"The Definition of Planet" (or, "The IAU is Dumb")

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"The Definition of Planet" (or, "The IAU is Dumb")

Postby King Author » Thu Dec 15, 2011 10:08 am UTC

The Pluto topic is two years old and Searching for "definition of planet" turns up only unrelated stuff (well, plus that Pluto topic), hence the new topic.

I just watching an episode of Nova on the Pluto controversy, and a thought struck me. The IAU definition of "planet" is based upon the appearance of our solar system at present time. Well, that seems temporally short-sighted and myopic. That'd be like classifying organisms based upon the way species look right now, rather than on their genetic history, common ancestors and so forth. I think a proper scientific definition of planet should be based upon the way celestial objects form.

Because think about this. If we consider a "planet" to be a celestial body that...
1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium and
3. has "cleared the neighbourhood" around its orbit.
...then during the evolution of a solar system, an object can go back and forth between being a planet. Which certainly doesn't make astronomy easier, and is patently silly.

For example, imagine if by some cosmic catacylsm, the Earth was knocked off its orbit and settled in the Kuiper Belt. According to the IAU definition, Earth would no longer be a planet, which is simply ridiculous. Additionally, if, say, Earth's moon escaped its orbit and settled into an orbit between Earth and Mars, it would become a planet, which, if you consider the formation of the moon, is again preposterous.

Here's what I think is a much better, more scientific definition of "planet." A celestial body that...
1. was formed in the accretion disc of a star and
2. has sufficient mass to assume hydrostatic equilibrium.

The first makes it clear that both small, rocky planets like Mercury and Earth and gas giants like Saturn and Jupiter are "planets," despite their extreme differences in composition, and delineates planets from natural satellites (and and it's not biased towards our solar system the way the IAU definition is -- my definition works for other solar systems as well). The second rules out asteroids, comets and the like.

Unlike the flawed IAU definition, this definition wouldn't classify an object differently based upon its location (which has no scientific usefulness and is ridiculous). So the Earth would still be a planet even if it was flung out to the Kuiper belt, or ejected from the solar system entirely, because it was formed in the accretion disc of the sun, and the Moon would still be a moon even if it escaped Earth and got its own orbit, because it formed in the accretion disc of the Earth after the Earth-Theia collision.

This would also help to settle the Pluto controversy, because we'd at least have a scientific way of saying it is or isn't a planet -- by determining how it formed (from the sun's accretion disc or not?), whereas the flawed IAU definition only spurs controversy because it isn't based on any hard scientific facts, it's just "what we say goes."

Most importantly of all, this definition spurs scientific endeavour -- let's find out how all the dwarf planets formed, so we can properly classify them!

Now, true, this deifinition does make it possible that our solar system has dozens of planets (which it essentially does, if you want to get right down to it; look at Ceres and Mercury in a void -- they're clearly the same species of thing, regardless of their size differences), but supporting the IAU definition because you don't like the idea of tons of planets in our solar system is asinine.

So, what do you think?
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby Gigano » Thu Dec 15, 2011 11:01 am UTC

"That'd be like classifying organisms based upon the way species look right now, rather than on their genetic history, common ancestors and so forth."

Just a short contribution: organisms ARE classified by what they look light right now. We haven't classified Bambuseae
or Escherichia coli or Pan troglodytes into plants, bacteria and mammals based upon their genetic history, rather by the features they currently display and share with similar organisms. Definitions are arbitrary by their very nature and once a definition has been set, borderline cases (e.g. are viruses alive, or is Pluto a planet?) tend to show how arbitrary definitions can be.

Honestly, I don't care Pluto got demoted: it's a piece of matter orbiting a star like most everything in our solar system.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby yurell » Thu Dec 15, 2011 11:27 am UTC

King Author wrote:The IAU definition of "planet" is based upon the appearance of our solar system at present time. Well, that seems temporally short-sighted and myopic. That'd be like classifying organisms based upon the way species look right now, rather than on their genetic history, common ancestors and so forth.


Ummmm, I think you'll find we do classify organisms based on the way the species look right now ... a human is a mammal, despite the fact we evolved from a reptile. We have names for species that simply didn't exist a few million years ago, so if we didn't base them on the way they looked now, they wouldn't even be named.

King Author wrote:I think a proper scientific definition of planet should be based upon the way celestial objects form.


Okay, and what are the consequences of this? What happens if I form a planet by your definition, and it gets worn down to the size of a pebble -- is that pebble still a planet? I think it's ludicrous to assume that it is. And what about artificially constructed planet? From your definition they wouldn't be considered planets, despite being identical to one in every conceivable way.

