NASA Scientists Propose New Definition of Planets, and Pluto Could Soon Be Back (sciencealert.com)

Rei writes: After several years of publicly complaining about the "bullshit" decision at the IAU redefining what comprises a planet, New Horizons program head Alan Stern and fellow planetary geologists have put forth a new definition which they seek to make official, basing planethood on hydrostatic equilibrium. Under this definition, in addition to Ceres, Pluto and other Kuiper Belt objects, large moons like Titan and Europa, as well as our own moon, would also become planets; "planet" would be a physical term, while "moon" would be an orbital term, and hence one can have a planetary moon, as well as planets that orbit other stars or no star at all (both prohibited under the current definition). The paper points out that planetary geologists already refer to such bodies as planets, citing examples such as a paper about Titan: "A planet-wide detached haze layer occurs between 300-350 km above the surface; the visible limb of the planet, where the vertical haze optical depth is 0.1, is about 220 km above the surface."

Comments

Maybe

[sexconker]

Maybe stop changing arbitrary definitions. Pluto was always a planet. Fuck you, NASA and shitty celebrity "scientists" like Neil Tyson.

Re:Maybe

[Rei]

Clearly given that people like Stern have regularly given interviews decrying the decision, and going so far as to call it "bullshit" (can you say that at NASA?), it's clearly not the storm in a teacup that you want to present it as.

What the proponents did was take a term widely used by planetary geologists and have it mean something completely different - akin to dentists suddenly declaring to doctors that the heart is no longer an organ and to stop referring to it as one. And contrary to your presentation of why they did it ("to make it easier to write journal articles") without fail every last supporter I've seen interviewed about their vote has given some variant of the following reason for why they voted the way they did: "I don't want my daughter having to memorize the names of hundreds of planets." Which is so blatantly unscientific it's embarrassing that such a thing would influence their decision at all on a scientific matter.

The IAU vote was narrow, at a conference only attended at all by a fraction of its membership, on the last day when a lot of the people opposed to the definition they passed had already left because it had looked up to that point like there was either not going to be a vote at all , or one on a hydrostatic equilibrium definition - all options that they were fine with. Only 10% of the people who attended were still around.

I have a lot of issues with the last vote, and that's just the start. Here's my full list:

1. Nomenclature: An "adjective-noun" should always be a subset of "noun". A "dwarf planet" should be no less seen as a type of planet than a "dwarf star" is seen as a type of star by the IAU.

2. Erroneous foundation: Current research agrees that most planets did not clear their own neighborhoods, and even that their neighborhoods may not always have been where they are. Jupiter, and Saturn to a lesser extent, have cleared most neighborhoods. Mars has 1/300th the Stern-Levison parameter as Neptune, and Neptune has multiple bodies a couple percent of Mars's mass (possibly even larger, we've only detected an estimated 1% of large KBOs) in its "neighborhood". Mars's neighborhood would in no way would be clear if Jupiter did not exist - even Earth's might not be. Should we demote the terrestrial planets as well?

Note that the Stern-Levison parameter does not go against this, as it's built around the ability of a planet to scatter a mass distribution similar to our current asteroid belt, not large protoplanets.

3. Comparative inconsistency: Earth is far more like Ceres and Pluto than it is like Jupiter, yet these very dissimilar groups - gas giants and terrestrial planets - are lumped together as "planets" while dwarfs are excluded.

4. Poor choice of dividing line: While defining objects inherently requires drawing lines between groups, the chosen line has been poorly selected. Achieving a rough hydrostatic equilibrium is a very meaningful dividing line - it means differentiation, mineralization processes, alteration of primordial materials, and so forth. It's also often associated with internal heat and, increasingly as we're realizing, a common association with subsurface fluids. In short, a body in a category of "not having achieved hydrostatic equilibrium" describes a body which one would study to learn about the origins of our solar system, while a body in a category of "having achieved hydrostatic equilibrium" describes a body one would study, for example, to learn more about tectonics, geochemistry, (potentially) biology, etc. By contrast, a dividing line of "clearing its neighborhood" - which doesn't even meet standard #2 - says little about the body itself.

5. Mutability: Under the IA definition, what an object is declared as can be altered without any of the properties of the object changing simply by its "neighborhood" changing in any of countless ways.

6. Situational inconsistency: (Related) An exact copy of Earth (what the vast majority of people would consider the prototype for what a planet s

Re:Maybe

[mysticgoat]

Wow.

