Our moon would be a planet under new definition of planethood

[jwl]

I think this is exactly what he was getting at when he mentioned double planets in his paper: the main idea of the thing is creating a definition of planetary status which allows you to easily work out whether you are looking at an exoplanet or exoplanetiod. When writing his paper, however, I don't think that he had thought of that the moon-earth system might be thought of as a double planet under his definition; he only realised this when the new scientist brought it up during interview.

This is the Pluto situation all over again. If a new definition fits the data better across more cases we should roll with it instead of clinging to traditional ideas of what constitutes a planet or a moon.

I think purple might have a valid point here: there's no need to jump down his throat. The fact is, people are not used to thinking of the moon, titan, and galiliean moons as part of a double planet system. So making sure the definition of a double planet is such that the moon isn't one might be something to take into consideration.

That kind of rigid thought isn't what I'd expect from anybody with a science background. The idea that we could think of our moon as another world isn't so far fetched either.

But what do you mean by "fits the data better"? It's pretty arbitrary which definition you decide to use to classify things into double planets vs moons. The main advantage the critical mass definition has over the barycentre definition or the "double planets don't exist" definition is not anything fundamental about the object, but simply that it is easier to measure in the case of exomoons. The pluto definition is slightly different because it is very difficult to conceive of a definition of a planet which includes Pluto but not a hundred other unnamed objects. People are not generally used to thinking of these hundred unnamed objects as planets; meanwhile Pluto's plantary status has been controversial for way before the 2006 iau definition. Also, say a proposed definition of "planet" was that it has to have sufficient mass to hold onto a hydrogen atmosphere. That might also make things easier for categorising exoplanets (since you could use spectroscopy to get direct verification), but it would be immediately rejected because it would rule out the earth as a planet.

[Jub]

But what do you mean by "fits the data better"? It's pretty arbitrary which definition you decide to use to classify things into double planets vs moons. The main advantage the critical mass definition has over the barycentre definition or the "double planets don't exist" definition is not anything fundamental about the object, but simply that it is easier to measure in the case of exomoons. The pluto definition is slightly different because it is very difficult to conceive of a definition of a planet which includes Pluto but not a hundred other unnamed objects. People are not generally used to thinking of these hundred unnamed objects as planets; meanwhile Pluto's plantary status has been controversial for way before the 2006 iau definition. Also, say a proposed definition of "planet" was that it has to have sufficient mass to hold onto a hydrogen atmosphere. That might also make things easier for categorising exoplanets (since you could use spectroscopy to get direct verification), but it would be immediately rejected because it would rule out the earth as a planet.

I think the mass definition for what is and isn't a planet is probably the best that we have for the time being. It is inclusive enough to cover all the required bases yet specific enough not to include a mass of planetoids. That certain objects we used to think of as Moons may now be classified as a binary planet shouldn't change the usefulness of our definition.

Also, as mentioned by Terralthra, the barycenter definition would make our solar system a binary system. That seems even less intuitive than the Moon as a planet and would result in far too many star systems being classed as binary to make it useful.

[LaCroix]

Also, as mentioned by Terralthra, the barycenter definition would make our solar system a binary system. That seems even less intuitive than the Moon as a planet and would result in far too many star systems being classed as binary to make it useful.

If that many systems would qualify, it would make sense revising our definition and acknowledging that binary systems of either two stars or a star and a brown dwarf are the norm. I don't see why it would make the definition less useful.

[Jub]

If that many systems would qualify, it would make sense revising our definition and acknowledging that binary systems of either two stars or a star and a brown dwarf are the norm. I don't see why it would make the definition less useful.

If that's the case shouldn't calling all planetoids planets and accepting that most solar systems have hundreds of planets be equally useful?

You want you definitions to be just broad enough to make predictive modeling easy, while staying narrow enough not to introduce unwanted noise.

[jwl]

But what do you mean by "fits the data better"? It's pretty arbitrary which definition you decide to use to classify things into double planets vs moons. The main advantage the critical mass definition has over the barycentre definition or the "double planets don't exist" definition is not anything fundamental about the object, but simply that it is easier to measure in the case of exomoons. The pluto definition is slightly different because it is very difficult to conceive of a definition of a planet which includes Pluto but not a hundred other unnamed objects. People are not generally used to thinking of these hundred unnamed objects as planets; meanwhile Pluto's plantary status has been controversial for way before the 2006 iau definition. Also, say a proposed definition of "planet" was that it has to have sufficient mass to hold onto a hydrogen atmosphere. That might also make things easier for categorising exoplanets (since you could use spectroscopy to get direct verification), but it would be immediately rejected because it would rule out the earth as a planet.

I think the mass definition for what is and isn't a planet is probably the best that we have for the time being. It is inclusive enough to cover all the required bases yet specific enough not to include a mass of planetoids. That certain objects we used to think of as Moons may now be classified as a binary planet shouldn't change the usefulness of our definition.

Also, as mentioned by Terralthra, the barycenter definition would make our solar system a binary system. That seems even less intuitive than the Moon as a planet and would result in far too many star systems being classed as binary to make it useful.

Well there are other ways of understanding that: you don't use the same definition for star systems, you suggest that Jupiter doesn't count because it isn't a star, or you do away with the "double planet" idea entirely and just say the smaller one is the moon.

[Broomstick]

That kind of rigid thought isn't what I'd expect from anybody with a science background. The idea that we could think of our moon as another world isn't so far fetched either.

Indeed.

If I recall correctly (and, let's be frank, it was a long time ago), when Neil Armstrong stepped off the Lunar Lander part of the live commentary described it as the first time a man set foot on another world.

Regardless of whether the Moon is a "planet", a "satellite", or both it is and will always be The Moon. That is its proper name in English. The fact we use "moon" as a synonym for "satellite" (which is the more proper term, really) is a contingency of history. Titan is a "moon" of Saturn because, like The Moon, it orbits a larger planet but regardless of size it is still a satellite.

So maybe we should stop calling satellites "moons" and call them satellites, and leave "moon" for the one that goes around the Earth, regardless of what it otherwise is.

[Jub]

Well there are other ways of understanding that: you don't use the same definition for star systems, you suggest that Jupiter doesn't count because it isn't a star, or you do away with the "double planet" idea entirely and just say the smaller one is the moon.

So you're suggesting we use different criteria for the term binary depending on the objects involved in the pairing, how does that make a lick of sense? Either the barycenter does affect how we name the pairing, in which case we live in a binary system or it doesn't and we get used to seeing the Moon as a satellite planet.

[Borgholio]

I have no issue with the Moon being a planet...were it not orbiting the Earth. It really would not be a problem to classify objects based on their mass as long as you had a few exceptions for things like the location of the barycenter, and whether the object being orbited is itself a planet or a star. Look at Ganymede for example. It is literally bigger than the planet Mercury. It would certainly be a planet were it directly orbiting the sun. But it's orbiting Jupiter so it's a moon.

[Jub]

I have no issue with the Moon being a planet...were it not orbiting the Earth. It really would not be a problem to classify objects based on their mass as long as you had a few exceptions for things like the location of the barycenter, and whether the object being orbited is itself a planet or a star. Look at Ganymede for example. It is literally bigger than the planet Mercury. It would certainly be a planet were it directly orbiting the sun. But it's orbiting Jupiter so it's a moon.

I think the best thing to do would be to class planet-sized moons as satellite planets with smaller moons as just satellites. However, that still doesn't really solve the binary exo-planet issue where determining the barycenter may not be possible. Nor does it solve the issue of classifying a binary system with a planet that is so close to the line of being a brown dwarf that has a barycenter outside of its system's primary star. It doesn't even solve the issue of our solar system being a Sun-Jupiter binary system if we simply measure by barycenters.

Given this state of affairs, it's probably easiest to get over ourselves and classify things like the Earth-Moon and Jupiter-Ganymede systems as binary planets.

[Borgholio]

It doesn't even solve the issue of our solar system being a Sun-Jupiter binary system if we simply measure by barycenters.

Well there really is no issue there. The sun is a star and Jupiter is not, so it's not a binary system. It only gets tricky if the Sun were a very small brown dwarf or if Jupiter was something like 10 times it's current mass. I agree that there needs to be a cutoff at some point but in a system that is already quite clearly defined by other criteria, there is no need.

Given this state of affairs, it's probably easiest to get over ourselves and classify things like the Earth-Moon and Jupiter-Ganymede systems as binary planets.

Except for that definition is fundamentally incorrect. See, if you were to use the definition of planet as "An object with a mass above a certain level", then you MUST have exceptions to that rule for things that are obviously not planets but that have very heavy masses. Black holes, neutron stars, regular stars, etc... In the case of Jupiter vs the Sun, it's not a binary planet system because a star is not a planet. That's one exception. If you check the Earth / Moon or Jupiter / Ganymede systems, then they do indeed get closer to the binary planet system, except for the whole barycenter issue which is something which should not simply be ignored offhand. That's another exception.

Don't get me wrong, I think that it could be a very useful definition of a planet to have a cutoff mass between dwarf planet and planet. But that can't be the only criteria used.