King Author wrote:...then during the evolution of a solar system, an object can go back and forth between being a planet. Which certainly doesn't make astronomy easier, and is patently silly.


Indeed they can. How does this make astronomy any harder? You don't seriously expect this definition to stand for millions of years while things change on astronomical time scales, do you? How is it patently silly to define objects based on their current state? For example, stars are classified based on how they are now -- should I be calling a white dwarf a yellow main-sequence star? Should I be calling a neutron star a core-collapse supernova? Or, if it's based on formation, should i simply be calling all stars Jeans-mass Nebulae?

King Author wrote:For example, imagine if by some cosmic catacylsm, the Earth was knocked off its orbit and settled in the Kuiper Belt. According to the IAU definition, Earth would no longer be a planet, which is simply ridiculous.


I don't think the definition of 'planet' will be much of a concern if that happens. A system doesn't have to make allowances for exceptionally unlikely circumstances, and if they are faced with ludicrous situations they can be changed.

King Author wrote:Additionally, if, say, Earth's moon escaped its orbit and settled into an orbit between Earth and Mars, it would become a planet, which, if you consider the formation of the moon, is again preposterous.


In what way is it preposterous? The moon is nearly six times more massive than Pluto, which you believe should be a planet. In fact, it's almost a quarter the mass of Mercury, making it by far large enough to qualify under the current definition of 'dwarf planet'.

King Author wrote:Here's what I think is a much better, more scientific definition of "planet." A celestial body that...
1. was formed in the accretion disc of a star and
2. has sufficient mass to assume hydrostatic equilibrium.

The first makes it clear that both small, rocky planets like Mercury and Earth and gas giants like Saturn and Jupiter are "planets," despite their extreme differences in composition, and delineates planets from natural satellites (and and it's not biased towards our solar system the way the IAU definition is -- my definition works for other solar systems as well). The second rules out asteroids, comets and the like.


For the record, the IAU definition was made deliberately for only our solar system. But quite apart from this, how does your definition help? Ganymede was formed in the accretion disk of the sun (more specifically, in the Jovian sub-nebula), so is it a planet? What about the rest of the moons that were formed in this manner? If you don't like sub-nebulae, let's consider Triton, which is believed to be a Kuiper-belt object (the same as Pluto), captured by Neptune's gravity. Under that definition, it's a planet. You're happy with this?
What if it's discovered that the Earth is actually the drawn-together remnants of a larger planet that got smashed, and so formed in isolation — would you then support its demotion from the role of planet?

King Author wrote:Unlike the flawed IAU definition, this definition wouldn't classify an object differently based upon its location (which has no scientific usefulness and is ridiculous).


Okay, here I call bullshit. It can have 'scientific usefulness' to define an object differently based on its location because it can be fulfilling a different role. After all, we are labelling them as part of their role in the current morphology of the solar system. Under your definition, a body that formed outside of the accretion disk of a star wouldn't be counted as a planet even if it was fulfilling the exact same role as Jupiter in our solar system. How is that not ridiculous?
An example of a role-change is a LEO satellite moving into geosynchronous orbit. It will come as no surprise to you that it stops being a LEO satellite once this happens ... should we object to this 'magical' transformation? Or is it just the fact that it formed the same way as a geosynchronous satellite that makes it alright? In which case, you must be having fits about the idea that the ISS and the moon are both satellites of the Earth, despite having completely different formations.

King Author wrote:So the Earth would still be a planet even if it was flung out to the Kuiper belt, or ejected from the solar system entirely, because it was formed in the accretion disc of the sun, and the Moon would still be a moon even if it escaped Earth and got its own orbit, because it formed in the accretion disc of the Earth after the Earth-Theia collision.


How can it be a moon if it's not orbiting a planet? Or are you changing the definition of 'moon' now? The current definition simply isn't designed to deal with your ludicrous scenarios, it was designed for ease of use in our current solar system. Sometimes ease of use is more important than completeness under every conceivable (if not plausible) circumstance.

King Author wrote:This would also help to settle the Pluto controversy, because we'd at least have a scientific way of saying it is or isn't a planet -- by determining how it formed (from the sun's accretion disc or not?), whereas the flawed IAU definition only spurs controversy because it isn't based on any hard scientific facts, it's just "what we say goes."


