Pluto as a Phoenix

by wj

Last week's Science News included an article It's time to redefine what qualifies as a planet. The proposal, by Will Grundy of Lowell Observatory and other planetary scientists: "It’s any round object in space that is smaller than a star."** That returns Pluto to the fold. Also about a hundred other objects in the solar system.

Personally, I think it wouldn't hurt to be a little more restrictive. Say exclude objects which orbit a planet which is more than 5(?) times their size. That keeps obvious moons out of the mix, while still allowing for double planets.

The argument got laid out at the Lunar and Planetary Science Conference in March. Whether these planetary scientists will (eventually) persuade the International Astronomical Union to re-revise their definition remains to be seen. But it appears that Pluto as a planet may yet rise from the ashes.

** I haven't dug down to see how the IAU formally defines "star." But I'm betting that it includes shining by its own light. Or perhaps being composed of a plasma.

Comments

[Hartmut]

Brown dwarfs would be problematic.

[Snarki, child of Loki]

"Brown dwarfs would be problematic."

ONE COMMENT, and you had to bring race and sizism into it! SHEESH!

[Matt McIrvin]

If I recall correctly, the IAU purposely did not address the question of defining a planet outside our solar system, which would have required consideration of the planet/star boundary. But since they've actually given official names to some exoplanets now, the time may be a little more ripe.

I'm fine with a definition of a planet that includes Pluto, Ceres and all of the round KBOs (that aren't moons). It'd be a simple change--just say that a dwarf planet is a kind of planet.

What doesn't make sense is to say that there are nine planets, unless you're just defining Pluto as a planet as a kind of historic landmark status.

[Ugh]

Nazi brown dwarfs for $600 Alex.

[Hartmut]

I was tempted to mention Bratzis (cf. https://en.wikipedia.org/wiki/Yoga_Hosers) explicitly but wanted to avoid this turning unserious.

[Fair Economist]

I didn't like demoting Pluto, but now that i realize there are possibly *hundreds* of Pluto-like object out past Neptune I don't think you should call them "planets". "Planets" traditionally are important, and the current definition captures that pretty well.

[hairshirthedonist]

wj just doesn't like Holst.

[McKinneyTexas]

Planetism is an ugly, ugly thing.

[wj]

Normally I would expect an ism to be advocated for by a member of the class. Which the scientists involved definitively are not. Not round enough, for one thing, nor heavy enough (I confidently assert without ever seeing them) to achieve a spherical shape with their gravity alone.

[Michael Cain]

Say exclude objects which orbit a planet which is more than 5(?) times their size.

Measured how? Diameter? Volume? Mass? By diameter, Earth is a bit under four times the size of the Moon.

[wj]

Criteria to be determined. I'm just thinking that there needs to be some provision for cases where what we have is a double planet, rather than a planet and moon.

[Matt McIrvin]

One of the draft proposals that was kicking around years ago distinguished between planets and moons by whether the system barycenter was beneath the surface--so Pluto-Charon would be a double planet rather than a planet and its largest moon. The problem I could see with that is that you could have a system (somewhere out in the Kuiper Belt, perhaps) where a body turns from a planet to a moon and back again over different parts of its orbit.

[wj]

Perhaps you could use the location of the barycenter when the objects are at their average distance.

[JakeB]

I have nothing to contribute to this discussion except http://rathergood.com/moon_song/

I favor 9 planets because that's how it was when I was a kid and therefore it is best.

[Michael Cain]

One of the draft proposals that was kicking around years ago distinguished between planets and moons by whether the system barycenter was beneath the surface--so Pluto-Charon would be a double planet rather than a planet and its largest moon.

Which leaves us with the problem of Jupiter -- the Jupiter-Sol barycenter is outside the accepted radius of the sun, making it a star/failed star double rather than a star/planet. Which has been convenient, as Jupiter has nicely managed to sweep up a lot of stuff that might have otherwise eventually hit Earth. Or, we could be inconsistent about what makes a planet rather than a failed star.

[wj]

No way to get to 9 and stop. There's more objects Pluto's size not much further out, so if it's in, then so are Sedna, et al.

[mike shupp]

What I'd like to do is to define Pluto and the Mercury-to-Neptune set as "classical planets" -- the point being we knew of their existence and called them planets before the October 4, 1957 launch of Sputnik.

[Matt McIrvin]

The term "classical planets" already exists, though--it refers to the ones known before the invention of the telescope, plus the Sun and Moon.

[Snarki, child of Loki]

"legacy planets"

The whole 'is it a planet?' thing is really stupid, and so perfect for blog arguments.

The distinction between 'real planets' and 'little lumpy orbity things that aren't real planets but play one on the internet' doesn't help understand either of them in any way. It's just the human tendency to stuff everything into a pigeonhole, whether it makes sense or not.

[Michael Cain]

What I'd like to do is to define Pluto and the Mercury-to-Neptune set as "classical planets" -- the point being we knew of their existence and called them planets before the October 4, 1957 launch of Sputnik.

As Matt McIrvin points out, the name is already taken. If you use 1957 as a cut-off, you need some other arbitrary value to exclude Ceres (discovered 1801) but still include Pluto. An obvious candidate is "discovered based on perturbations of Uranus's orbit", but today we know that the "perturbations" Lowell used were caused not by Pluto, but by incorrect estimates of Neptune's mass. Lowell's calculations pointing to the right part of the sky were one of those odd coincidences that happen from time to time.

[Michael Cain]

I haven't dug down to see how the IAU formally defines "star." But I'm betting that it includes shining by its own light. Or perhaps being composed of a plasma.

Bodies made up of mostly hydrogen and helium and massive enough to support hydrogen-hydrogen fusion (roughly 65 Jupiter masses). Brown dwarfs are such bodies massive enough to support lithium or deuterium fusion, but not hydrogen-hydrogen (roughly 13-65 Jupiter masses). Gas giants are such bodies too small to support fusion at all (<13 Jupiter masses).

[Cranky Observer]

It is only a planet if the name can be written in cursive script.

[Matt McIrvin]

The whole dwarf-planet saga played out in a similar manner with the asteroids in the early 19th century. For a while, Ceres, Juno, Vesta and Pallas were listed as planets in astronomy texts, with fun symbols associated with them and everything. As the number of discovered asteroids began to increase, astronomers decided that "minor planets" weren't planets.

You could, however, definitely make a reasonable case for Ceres being a planet, if not the others. Any definition of a planet beyond "if it's good enough for granddad" that includes Pluto should probably include Ceres as well as all the big round KBOs.

[Marty]

I think it should be a planet if it makes the mobile of the solar system look cooler.

[wj]

The distinction between 'real planets' and 'little lumpy orbity things that aren't real planets but play one on the internet' doesn't help understand either of them in any way.

Actually, if you use the "round" definition you do have something useful. If nothing else, the geology is going to be different if the object has sufficient gravity to achieve that.

[cleek_with_a_fake_beard]

shape and orbit-clearing are about the only things the planets have in common. the gas giants are really nothing like the rocky planets.

[wj]

And orbit clearing is chancy. After all, even earth has stuff that is arguably not cleared from its orbit. And once you get beyond Neptune, there's just way too much real estate for clearing to happen.

[cleek_with_a_fake_beard]

the stuff in earth's orbit is just minor junk that's in the process of being cleared.

but orbit-clearing does exclude the asteroids.

we could also require that planets orbit on the same plane as one another. which gets rid of Pluto.

[wj]

But orbital plane requirements leave you with the question of how you deal with something earth-size which isn't actually orbiting a star.

Not that you couldn't just make up a new classification, of course. But what purpose would it serve to have different classifications for things that are otherwise identical?

[cleek_with_a_fake_beard]

it's all arbitrary. might as well come up with the right set of arbitrary criteria that fits with popular sentiment.

[Michael Cain]

but orbit-clearing does exclude the asteroids

And with what we know today, Jupiter. Estimates are that the leading and trailing trojan groups that share Jupiter's orbit include as many bodies as there are in the asteroid belt.

I can accept cleek's "popular sentiment" argument: nine planets, with Pluto grandfathered in because tradition.

[Hartmut]

Given the three-dimensional pool billiard that takes place during the formation of a planetary system, I would not even put that much value on the orbital plane. Would a planet turn into a non-planet, when a series of involontary orbital maneuveres shifts its orbit to one that in an extreme case would go over the solar poles?

[Lurker]

Michael Cain,

I understand the point about Pluto needing to be grandfathered in. However, it is clear that Pluto is, by no means, unique. There are a lot of objects like it, but none of those objects is really a planet.

The eight planets of the current IAU definition are clearly different from everything else in the solar system. Talking about the planet-sized moons of the gas planets is a bit misleading, because those objects orbit larger planets.

Finally, having eight planets and not a hundred is good for paedagogical reasons. You can have children learn eight planets by heart. If there are a hundred planets, they will not even learn Venus, Mars and Jupiter.

[joel hanes]

What's Montague? It is nor hand, nor foot, Nor arm, nor face, nor any other part Belonging to a man. O, be some other name! What's in a name? that which we call a rose By any other name would smell as sweet;

Pluto is what it is, and all our shuffling of categories matters not a bit.

These two of the great Gene Spafford's observations about Usenet seem apposite :

Corollary #1:
Attempts to change the real world by altering the structure of the Usenet is an attempt to work sympathetic magic -- electronic voodoo.
Corollary #2:
Arguing about the significance of newsgroup names and their relation to the way people really think is equivalent to arguing whether it is better to read tea leaves or chicken entrails to divine the future.

