, planets are any worlds not orbiting a barycentre inside another world, and moons are any worlds orbiting a barycentre inside another world. Major planets are planets large enough to have a moon; minor planets are planets not large enough to have a moon. Natural satellites are objects too small to be in hydrostatic equilibrium which orbit a barycentre inside a world.

Problem with the Major/Minor distinction:

There is no such thing as a planet not large enough to have a moon, as some asteroids have them. Perhaps the major/minor distinction could be the rule where the local orbit has been swept of large objects.

[sevenperforce]

28 minutes ago, Bill Phil said:

Problem with the Major/Minor distinction:

There is no such thing as a planet not large enough to have a moon, as some asteroids have them. Perhaps the major/minor distinction could be the rule where the local orbit has been swept of large objects.

You missed the point above where a moon is defined as a world orbiting a barycentre inside another world, and a world is a non-stellar body large enough to achieve hydrostatic equilibrium. So it goes with the same notion of hierarchy. 

[Bill Phil]

14 minutes ago, sevenperforce said:

You missed the point above where a moon is defined as a world orbiting a barycentre inside another world, and a world is a non-stellar body large enough to achieve hydrostatic equilibrium. So it goes with the same notion of hierarchy. 

The problem is that there are incredibly small asteroids with natural satellites (with a barycenter within said asteroid, as well). But my point has to do with the fact that all planets are "large enough" to have a moon, since there are bodies which are fall smaller which also have them. This implies that all objects large enough to have a moon includes very small objects. Objects which aren't, under the system you propose, even worlds. The point is that this includes ALL planets as well. Meaning minor planets would not exist. All planets would be major planets. Since a "world" could be just a few hundred kilometers across. Any planet could then have a moon.

Also, there are definitely moons that aren't in hydrostatic equilibrium... Iapetus springs to mind. That would no longer be a moon under this system.

 

[kerbiloid]

A nice definition was proposed by somebody on this forum a year ago.

Briefly:
"moon" - a hydrostatically spherical (rather than just a satellite) thing orbiting a planet.
"planet" - something orbiting star enough large to have a "moon"

[Gargamel]

9 hours ago, kerbiloid said:

A nice definition was proposed by somebody on this forum a year ago.

Briefly:
"moon" - a hydrostatically spherical (rather than just a satellite) thing orbiting a planet.
"planet" - something orbiting star enough large to have a "moon"

That excludes Mars, Mercury, and Venus, either their moons aren't round, or they have no moons, and includes things like Pluto. 

 

19 hours ago, sevenperforce said:

The interaction thing is also tricky to parse. Are you trying to eliminate small planets according to the hierarchy approach: e.g., if it's too small to have a round moon, it shouldn't be grouped with the "real" planets?

 

No, it would in essence set a minimum radius for a planet size.    If an object is large enough be considered a planet, and we add another body to the system that is the minimum size, where will the barycenter be?  From that, a minimum size of the planet can be determined.

 

But  I see what your saying about distance, I'll have to think about that. 

Edited March 29, 2017 by gargamel

[kerbiloid]

2 minutes ago, gargamel said:

That excludes Mars, Mercury, and Venus, either their moons aren't round, or they have no moons

Ability doesn't mean necessity. Mars, Mercury and Venus are enough large, Pluto is also.
In fact, this criterion is equal to minimal critical mass of a body depending on its composition.

Edited March 29, 2017 by kerbiloid

[sevenperforce]

9 hours ago, kerbiloid said:

A nice definition was proposed by somebody on this forum a year ago.

Briefly:
"moon" - a hydrostatically spherical (rather than just a satellite) thing orbiting a planet.
"planet" - something orbiting star enough large to have a "moon"

That was me.

5 minutes ago, gargamel said:

That excludes Mars, Mercury, and Venus, either their moons aren't round, or they have no moons, and includes things like Pluto. 

Not quite. It doesn't say that a world must have a round moon to be a planet; it says a world must be large enough to have a round moon. The size threshold for gravitationally-rounded hydrostatic equilibrium is somewhere around Vesta or Mimas. Thus, if a given world is large enough to be orbited by Vesta/Mimas, it's a planet. If it's so small that it would be in a binary pair with Vesta/Mimas, it's a dwarf/minor planet.

11 minutes ago, gargamel said:

No, it would in essence set a minimum radius for a planet size.    If an object is large enough be considered a planet, and we add another body to the system that is the minimum size, where will the barycenter be?

Difficulty here is that the barycentre depends on the size of the orbit. Let's take a world like Mercury. Mimas could orbit a barycentre inside of Mercury, so we know Mercury is a major planet. But for Mimas to orbit a barycentre inside of Ceres, it would have to be so close as to render the system unstable or tidally locked. The trick is figuring out how to mathematically define system stability. But at least it is a more intuitive approach then "clearing one's orbit", even if the math for the planetary discriminant is fairly straightforward.

11 hours ago, Bill Phil said:

The problem is that there are incredibly small asteroids with natural satellites (with a barycenter within said asteroid, as well). But my point has to do with the fact that all planets are "large enough" to have a moon, since there are bodies which are fall smaller which also have them. This implies that all objects large enough to have a moon includes very small objects. Objects which aren't, under the system you propose, even worlds. The point is that this includes ALL planets as well. Meaning minor planets would not exist. All planets would be major planets. Since a "world" could be just a few hundred kilometers across. Any planet could then have a moon.

Also, there are definitely moons that aren't in hydrostatic equilibrium... Iapetus springs to mind. That would no longer be a moon under this system.

Yes, that's part of the terminology. A body orbiting another body is only considered a moon if it is large enough to be gravitationally rounded. If it's too small to be gravitationally rounded, it's merely a natural satellite.

This is another nod to hierarchy. In common usage, both by the lay public and in most fiction, a "moon" is a world. It's large enough to walk around on. It's round.

Iapetus would be a natural satellite rather than a moon, yes. As would Phobos and Deimos.

[sevenperforce]

28 minutes ago, kerbiloid said:

This criterion is equal to minimal critical mass of a body depending on its composition.

I much prefer a purely size-based determinant rather than the whole "clearing one's orbit" business. Mercury or even Mars and Earth would cease to be planets if they were simply farther from the sun. That doesn't make any intuitive sense. 

One advantage here is that setting the definition of a planet (or major planet, if we use that term) based on the ability to retain a round moon overlaps closely with the critical size at which a world can begin to retain a stable atmosphere. This also matches the idea of a hierarchy. The public generally understands that not all planets have atmospheres, but there's a general sense of "large enough".