TL;DR but I got through enough of it to realize that most, and maybe all, the points are cogent. Above post should be stuffed down the throats of every IAU member who voted for their absurd definition of planet until they can regurgitate those points, with meaning.

Some astronomers are stupid. The phrase "educated beyond the level of their intelligence" comes to mind. This idiots should have been taught somewhere along the way that their expertise in one narrow field does not endow them with the authority to mess about in other disciplines like linguistics.

Re:Wow

[ShanghaiBill]

Things are finally going back to normal.

Nope. I am fine with Pluto going back to being a planet, but MOONS ARE NOT PLANETS. The concept of a "planet" has been around for millennia, and the moon (and later Jupiter's Galilean moons) have never been considered planets. We need to stop with the arbitrary redefinition of common words. If astronomers want a term to refer to hydrostatically stable bodies, they should make up a new word rather than trying to steal one that is already in use.

Question...

[Freischutz]

Personally I think the whole Pluto being a planet vs it being a dwarf planet makes about as much sense as arguing about whether American football deserves being called a football because players spend most of their time holding the ball and running around with it. Having said that, Pluto is a fascinating place regardless of it's label and, and since I'm not an astronomer, I am left wondering: Is the fight to make Pluto a planet again (or for that matter the original decision to demote it) based on sound scientific reasoning or is it just an ego driven pissing contest born injured national pride because Pluto is the only planet in the Sol system found by an American?

Re:And what are the other terms?

[silentcoder]

Indeed, he is flagrantly biassed. He was the head of the New Horizon's project - which just sent us a huge amount of data to study on Pluto. Studies which cost money, and having Pluto not deemed a planet anymore has, he says, made it harder to drum up funding for that research.

The answer of course is: because everything in our solar system should be studied - not just planets. One of the greatest achievements in recent astronomy has been landing a probe on a comet, because it taught us a great deal about comets (things we couldn't learn from the remnants that sometimes fall on earth). Studying mars is cool - but we SHOULD be putting probes on Eris as well.

Good probes to Titan, Europa and Ganymede are important studies to undertake in the near future - they are among the highest likelihood cases for life elsewhere in the solar system. Europan bacteria would be a fantastic discovery - Europan 'fish' an unlikely but amazing bonus - but we will never know if we don't go look.

If anything looking at planets is near the bottom of the priority list. Most of the planets are gas giants - there's only so much you can learn from something you can't land on.

Re:That's no moon

[mysticgoat]

Any "space-trash" that demands to be listed as something else needs to be immediately identified as a "sentient being", and on behalf of all of us Earthlings the UN needs to publicly apologize to him/her/it. That is simple playground rules: you don't want to insult anybody that much bigger than you are.

As to everything else, I think the planetary geologists have it right. If it is big enough to be rounded of its own volition, it is a planet. And planets that go around another planet more quickly than they go around their star are also moons.

Corollary: that makes Earth the larger part of a binary planetary system. Which puts proper emphasis on the way the Moon creates tides that keeps the hydrosphere stirred up, which has had a major impact on how life has evolved here. Exoplanetary explorers should look for other binary planets in the Goldilocks zone as these are much more likely to have life that is similar to Earth life.

(Is a "bazinga!" called for here? Was this just another Sheldon impersonation, or did I accidentally say something insightful?)

Re:The definition is fine

[Rei]

Saying pluto is a dwarf planet seems pretty good to me as it gives it a special place among planet like objects already.

If they had simply stopped there, that wouldn't have been a problem. The problem is that they didn't. They declared that dwarf planets aren't planets at all - which is nonsense. Mars has far more in common with Pluto than, say, Jupiter. If anything should have been separated out, it's the gas and ice giants from the rocky/icy planets.

Hydrostatic equilibrium is a very meaningful dividing line to split groupings on. If a body is in hydrostatic equilibrium, it's experienced dramatic geologic change in its history - differentiation, tectonics, internal heating, generally fluids (particularly liquid water), and on and on. It's the sort of place you go if you want to learn about planetary evolution or search for life. If a body is not in hydrostatic equilibrium, it's made of primordial materials, preserved largely intact. It's the sort of place you go to learn about the formation of our solar system and its building blocks.

It's rare that nature gives you such clear dividing lines, but when it comes to planets, it has. It's not perfect - you can (and do) have bodies that straddle the border and are only partially or slightly differentiated. But in general, nature has drawn an obvious line in the sand, and we should respect that.

if the object is really big and clear

Is Earth's orbit clear? No, we have a huge massive object co-orbiting with us. Is Neptune's orbit clear? No, it has Pluto in it. They try their hardest to pretend that the IAU actually chose a "gravitationally dominant" standard, but that's not what they actually put in the definition. The standard in the definition is "cleared the neighborhood".

And it's based on a false premise - that each planet cleared its own neighborhood. Which is just pseudoscience. All of our models show that Jupiter, and to a lesser extent Saturn, cleared most of the solar system, including the vast majority of the clearing around Mars, and a good fraction around Earth (lesser around Venus). Mars did not clear its own neighborhood. Nor is it gravitationally dominant in its neighborhood; the vast majority of asteroids are in orbital resonance with Jupiter and not Mars.

And I've heard some people try to sneak around this by saying "Okay, maybe it isn't gravitationally dominant / cleared its neighbood now, but it has enough of a Stern-Levison parameter that it would have been had Jupiter not existed". First off, that's changing the definition yet again (to "would have cleared its neighborhood if no other planets were there"). But beyond that, it's abuse of the Stern-Levison parameter. The Stern-Levison parameter is built around a body's ability to clear asteroids - bodies with the current size and orbital distribution of our asteroid belt. Not protoplanets. In the early solar system it was the ability to clear protoplanets that caused neighborhoods to be cleared. Jupiter got rid of some really massive things that were forming in and near the inner solar system. There's a reason why our planetary system has such an unusual size distribution: the inner planets start getting bigger, the stop getting bigger, then get small, then debris, then something huge. That "something huge" stripped the building blocks out of the inner solar system, preventing it from becoming dominated by super-Earths. Saturn appears to have been our savior - its (delayed) formation appears to have stopped Jupiter's inward migration.

And even just going with the Stern-Levison parameter - Neptune has a Pluto-sized body in its "neighborhood". Now, Pluto may be small compared to Neptune, but compared to Mars it wouldn't be - yet Mars has a much lower Stern-Levison parameter than Neptune. Again: the only reason Mars doesn't experience stuff like this is because Jupiter cleared its neighborhood for it.

Re:TL;DR something you claim is cogent...?

[Rei]

The IAU spend months in total hashing out this issue and three days talking in meetings before the vote

That's just the issue: that's not what happened. The IAU discussion was a disaster. Here's the timeline:

2005: The IAU appoints a committee to investigate the issue and generate a proposal. The committee investigated the issue for a year.

The IAU meeting is scheduled from 14-25 August 2006.

16 August: The committee recommends a definition based on hydrostatic equilibrium. No "cleared the neighborhood" nonsense. They publish their draft proposal.

18 August: The IAU division of planetary sciences (aka, the people who actually deal with planets) endorses the proposal.

Also 18 August: A subgroup of the IAU formed which opposed the proposal. An astronomer in the group (aka, someone who studies stars, not planets) - Julio Ángel Fernández - made up his own "cleared the neighborhood" definition. While most of the membership starts to trickle away over the next week, they remain determined to change the definition.

22 August: The original, hydrostatic equilibrium draft continued to be the basis for discussion. There were some tweaks made (some name changes and adjusting the double-planet definition), but it remained largely the same.

Late on 22 August: Fernández's group manages to get to just over half of the attendance at the (open) drafting meeting, leading to a very "heated" debate between the two sides.

22 to 24 August: The drafting group begins to meet and negotiate in secret. The last that the general attendance of the conference knew, they'll either end up with a vote on a purely hydrostatic definition, or (more likely) no vote at all due to the chaos. Attendence continues to dwindle, particularly among those who are okay with either a hydrostatic definition or none at all.

24 August: The current "cleared the neighborhood" definition is suddenly proposed and voted on on the same day. Only 10% of the conference attendance (4-5% of the IAU membership) is still present, mainly those who had been hanging on trying to get their definition through. They pass the new definition.

It's not generally laypeople who are upset about how it went down, it's IAU members. Many have complained bitterly about it to the press. The IAU's own committee of experts was ignored, in favour of a definition written in secret meetings and voted on by a small, very much nonrandom fraction of people, the vast majority of whom do not study planets.

If there's one thing I hate, it's people who pretend that anyone who opposes the IAU definition does so because they're ignorant morons overcome by some emotional attachment to Pluto, when in reality it's been planetary scientists themselves who have been the definition's harshest critics, because it's an internally self-inconsistent, linguistically flawed, false-premise-based definition that leads to all sorts of absurd results and contradicts terminology that was already in widespread use in the scientific literature.

Re:Mike Brown was the Clown Responsible

[Quirkz]

I find that title hilarious.

Re:Jesus saves

[rgbatduke]

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