Significant figures

[LGTrotter]

That Dr Maggie Aderin-Pocock sounds just like Patrick Moore.

[jdaw1]

whether or not Charon is also a Planet.

In the expert opinion of Professor Turnbull, is the earth’s moon a planet?

[DRT]

whether or not Charon is also a Planet.

In the expert opinion of Professor Turnbull, is the earth’s moon a planet?

No.

The question over Charon arises from the potential classification of Pluto and Charon as a binary system rather than an object with a satellite. This concept was raised during the Sky at Night episode referred to by Owen which was presented by real experts. If these two bodies are a binary system, and both geologically active, then both could be considered to be the same class of object. Once the world wakes up to the fact that Pluto is a planet that would also make Charon a planet.

[LGTrotter]

Yeah! Go Derek. Binary planet, KO to Derek and Glenn!

[DRT]

Yeah! Go Derek. Binary planet, KO to Derek and Glenn!

Are the Nobel Prize Selection Committee likely to see this thread or should we send a link?

[jdaw1]

Why is the moon not a planet?

[LGTrotter]

Well Julian, if you had followed these things, like me and Derek and Brian May then you would know about the centre of mass between two objects, which has a posh term which I didn't quite catch, but ex-rockandroller Dr May will know about for sure. *sings* "We are the Champions..."

[DRT]

Why is the moon not a planet?

Because it is a satellite of a Planet.

[jdaw1]

Why is the moon not a planet?

Because it is a satellite of a Planet.

Are you saying that the moon-planet distinction depends on, ahem, ahem ahem, location (at least relative to other things)? If not, please explicitly deny.

[DRT]

Why is the moon not a planet?

Because it is a satellite of a Planet.

Are you saying that the moon-planet distinction depends on, ahem, ahem ahem, location (at least relative to other things)? If not, please explicitly deny.

No, that is not what I am saying.

A moon is a type of object orbiting a planet. A planet is a type object orbiting a star. A moon is still a moon regardless of how far away the planet that it is orbiting is away from its star.

That all seems pretty clear to me.

[jdaw1]

A moon is a type of object orbiting a planet.

Yes, you rightly agree that location matters. Our moon in an Ceres-ish orbit would be a planet (Λ ≈ 5.05, so just large enough to have swept away the asteroids). Our moon in an orbit around the Earth, same rock, same composition, same geological history, same Wensleydale, isn’t a planet. Yes, DRT says that location matters: rock around sun = planet; rock around larger rock = moon.

Progress!

[DRT]

A moon is a type of object orbiting a planet.

Yes, you rightly agree that location matters. Our moon in an Ceres-ish orbit would be a planet (Λ ≈ 5.05, so just large enough to have swept away the asteroids). Our moon in an orbit around the Earth, same rock, same composition, same geological history, same Wensleydale, isn’t a planet. Yes, DRT says that location matters: rock around sun = planet; rock around larger rock = moon.

Progress!

Progress indeed. But your description works with or without the formula that you keep quoting in the hope that someone else will believe it is of relevance, which it isn't.

Without the formula Pluto is a planet. The only reason you think it isn't is because of the existence of the formula which is just someone's way of drawing a line in an arbitrary place with no real meaning.

I do agree that the existence of the formula helps the unimaginative jobsworth bureaucrats who cannot contemplate the notion that the Sun might have hundreds of planets. Personally I think it is irrelevant how many planets the Sun has providing there is logic in the definition of what a planet is. The current logic just draws a line to limit the number, not to define what the objects actually are and how they materially differ from things that are not planets (e.g. dust, meteorites, comets, asteroids and moons). We do not need a mathematical formula, we need an agreed description of the physical properties that a planet must have in order to be a planet.

[jdaw1]

Earlier you said

given that Mercury and Pluto are much more similar objects than either is to Jupiter (and both by an enormous margin), why would one not be a planet just because it is farther away from the Sun

which I interpreted, perhaps mis-interpreted, as you arguing that location should not be an allowed parameter in making the planet/non-planet distinction.

It must have been a mis-interpretation, because you now seem to be allowing the use of location — albeit not necessarily as in Stern-Levison’s Λ.

So why Λ? It encapsulates the outcome of a real physical process, which gives it enough plausibility to be a candidate. What are the other candidates?

There is a location-free argument: if big enough to be rounded, then big enough to be a planet. Rephrased, as is except that all dwarf planets are promoted to planets, and there will be many planets in the Kuiper Belt. Fine, but then Ceres is a planet, as it was long ago. Are you happy with that?

If not happy with that, suggest a distinction, ideally one that encapsulates a real physical process rather than a made-up squiggly line, that keeps Pluto in and Ceres out.

[PopulusTremula]

For imbeciles like me the Stern line in the sand by Levi.. umm, Leviticus? has appeal in its binary-ness. It sort of "keeps the riff-raff out of Waitrose" to put it in plain and easily understood English.

Otherwise where would we be? Could a comet be a planet? A star? What about asteroids crossing the path of say, Jupiter, and get chucked out of the solar system? No, ordnung muss sein i say!

[DRT]

Ceres a planet? Fine with me.