[Fair Economist]

And once you get beyond Neptune, there's just way too much real estate for clearing to happen.

Not true at all; the likely Planet X had its approximate orbit identified by how objects all over the outer solar system have been cleared or moved, in spite of being about 20 times further out than Neptune.

[Jim Parish]

but orbit-clearing does exclude the asteroids

And with what we know today, Jupiter. Estimates are that the leading and trailing trojan groups that share Jupiter's orbit include as many bodies as there are in the asteroid belt.

From the Wikipedia article on "Clearing the neighborhood":

"The phrase refers to an orbiting body (a planet or protoplanet) 'sweeping out' its orbital region over time, by gravitationally interacting with smaller bodies nearby. Over many orbital cycles, a large body will tend to cause small bodies either to accrete with it, or to be disturbed to another orbit, or to be captured either as a satellite or into a resonant orbit. As a consequence it does not then share its orbital region with other bodies of significant size, except for its own satellites, or other bodies governed by its own gravitational influence. This latter restriction excludes objects whose orbits may cross but that will never collide with each other due to orbital resonance, such as Jupiter and its trojans"

[Snarki, child of Loki]

wj: "Actually, if you use the "round" definition you do have something useful. If nothing else, the geology is going to be different if the object has sufficient gravity to achieve that."

But that "useful" part has ZERO to do with the "lumpy orbity thing"'s relationship with other "lumpy orbity things", so all the 'clearing the neighborhood', 'not in orbit around a gas giant', 'in a stellar orbit' stuff is irrelevant.

This is all about trying to shoehorn the Universe into arbitrary human linguisting categories.

Re: Spafford: this is WHY I mentioned that "arguing about what is and is not a planet is the PERFECT subject for internet arguments".

When we start to get spammy telemarketing calls from Pluto because we gave them their own internet domain, we can revisit the issue.

[Jim Parish]

Some years back, when the Pluto kerfuffle was in the news, I asked a friend of mine who had some expertise in the area what the use of the concept "planet" was; I found his response interesting. It's the third comment in this LiveJournal entry:
http://pompe.livejournal.com/121086.html

[wj]

So what would be the utility of differentiating between stars and planets? Or between planets and comets? Is it all just "trying to shoehorn the Universe into arbitrary human linguistic categories"?

The definition of a "planet" falls into the same category. It's a category which helps us understand a class of things, which class differs from other classes of things.

[Fair Economist]

Whether something is round or not isn't very "useful" in a scientific sense. Whether it significantly affect orbits of other objects is. The possible effects of Planet X has almost become a sub-field of astronomy (for example, the predicted orbit would explain why the axes of the planets' orbits are tilted from the Sun's rotational axis). That's pretty darn interesting. If it exists, it's going to be round, but who cares about that?

[wj]

Being round is not, in and of itself, interesting from a scientific standpoint. But that doesn't mean that it is scientifically irrelevant.

If an object is round, that's because of its mass. Its gravity. And the result is that it probably has geology -- which is scientifically interesting.

For openers, it's interesting to see what kinds of geological layers gravity has produced. An object too small to be pulled into a sphere is necessarily just a hodgepodge of whatever stuff came together to make it. But a round object can not only have differentiated layers, the movement of its components can produce rocks which do not exist outside planets. And which ones are present is also interesting . . . at least to a geologist.

[Scott P.]

"It's all arbitrary. might as well come up with the right set of arbitrary criteria that fits with popular sentiment."

I expect you would hesitate to use that method to determine who gets to be a US citizen, so why should we use it as an arbiter of a scientific definition?

[Snarki, child of Loki]

The IAU, the people that decided that Pluto would no longer be "officially" a planet, really missed an opportunity with the nomenclature used for Pluto and the "lumpy orbity things" beyond.

Because the IAU are the ones that decide on NAMES for those bodies, like "Sedna" and "Quoar?" etc.

See, they should have named the first one after Pluto "Micky", followed by "Goofy", "Donald", etc.

Collectively, they could be termed the "Disney Planets". And likely get some significant research funding from DisneyCorp.

How else are we going to explore the heavenly bodies in the Princess Belt, I ask you?

[CharlesWT]

Who cares about Pluto anyway since there can be no sexual relationships there...

[formerly known as cleek]

why should we use it as an arbiter of a scientific definition?

well, i did write "popular sentiment", not "scientific definition".

[Donald]

Using Pluto as an open thread.

The following is kind of ridiculous-- I feel like we are asking the Czar's daughter to use her tear ducts to get him to notice something. Probably won't work, but with Trump maybe the sheer chaos of his thought processes could be bent in the right direction once in a while.

http://petitions.moveon.org/sign/ivankatrump-watch-yemen

[wj]

I feel like we are asking the Czar's daughter to use her tear ducts to get him to notice something.

Most likely, we are. If only because it appears to be the best (only?) option available. At least, if there is a better option available, I am too dense to have picked up on it.