How would it help the 'controversy' at all? We do have a scientific way of saying whether it is or not a planet and the 'controversy' still continues. What makes you think it will be better your way? What, everyone who thinks Pluto shouldn't be a planet simply accept that it is the exact way people who currently think it should be haven't?
And saying the IAU's definition isn't based on scientific facts is ludicrous — they're defining something, and they've selected the characteristics for this definition. They're not arbitrarily labelling things 'planet, not planet, planet, hmmm, maybe a star', which is how it was done before this definition. They now have a rule that they apply consistently to determine what a planet is, and to say it's not 'based on scientific facts' in this instance is akin to saying 'I don't like your definition ergo it's wrong.' It is based on scientific facts, you just disagree with their decision.

King Author wrote:Most importantly of all, this definition spurs scientific endeavour -- let's find out how all the dwarf planets formed, so we can properly classify them!


You're really scraping the bottom of the bucket here. You seriously think changing the definition of planet will seriously make people more eager to go looking for them? Remember all the time biologists changed definitions, and it made people rush out to find new species? Neither do I.

King Author wrote:Now, true, this deifinition does make it possible that our solar system has dozens of planets (which it essentially does, if you want to get right down to it; look at Ceres and Mercury in a void -- they're clearly the same species of thing, regardless of their size differences)


Regardless of their size difference? Look at the bloody moon and Mercury in a void — they're clearly the same species of thing, and almost identical in size. Look at Ganymede and Mercury in a void, are you seriously telling me one's a planet and the other isn't if they're both in a void? Hell, you're saying Pluto (and presumably Charon) should be planets, and yet the moon is more similar to Mercury than Pluto is in a void.

King Author wrote:but supporting the IAU definition because you don't like the idea of tons of planets in our solar system is asinine.


In what way is it asinine? What's wrong with defining 'planet' in a way that allows there to be few enough to be memorable, and leaving the rest as dwarf planets? Can you name all the wonders of the ancient world? Probably. Can you name all the ancient buildings ever built that have survived? Probably not. Is it asinine that we should have these as a separate 'category' to all the other monuments?
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby BlackSails » Thu Dec 15, 2011 1:53 pm UTC

I really dont see a problem with things being able to become planets, then not become planets. People can alternate between sick and not sick.

Lets say the moon were to be knocked out of our solar system entirely and end up in orbit around another star. Why would it not be a planet?

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby SlyReaper » Thu Dec 15, 2011 2:41 pm UTC

BlackSails wrote:I really dont see a problem with things being able to become planets, then not become planets. People can alternate between sick and not sick.

Lets say the moon were to be knocked out of our solar system entirely and end up in orbit around another star. Why would it not be a planet?

It could already be considered a planet under some definitions. The dominant gravitational force acting on it comes from the sun, not the earth. It's actually orbiting the sun, while the earth perturbs that orbit in such a way as to create the illusion that it's orbiting us. So you could say it's a planet orbiting the sun in 1:1 resonance with the earth. The only reason it's ambiguous is because we don't have a solid definition that draws a firm line between a moon and a planet.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby gmalivuk » Thu Dec 15, 2011 3:35 pm UTC

SlyReaper wrote:It's actually orbiting the sun, while the earth perturbs that orbit in such a way as to create the illusion that it's orbiting us.
Um, no it's not.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby yurell » Thu Dec 15, 2011 3:43 pm UTC

How does one determine something like that? I just quickly ran through some of the maths and the sun is applying about twice as much force to the moon as the Earth is. Do we just accept that it's simultaneously orbiting both?
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby gmalivuk » Thu Dec 15, 2011 3:46 pm UTC

http://en.wikipedia.org/wiki/Sphere_of_ ... ynamics%29 and http://en.wikipedia.org/wiki/Hill_sphere both put the Moon quite comfortably within Earth's relevant spheres.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby yurell » Thu Dec 15, 2011 3:56 pm UTC

I ... can't believe I forgot about the Hill sphere, I had to derive that last semester *facepalms*
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby LaserGuy » Thu Dec 15, 2011 4:17 pm UTC

King Author wrote:Here's what I think is a much better, more scientific definition of "planet." A celestial body that...
1. was formed in the accretion disc of a star and
2. has sufficient mass to assume hydrostatic equilibrium.


Just gonna leave this here. From Wikipedia:

Also many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. There are 19 moons in the Solar System that have achieved hydrostatic equilibrium and would be considered planets if only the physical parameters are considered. Both Jupiter's moon Ganymede and Saturn's moon Titan are larger than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede has a magnetic field. Just as stars in orbit around other stars are still referred to as stars, so some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets.[66][67][68] Indeed Mike Brown makes just such a claim in his dissection of the issue, saying:[59]

It is hard to make a consistent argument that a 400 km iceball should count as a planet because it might have interesting geology, while a 5000 km satellite with a massive atmosphere, methane lakes, and dramatic storms (Titan) shouldn't be put into the same category, whatever you call it.


And indeed, based solely on hydrostatic equilibrium:

The qualifying feature of dwarf planets is that they must "have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical shape)."[1][2][3] Those dwarf planets lying beyond the orbit of Neptune are termed "plutoids", after Pluto. Except for Pluto and Ceres, observations are insufficient for direct classification. However, based on present knowledge of how icy bodies gravitationally relax into equilibrium shapes, there are currently about 73 potential candidates amongst the population of trans-Neptunian objects (TNOs).[4] It is estimated that there are around 200 dwarf planets in the Kuiper belt and up to 2000 in the region beyond.[4]


It's worth pointing out that most, if not all Kuiper belt objects (as well as the aforementioned moons) are almost certainly formed from the accretion disk of the Sun, and would therefore certainly quality. It's not exactly clear to me which objects in the Solar system wouldn't in fact meet this criterion, in fact.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby SlyReaper » Thu Dec 15, 2011 4:23 pm UTC

gmalivuk wrote:
SlyReaper wrote:It's actually orbiting the sun, while the earth perturbs that orbit in such a way as to create the illusion that it's orbiting us.
Um, no it's not.


From what I've read, the sun's force on the moon is more than twice as strong as the Earth's force on the moon.

In the case of the Earth's Moon, the Sun actually "wins" the tug of war with a value of only 0.46, which means that Earth's hold on the Moon is less than half the Sun's hold. Since the Sun's gravitational effect on the Moon is more than twice that of Earth's, Asimov reasoned that the Earth and Moon must form a double-planet system. This was one of several arguments in Asimov's writings for considering the Moon to be a planet rather than a satellite.[4]

We might look upon the Moon, then, as neither a true satellite of the Earth nor a captured one, but as a planet in its own right, moving about the Sun in careful step with the Earth. To be sure, from within the Earth-Moon system, the simplest way of picturing the situation is to have the Moon revolve about the Earth; but if you were to draw a picture of the orbits of the Earth and Moon about the Sun exactly to scale, you would see that the Moon's orbit is everywhere concave toward the Sun. It is always "falling toward" the Sun. All the other satellites, without exception, "fall away" from the Sun through part of their orbits, caught as they are by the superior pull of their primary planets – but not the Moon.[4]
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If there's some subtlety I'm missing here, what is it?
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby thoughtfully » Thu Dec 15, 2011 4:41 pm UTC

A few remarks on Pluto.

Pluto has the highest orbital eccentricity and inclination to the ecliptic of any of the "nine planets". By a fair bit.
Part of Pluto's orbit actually passes closer to the Sun than Neptune's. From 1979-1999, it was actually the eighth planet!
Compositionally (and orbitally), it resembles a comet more than anything else.
Any well-informed eight year old can see that this thing is not like the others.
http://en.wikipedia.org/wiki/Pluto#Orbit_and_rotation

Historically, it's status as a planet was largely due to it being found while searching for a planet. A fictitious one, even, that arose from a combination of imprecise measurements and/or inadequate analysis.

Kids get attached to the planets at an early age, but usually aren't so savvy with the details. Hence all the wailing and gnashing of teeth.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby gmalivuk » Thu Dec 15, 2011 5:03 pm UTC

SlyReaper wrote:From what I've read, the sun's force on the moon is more than twice as strong as the Earth's force on the moon.
Yep. But the radius of Earth's sphere of influence is more than twice the size of the Moon's orbit, and its Hill sphere is 4 times that radius. What this means is that a body's path can be described quite accurately as a conic section with Earth as a focus even if the Sun's gravity were 8 times stronger there than Earth's, instead of only twice as strong.

I mean, yes, we could also treat this as a case of everything orbits the sun but some things have larger or smaller perturbations added to that orbit by other nearby masses, but it seems far simpler to treat most things as being in orbit around whichever body lies closest to the focus of their orbital trajectories, from its reference frame.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby SlyReaper » Thu Dec 15, 2011 5:11 pm UTC

How does that work? Isn't the earth's sphere of influence the volume where the dominant gravitational force is the earth? How can something be inside this and yet be more strongly influenced by the sun?
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby gmalivuk » Thu Dec 15, 2011 6:10 pm UTC

No, it's the area where the shape of something's orbit can be well described with a simple two-body solution that ignores the Sun entirely.

Edit: I'd imagine that this is because, within a secondary body's sphere of influence, the force of gravity from the primary has a fairly constant magnitude and direction, even if that magnitude happens to be stronger than the gravity from the secondary. As such, it's not going to have a varying influence on the direction of tertiary bodies orbiting the secondary body, so it won't appreciably affect their orbits from the perspective of the secondary.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby mfb » Thu Dec 15, 2011 10:42 pm UTC

That is the important point. You have to compare how inhomogeneous the gravitational fields from earth and sun are. And there, the earth wins as that runs with 1/r^3 and not with 1/r^2.
The sun attracts earth+moon nearly with the same force and in the same direction.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby gmalivuk » Thu Dec 15, 2011 11:47 pm UTC

As a related aside, my roommate just suggested that the criterion for distinguishing between a planet+satellite system and a double planet system could be whether the barycenter around which both bodies orbit is within one of them. Thus, massive as it may be, Earth+Moon is clearly a planet with a satellite, whereas Pluto+Charon is arguably a double (dwarf) planet (with three additional satellites).

Of course, the fact that the Sun+Jupiter barycenter is outside the sun might make people like King Author whine that this definition is somehow flawed, too, but I think we've already established that his argument in the OP leaves rather a lot to be desired.

Edit:
LaserGuy wrote:From Wikipedia:

Just as stars in orbit around other stars are still referred to as stars, so some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets.
This, too, could be resolved by considering the characteristics of said orbits. Stars in binary (or more) configurations invariably(?) orbit around barycenters that are in the middle of empty space, whereas all the planet+satellite systems in around the Sun have barycenters that are firmly within the surface of the planet.

Again, the fact that the Sun+Jupiter center is outside the Sun might throw a wrench in the works. Or would, if Jupiter were massive enough to actually become a star. Which it isn't, so I'm not going to lose any sleep over it.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby yurell » Thu Dec 15, 2011 11:57 pm UTC

gmalivuk wrote:As a related aside, my roommate just suggested that the criterion for distinguishing between a planet+satellite system and a double planet system could be whether the barycenter around which both bodies orbit is within one of them. Thus, massive as it may be, Earth+Moon is clearly a planet with a satellite, whereas Pluto+Charon is arguably a double (dwarf) planet (with three additional satellites).

Of course, the fact that the Sun+Jupiter barycenter is outside the sun might make people like King Author whine that this definition is somehow flawed, too, but I think we've already established that his argument in the OP leaves rather a lot to be desired.


That's the definition I tend to use, although I only apply it to 'planets' and not stars (which are entirely different beasts in terms of mass dominance ... Jupiter is only a really tiny proportion of the sun's mass, but is really far away). I don't think the IAU has actually defined 'moon', but they don't accept Charon as a dwarf planet for some reason.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby mfb » Fri Dec 16, 2011 2:19 pm UTC

The barycenter definition can become tricky if you have 3 candidates in hydrostatic equilibrium. If the barycenter is always within one object (and the system cleaned its orbit), this is a planet and I think it is fine to call all other objects "moon". But what happens if that is not the case? Imagine Pluto/Charon with an additional large third object in an orbit around the system of both.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby yurell » Fri Dec 16, 2011 2:25 pm UTC

mfb wrote:The barycenter definition can become tricky if you have 3 candidates in hydrostatic equilibrium. If the barycenter is always within one object (and the system cleaned its orbit), this is a planet and I think it is fine to call all other objects "moon". But what happens if that is not the case? Imagine Pluto/Charon with an additional large third object in an orbit around the system of both.


How large? If it's large enough to displace the barycentre significantly, then what's wrong with calling it a ternary system? Although I really doubt such a system would be stable.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby idobox » Fri Dec 16, 2011 3:21 pm UTC

Also, the definition of a star is pretty straightforward. If it undergoes, or did, significant amounts of fusion, then it's a star. Drawing the line between a tiny brown dwarf and a giant super-Jupiter might be a bit tricky, though.

By the way, I remember reading about hypothetical lone planets that escaped their star and travel without ever orbiting. Did someone ever bother defining what piece of matter deserves to be called a planet in this case?
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby mfb » Fri Dec 16, 2011 6:30 pm UTC

yurell wrote:How large? If it's large enough to displace the barycentre significantly, then what's wrong with calling it a ternary system? Although I really doubt such a system would be stable.

And what is "significantly"? Or "large"?
Of course, you can compare all displacements to the radius of the largest object, but that looks a bit arbitrary.

Another general issue is that the definition via the barycenter is really sensitive to the distance. Two objects in a really close orbit cannot be a double planet, while one smaller object far away can be a part of that.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby LaserGuy » Fri Dec 16, 2011 7:57 pm UTC

mfb wrote:
yurell wrote:How large? If it's large enough to displace the barycentre significantly, then what's wrong with calling it a ternary system? Although I really doubt such a system would be stable.

And what is "significantly"? Or "large"?
Of course, you can compare all displacements to the radius of the largest object, but that looks a bit arbitrary.

Another general issue is that the definition via the barycenter is really sensitive to the distance. Two objects in a really close orbit cannot be a double planet, while one smaller object far away can be a part of that.


Barycenter is dependent on mass as well. Two planets of equal mass and similar volumes could quite easily have a barycenter that is in neither planet even if the distance between them is vanishingly small.

Incidentally, under the barycenter definition, the Moon will, in some 4 million of years, presumably get a status upgrade to a planet or dwarf planet, since the Earth and Moon are receding from each other at a rate of about 4 cm per year* and after about that time, the barycenter will then be outside of the Earth.

[*]There is no reason to believe that this rate will actually stay constant, though.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby yurell » Fri Dec 16, 2011 11:38 pm UTC

mfb wrote:And what is "significantly"? Or "large"?
Of course, you can compare all displacements to the radius of the largest object, but that looks a bit arbitrary.


You could use the current limit for cleared their orbit. If it's larger than the limit, then it's a dwarf planet (so long as it's been pulled into a rough sphere, of course), and f it's smaller then it's a moon orbiting Pluto / Charon.
Of course, this is just conjecture since we don't actually have a definition of 'moon' afaik.

LaserGuy wrote:Incidentally, under the barycenter definition, the Moon will, in some 4 million of years, presumably get a status upgrade to a planet or dwarf planet, since the Earth and Moon are receding from each other at a rate of about 4 cm per year* and after about that time, the barycenter will then be outside of the Earth.


If the definition proves so good and useful that it lasts for four million years, I'm pretty sure this is an acceptable sacrifice.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby mfb » Sat Dec 17, 2011 1:39 pm UTC

LaserGuy wrote:Barycenter is dependent on mass as well.

I want to have that the definitions are mass-dependent of course, so that is fine.

>> You could use the current limit for cleared their orbit.
Their orbit around pluto? Interesting definition... that may work. But only together with the barycenter in general, otherwise most large moons in the solar system would qualify as planet.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby AvatarIII » Sat Dec 17, 2011 3:02 pm UTC

idobox wrote:Also, the definition of a star is pretty straightforward. If it undergoes, or did, significant amounts of fusion, then it's a star. Drawing the line between a tiny brown dwarf and a giant super-Jupiter might be a bit tricky, though.

By the way, I remember reading about hypothetical lone planets that escaped their star and travel without ever orbiting. Did someone ever bother defining what piece of matter deserves to be called a planet in this case?


if it undergoes any fusion at all, it's a star. if it doesn't, it's a planet. Surely?

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby idobox » Sat Dec 17, 2011 3:18 pm UTC

AvatarIII wrote:if it undergoes any fusion at all, it's a star. if it doesn't, it's a planet. Surely?

If you have enough hydrogen, even at low pressure and temperature, some fusion ought to occur, even if it's on the order of a few reactions per cubic km per year.
Significant doesn't mean a lot in my mind, but significant in comparison to the heat acquired during accretion, and the power received from outside.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby AvatarIII » Sat Dec 17, 2011 3:50 pm UTC

idobox wrote:
AvatarIII wrote:if it undergoes any fusion at all, it's a star. if it doesn't, it's a planet. Surely?

If you have enough hydrogen, even at low pressure and temperature, some fusion ought to occur, even if it's on the order of a few reactions per cubic km per year.
Significant doesn't mean a lot in my mind, but significant in comparison to the heat acquired during accretion, and the power received from outside.


but you need some sort of limit and saying "any" makes more sense than some arbitrary number of reactions per whatever.

perhaps just say if it has a positive net energy output?

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby Mr_Rose » Sat Dec 17, 2011 6:03 pm UTC

AvatarIII wrote:
idobox wrote:
AvatarIII wrote:if it undergoes any fusion at all, it's a star. if it doesn't, it's a planet. