Experts meet to decide Pluto fate, Finally we'll know what a 'planet' is... Options

[ljk4-1]

Just how binding is the IAU decision on astronomers, both professional and
amateur?

Does it have to be taken any more seriously than, say, the UN Outer Space
Treaties are?

People are already buying up lunar property. Just wait until corporations start
landing there to mine the regolith and see how quickly and easily their lawyers
circumvent that dated bunch of rules.

As for an example right here on Earth, the Antarctic Treaty is frequently
violated and ignored by the numerous countries which claim various sections of
the southernmost continent, which they have sliced up like a pie. There are even
ongoing disputes over who owns certain parts of Antarctica.

In their efforts to keep Pluto an "official" planet, the IAU has made the issue even
more complicated for future generations. We still know so little about "Xena" and
you know there are even bigger worlds out there just waiting to be found and
argued over.

And what about all those objects orbiting other stars?

They should have gone with planetoids.

[rogelio]

Dateline 2015: U.S. Postal Service issues revised planetary exploration postage stamp series ending with “Xena –Not Yet Explored (or formally named)”...

[Ames]

How about: Many Vexed Experts Make Confusing Judgment So Us Normal People Say 'XXXX'

I like it

Nick

[Alan Stern]

Hmm... We seem to be running in circles here, so to speak. Didn't they say a body needs to orbit the Sun, not another body, in order to be classified as a planet? If so, how can Charon (and for that matter Pluto as well!) be a planet?

Owing to its high mass relative to Pluto, Pluto-Charon's barycenter is in free space,
which means Charon is technically in orbit about the Sun, not Pluto. This is not a debatable
fact, it's just the way the dynamics works.

[ngunn]

Owing to its high mass relative to Pluto, Pluto-Charon's barycenter is in free space,
which means Charon is technically in orbit about the Sun, not Pluto. This is not a debatable
fact, it's just the way the dynamics works.

The barycenter of the solar system is also in free space. Does this mean that technically Jupiter is in orbit about the Milky Way, not the Sun?

[maycm]

My kids have a video of "Blues Clues" where 'Steve' sings the following song to teach them about the planets.

Oh the Sun's a hot star
And Mercury's hot too
Venus is the brightest planet
And Earth is home to me and you
Mars is the red one
And Jupiter's most wide
Saturn's got those icy rings,
and Uranus spins on it's side
Neptune's really windy And Pluto's really small

Well we wanted to name the planets and now we named them all.

...seems it will need updating. Any suggestions? I know there are some creative people here

[ugordan]

Owing to its high mass relative to Pluto, Pluto-Charon's barycenter is in free space,
which means Charon is technically in orbit about the Sun, not Pluto.

Why would that mean Charon is technically orbiting the sun? What difference does it make whether or not the barycenter is below the surface of the primary body? What's so special about surface radius, dynamically speaking?
Shouldn't the fact which object exerts a greater force on the moon determine what it's technically orbiting? Similar to the case with our Luna -- I don't know the numbers or if it's exactly true, but it's been stated the Sun exerts a greater pull on it than Earth does. So technically the Moon orbits the Sun.
This barycenter-based definition sounds pretty vague and useless to me.

[David]

Owing to its high mass relative to Pluto, Pluto-Charon's barycenter is in free space,
which means Charon is technically in orbit about the Sun, not Pluto. This is not a debatable
fact, it's just the way the dynamics works.

Of course Charon is in orbit around the Sun, but then so is Earth's Moon.

Here's another way to look at it: if you sketch the ellipse of Charon's orbit about the Pluto-Charon barycentre, Pluto is always within Charon's orbit; if you sketch the ellipse of Pluto's orbit, Charon is never found inside it:


Pluto's orbit is so close to the barycentre that saying that Charon doesn't orbit Pluto (but rather a bit of empty space close to Pluto), though technically correct, seems whimsical, sort of like denying that Jupiter revolves around the Sun.

[djellison]

I must admit - I was explaining all this to my ever fascinated colleague Josh ( for those that watch QI I said "How many planets are there Josh" "Nice?" "BZZZZZZZZZ -awooga...awoooga" ) and we debated the point of Charon getting an upgrade.

Why should the movement of the barycenter from 1m below the mean radius to 1m above reclassify the system from being planet + moon to being binary?

Systems evolve, that reclassification could occur during the lifetime of a system, and I don't think a body should be reclassified from moon to planet just because it got a little further away.

A body is what a body is....it doesnt matter where it is (as the rest of these new rules would have us believe)...and I think it makes a bit of a farce of the system if suddently we have to go back to our text books because a system has evolved and scrub out 'moons...1' and instead right "binary planet"

BUT....then...there ARE likely to be binary systems out there, so at what point do you say "ok - this is now a binary system, not planet+moon" - mass, radius, etc etc....you do need a cut off, but I don't think it should be one that can change.

Doug

[Tom Tamlyn]

Today's New York Times has a good article on the controversy, with quotes from Alan Stern, as well as an op-ed piece by Mike Brown.

TTT

[David]

Systems evolve, that reclassification could occur during the lifetime of a system, and I don't think a body should be reclassified from moon to planet just because it got a little further away.

I'm just imagining:

1) A "lumpy" primary (shaped something like Iapetus) where the barycentre is sometimes below the actual surface and sometimes above it

2) An ellipsoidal primary, where the barycentre is sometimes below the actual or notional surface of the ellipsoid and sometimes above it

3) A primary with a thick atmosphere, where the barycentre is suspended somewhere "in the middle of the air"

4) A primary with an atmosphere whose height changes seasonally, so that the barycentre is sometimes in the atmosphere and sometimes in space

You can have all sorts of fun with this, the more so if you happen to live on such a world. "Is that the Moon?" "No, it's a planet... today..."

[ngunn]

I like it

Nick

Grand! However I now realise my mnemonic needs editing. I was following 'the other Doug' who inadvertently inserted an S (for Sharon?) instead of a C for Charon. My definitive version is now as follows:

Many Vexed Experts Make Confusing Judgment So Us Normal People Cry 'XXXX'

The wor(l)d denoted by XXXX has not yet been fully explored.

Incidentally I see that the word 'farce' has just entered this discussion. . . quite so.

[Greg Hullender]

It does seem that it would have been useful to create a more general term (e.g. planetoid) to cover any non-fusing body rounded by its own gravity (absent effects of rotation or tides) but ignoring its orbital dynamics. Eath, Luna, Ceres, Titan, etc. would all be planetoids. Everything smaller could be an asteroid (or maybe a new name), while everything larger would be a star. Only a truly transforming cataclysm could transform one into another.

A multiple planetoid would just be any long-term stable, gravitationally bound set of planetoids. (Likewise you could have a multiple asteroid I guess.)

The planet vs. moon distinction still comes down to whether one member of the system sufficiently dominates the rest. I suppose the barycenter definition is as good as any for this purpose, although it bothers me that it depends on the density of the planet(oid).

This also suggests that we might usefully make a distinction between a "moon," which would have to be a planetoid, and a "moonlet" (is there a better name) which is just an asteroid that's gravitationally bound to a planetoid.

[mcaplinger]

The barycenter rule is laughable, IMHO. You'd think they could have tried a little harder if the intent was to handle extrasolar double planets in the future, unless somebody had some political agenda to make Charon a planet. I'd have tried to make the minimum barycenter distance some function of the body radii so as to exclude Charon.

I also wonder how well the hydrostatic rule will work in practice around the low end, something we are likely to see either for KBOs or even for the larger asteroids.

Leave it to the IAU to overcomplicate what was a seemingly simple question.

[ljk4-1]

A camel: A horse designed by committee.