Lots of Kuiper Belt objects are planets? Fine with me provided that are not just oddly shaped rocks but have physical properties similar to the other planets and they are orbiting the Sun, not just passing through.

[DRT]

It sort of "keeps the riff-raff out of Waitrose"

That nicely encapsulates its purpose and its problem. History is littered with rules that created an elite and excluded the unwelcome.

[jdaw1]

There is a location-free argument: if big enough to be rounded, then big enough to be a planet. Rephrased, as is except that all dwarf planets are promoted to planets, and there will be many planets in the Kuiper Belt. Fine, but then Ceres is a planet, as it was long ago. Are you happy with that?

Ceres a planet? Fine with me.

Lots of Kuiper Belt objects are planets? Fine with me provided that are not just oddly shaped rocks but have physical properties similar to the other planets and they are orbiting the Sun, not just passing through.

Now I understand you—hopefully. You object to the making of a distinction between Ceres/Pluto/KBOs and the überplanets. It isn’t the location of the line that bothers you, it’s its existence.

Please confirm.

[DRT]

Correct.

[Glenn E.]

At what point should I throw the wrench labeled "Jupiter and Sol are a binary system" into the fray?

Yes, yes, Jupiter isn't on fire. Duly noted. But it and Sol jointly orbit a point outside of Sol in the same way that Pluto and Charon orbit a point outside of Pluto.

So does that mean Jupiter isn't a planet?

*runs and hides*

[DRT]

I don't think that presents a problem, Glenn. We can just draw a bright line around Jupiter and call it a Dwarf Main Sequence Star. The big problem comes from the fact we now have 67 more planets, or 66 if we draw the next bright line in the appropriate place.

[jdaw1]

I’ve argued that Stern-Levison’s Λ is a sensible bright line between überplanets and unterplanets — a separate matter from whether such a line is desirable.

So let’s move to question about which I want an opinion, but don’t have certain opinion†. What’s a moon? Obviously something orbiting something orbiting sol. But is every scrap of gin-and-tonic-free ice in Saturn’s rings a separate moon? Is every scrap of dust in the Jovian rings a separate moon? If not, what is the boundary?

† Well, maybe I do. My Nobel-prize-winning answer is prepared, with which you will surely agree, but first I want to hear your opinions.

[DRT]

I have been contemplating this today after seeing pictures of Pluto's lesser moons.

I think a moon should be near spherical in that its own gravitational forces have overcome the inherent structure of the particles and rocks that formed it so that those materials have morphed into a single near spherical object. I consider objects such as Nix to be captive asteroids, not moons.

[jdaw1]

I consider objects such as Nix to be captive asteroids, not moons.

Is moon-ness dependent on origin? Are ex-asteroids de-barred from moon status? Speak of Triton. Speak of Jovian retrograde moons. Indeed, your definition suggests that there are there only four Jovian moons: comfortable?

[DRT]

I consider objects such as Nix to be captive asteroids, not moons.

Is moon-ness dependent on origin? Are ex-asteroids de-barred from moon status? Speak of Triton. Speak of Jovian retrograde moons. Indeed, your definition suggests that there are there only four Jovian moons: comfortable?

I think of Triton as being a former planet that was captured by Neptune and therefore became a moon.

I care not for the number of objects that result from a definition. See Pluto argument. If only four out of sixty-seven things orbiting Jupiter meet the criteria I set out above then four it is. I see nothing problematic with a definition that results in "Jupiter having sixty-seven known satellites, four of which are moons".

[jdaw1]

I consider objects such as Nix to be captive asteroids, not moons.

Is moon-ness dependent on origin? Are ex-asteroids de-barred from moon status? Speak of Triton. Speak of Jovian retrograde moons. Indeed, your definition suggests that there are there only four Jovian moons: comfortable?

I think of Triton as being a former planet that was captured by Neptune and therefore became a moon.

Please explain why captured ‘planets’ do become moons, but captured asteroids don’t.

I care not for the number of objects that result from a definition. See Pluto argument. If only four out of sixty-seven things orbiting Jupiter meet the criteria I set out above then four it is. I see nothing problematic with a definition that results in "Jupiter having sixty-seven known satellites, four of which are moons".

Very fair, I was merely checking your comfort with the consequences of your proposal.


So, speak of Hyperion. Is it a moon?

[DRT]

Please explain why captured ‘planets’ do become moons, but captured asteroids don’t.

I simply think of these as being fundamentally different things. An asteroid is a rock, or a collection of rocks, with no particular form and no history of geological activity. Spherical rocky objects have a history of geological process driven by their own gravity that causes the colliding rocks that formed them to change form and through these processes end up with the classical core, mantle and crust formation. I think this makes them fundamentally different to rocks that just happened to stray into the gravitational path of a planet and become trapped and forever unchanged other than a few more craters over time.

So, speak of Hyperion. Is it a moon?

Hyperion is a fragment of rock of unknown origin. I think it falls into the captured asteroid category.

[jdaw1]

So the DRT classification is as follows.
• Things big enough to be rounded:
◊ If in orbit around the sun are ‘planets’ (about a dozen are known, there might be hundreds);
◊ If in orbit around something orbiting the sun are ‘moons’ (there being about 19, the smallest perhaps being Mimas).
• Things too small to be rounded are ‘rocks’, ‘pebbles’ or ‘dust’.

Please confirm.

[DRT]

A fair summary of my opinion.

[flash_uk]

So the DRT classification is as follows.
• Things big enough to be rounded:
◊ If in orbit around the sun are ‘planets’ (about a dozen are known, there might be hundreds);
◊ If in orbit around something orbiting the sun are ‘moons’ (there being about 19, the smallest perhaps being Mimas).
• Things too small to be rounded are ‘rocks’, ‘pebbles’ or ‘dust’.

I may regret asking this, but what is it about the 'rules' above that some disagree with, and thus conclude that Pluto is not a planet? That sort of means I am assuming Pluto meets the definition above of being rounded and orbiting the sun...is that true?

[DRT]

So the DRT classification is as follows.
• Things big enough to be rounded:
◊ If in orbit around the sun are ‘planets’ (about a dozen are known, there might be hundreds);
◊ If in orbit around something orbiting the sun are ‘moons’ (there being about 19, the smallest perhaps being Mimas).
• Things too small to be rounded are ‘rocks’, ‘pebbles’ or ‘dust’.

I may regret asking this, but what is it about the 'rules' above that some disagree with, and thus conclude that Pluto is not a planet? That sort of means I am assuming Pluto meets the definition above of being rounded and orbiting the sun...is that true?

The classification above relates to a discussion on what constitutes a moon, not what constitutes a planet.

The debate about planets is set out on the previous three pages of this thread but I think can be fairly summarised as follows:

1. Some people, DRT included, think that being a sphere and orbiting the Sun is enough to be called a planet.

2. Others, jdaw1 included, think that some spheres are too numerous, far away and small and should therefore not be called planets.

Pluto falls into category 1 as a planet and category 2 as not a planet.

[DRT]

My Nobel-prize-winning answer is prepared, with which you will surely agree, but first I want to hear your opinions.

{bump}

[jdaw1]

To be a moon, a body must be:
• In direct orbit around a body too small to be a star;
• Have been sighted multiple times within a mapped orbit;
• Be larger than a chunk of Westcombe Cheddar as served by the Boot & Flogger.

[DRT]

To be a moon, a body must be:
• In direct orbit around a body too small to be a star;
• Have been sighted multiple times within a mapped orbit;
• Be larger than a chunk of Westcombe Cheddar as served by the Boot & Flogger.

I agree with your first point.

I disagree with your second point on the basis that a moon is still a moon even if we or another species have not yet seen it.

I disagree with your third point on the basis that there is too wide a gap between a chunk of cheese and something large enough to be considered a moon. Your description implies that every piece of space junk currently orbiting the earth is a moon, along with all of the rocks too small to be detected.

[Glenn E.]

I find myself nodding in agreement as I read DRT's posts. Well said, sir, well said.

[DRT]

I find myself nodding in agreement as I read DRT's posts. Well said, sir, well said.

Thank you. I am preparing my acceptance speech now.

[Glenn E.]

To be a moon, a body must be:
• In direct orbit around a body too small to be a star;
• Have been sighted multiple times within a mapped orbit;
• Be larger than a chunk of Westcombe Cheddar as served by the Boot & Flogger.

I agree with your first point.

I disagree with your second point on the basis that a moon is still a moon even if we or another species have not yet seen it.

I disagree with your third point on the basis that there is too wide a gap between a chunk of cheese and something large enough to be considered a moon. Your description implies that every piece of space junk currently orbiting the earth is a moon, along with all of the rocks too small to be detected.

I agree with DRT on all points.

But that does pose a further question: what to call the (now former) moons of Mars? Phobos and Deimos are not round, so do not qualify as moons. I've always thought it weird that they were considered moons. They are thought to be captured asteroids. Do we now refer to them as captured asteroids? Or do we simply expand what "asteroid" means to include non-moon planet-orbiting bodies? Or are they dwarf moons? Or do we simply grandfather them in as moons?

To me there is a clear category of planet-orbiting objects larger than junk/rocks/the components of Saturn's rings yet smaller than a true moon.

[DRT]

I do not claim that my definition is or should be universally accepted, I just think that planet-like moons are so materially different to the captured asteroid or big rock type moons that they deserve a special category.

I have not studied the moons of Mars but do realise they are not spheres. Could it be that one day they will be? Are they of sufficient size that their own gravity and the forces exerted upon them by their host planet will pull them into shape?

[Glenn E.]

I have not studied the moons of Mars but do realise they are not spheres. Could it be that one day they will be? Are they of sufficient size that their own gravity and the forces exerted upon them by their host planet will pull them into shape?

You do realize that you are asking for jdaw1 to quote math at us again, right?

[DRT]

I have not studied the moons of Mars but do realise they are not spheres. Could it be that one day they will be? Are they of sufficient size that their own gravity and the forces exerted upon them by their host planet will pull them into shape?

You do realize that you are asking for jdaw1 to quote math at us again, right?

I've been trying to tempt him for a whole page of posts and you've just scared him off again!

Sit tight, he'll not be able to resist forever