[Terralthra]

It doesn't even solve the issue of our solar system being a Sun-Jupiter binary system if we simply measure by barycenters.

Well there really is no issue there. The sun is a star and Jupiter is not, so it's not a binary system. It only gets tricky if the Sun were a very small brown dwarf or if Jupiter was something like 10 times it's current mass. I agree that there needs to be a cutoff at some point but in a system that is already quite clearly defined by other criteria, there is no need.

Why is "the sun is a star and a planet isn't" meaningful at all? If two bodies orbited a common center equidistant from both, with one a small star (a red dwarf with proportionally high levels of hydrogen, let's say) and one a very large planet (a block of iron the size of a white dwarf; huge, but incapable of self-sustaining fusion), would that not be a binary system? You'd say the planet is orbiting the star, despite being equal mass? Don't be silly.

[Borgholio]

Perhaps I should rephrase. I meant to say binary PLANET system. Of course the situation you describe would be a binary system but not a binary planet system.

The jist of what I was saying is that we should not rely on mass alone to determine if something is a planet or not. There need to be other considerations too.

[Jub]

Except for that definition is fundamentally incorrect. See, if you were to use the definition of planet as "An object with a mass above a certain level", then you MUST have exceptions to that rule for things that are obviously not planets but that have very heavy masses. Black holes, neutron stars, regular stars, etc... In the case of Jupiter vs the Sun, it's not a binary planet system because a star is not a planet. That's one exception. If you check the Earth / Moon or Jupiter / Ganymede systems, then they do indeed get closer to the binary planet system, except for the whole barycenter issue which is something which should not simply be ignored offhand. That's another exception.

Don't get me wrong, I think that it could be a very useful definition of a planet to have a cutoff mass between dwarf planet and planet. But that can't be the only criteria used.

First, I wasn't saying that the Sun-Jupiter system was a binary planet system. I was saying that it makes our system a Star-Planet binary system, something that is going to sound wrong to a lot of people.

The proposed definition for a planet also isn't as inclusive as you've claimed it is. The definition was written in the paper linked by the OP, you have no excuse not to have read said paper before making your comments. However seeing as haven't done so, let me post the proposed definition for you:

"A planet is a celestial body that (a) is in orbit
around one or more stars or stellar remnants,
(b) has sufficient mass to clear [or dynamically
dominate] the neighbourhood around
its orbit, i.e., Π ≥ 1, (c) has a mass below
13 Jupiter masses, a nominal value close to
the limiting mass for thermonuclear fusion of
deuterium.
For single-star systems, Π ≥ 1 when
M[p] / M[E] ≳ 1.2 × 10−3 (M / M)5/2 (a[p] / 1 au)9/8,
where M is mass, a is semi-major axis, and
letters in brackets [p], , [E], refer to the planet, star,
Earth, and Sun, respectively."

[jwl]

On his website Margot is now saying tha the moon is not a planet and that his proposal never mentioned the moon. While it is true that it doesn't mention the moon, section 7.6 looks to me like it is implying that kind of criterion, and while their headlines can sometimes be off I don't that the NS is generally in the habit of simply making stuff up about an interview.

But suffice to say, if he ever did think the moon-earth system could be considered a double planet he's changed his mind now.

[Purple]

Well there are other ways of understanding that: you don't use the same definition for star systems, you suggest that Jupiter doesn't count because it isn't a star, or you do away with the "double planet" idea entirely and just say the smaller one is the moon.

So you're suggesting we use different criteria for the term binary depending on the objects involved in the pairing, how does that make a lick of sense? Either the barycenter does affect how we name the pairing, in which case we live in a binary system or it doesn't and we get used to seeing the Moon as a satellite planet.

It makes as much sense as trying to force the same the opposite. Different terms can and should mean different things in different contexts. And the context of stars and planets is in fact different. Trying to force things into compliance as you would simply leads to a lot of messed up logic tied in knots once you start using it.

[Borgholio]

First, I wasn't saying that the Sun-Jupiter system was a binary planet system. I was saying that it makes our system a Star-Planet binary system, something that is going to sound wrong to a lot of people.

Yes but in addition, you mentioned Jupiter / Ganymede and Earth / Luna as binary planet systems. That was the point I was arguing. I agree that people who don't know what a barycenter is may be confused by the new terminology, but it's easily explained.

The proposed definition for a planet also isn't as inclusive as you've claimed it is.

You missed my point. Let me try again.

You said that Earth / Moon and Jupiter / Ganymede should be classified as binary planets. Based on your preceding paragraph, it was implied to me that the classification should be based on size. My argument was that other factors should be taken into consideration when determining if it's a binary planet system or a planet - moon system. The article doesn't mention anything about barycenters, for example.

[Jub]

On his website Margot is now saying tha the moon is not a planet and that his proposal never mentioned the moon. While it is true that it doesn't mention the moon, section 7.6 looks to me like it is implying that kind of criterion, and while their headlines can sometimes be off I don't that the NS is generally in the habit of simply making stuff up about an interview.

But suffice to say, if he ever did think the moon-earth system could be considered a double planet he's changed his mind now.

By his proposed criteria it is, but that could change when the term satellite is defined in 2018.

It makes as much sense as trying to force the same the opposite. Different terms can and should mean different things in different contexts. And the context of stars and planets is in fact different. Trying to force things into compliance as you would simply leads to a lot of messed up logic tied in knots once you start using it.

Please explain the logic behind a Star-Planet binary using different rules than any other binary system.

Yes but in addition, you mentioned Jupiter / Ganymede and Earth / Luna as binary planet systems. That was the point I was arguing. I agree that people who don't know what a barycenter is may be confused by the new terminology, but it's easily explained.

I mention the Sun-Jupiter system as it adds context to the discussion, given the defined definition I obviously wasn't calling that a dual planet system as your reply inferred.

You missed my point. Let me try again.

You said that Earth / Moon and Jupiter / Ganymede should be classified as binary planets. Based on your preceding paragraph, it was implied to me that the classification should be based on size. My argument was that other factors should be taken into consideration when determining if it's a binary planet system or a planet - moon system. The article doesn't mention anything about barycenters, for example.

My argument used the definition provided by the research paper the article was based on. A document that was linked in the OP and that I would consider required reading as far as this thread is concerned. That you failed to read it and skimmed the article is not my problem.

[Borgholio]

I mention the Sun-Jupiter system as it adds context to the discussion, given the defined definition I obviously wasn't calling that a dual planet system as your reply inferred.

Again, I'm not referring to that.

This:

Given this state of affairs, it's probably easiest to get over ourselves and classify things like the Earth-Moon and Jupiter-Ganymede systems as binary planets.

That you failed to read it and skimmed the article is not my problem.

No, your problem is that you fail to see my point that other criteria beyond the ones listed in the article are necessary for planetary classification to make sense.

[Jub]

I mention the Sun-Jupiter system as it adds context to the discussion, given the defined definition I obviously wasn't calling that a dual planet system as your reply inferred.

Again, I'm not referring to that.

This:

Given this state of affairs, it's probably easiest to get over ourselves and classify things like the Earth-Moon and Jupiter-Ganymede systems as binary planets.

That you failed to read it and skimmed the article is not my problem.

No, your problem is that you fail to see my point that other criteria beyond the ones listed in the article are necessary for planetary classification to make sense.

Given the definition published, which I agree with, those are binary planets. Both objects are of the required mass and meet all other requirements for planethood as outlined in the quoted paper. If we're going strictly by the barycenter of a two body system, something not part of the paper, then they are not binary planets, but our solar system should be classified as a binary system consisting of the Sun and Jupiter. I feel the classification of our solar system as a binary system creates more issues than it solves and thus the Earth-Moon and Jupiter-Ganymede should stay as Binary planets.

[Purple]

Any system of classification that classifies a star system as binary when it does not have two stars is highly flawed.

[Adam Reynolds]

Any system of classification that classifies a star system as binary when it does not have two stars is highly flawed.

It would be a binary planet rather than a binary star.

[Jub]

Any system of classification that classifies a star system as binary when it does not have two stars is highly flawed.

Can you explain your logic? I tend to agree, but if you classify binary systems by barycenter, I don't see why a Star-Planet binary system is any different than a Star-Star or a Star-Black Hole binary system.

[Purple]

Any system of classification that classifies a star system as binary when it does not have two stars is highly flawed.

Can you explain your logic? I tend to agree, but if you classify binary systems by barycenter, I don't see why a Star-Planet binary system is any different than a Star-Star or a Star-Black Hole binary system.

For the same reason why Pluto should not be a planet. It creates a category that's too broad to be functionally useful. And than we have to invent different special case subcategories for every possible combination of N bodies orbiting around one another. So you'll end up with planet-star-planet-star n-ary system or something. For classifications to make sense there need to be clearly defined boundaries and it has to be easy for humans to use and navigated. So anything that resembles a search tree that gets exponentially wider as you go down is just not going to do.

[jwl]

BTW most g-type stars are part of binary systems (or star-star binaries, if you prefer).

[Broomstick]

Or call them "binary star systems". As opposed to "binary planet systems" or "binary mixed systems". I think it is possible to come up with something suitable after some further thrashing about.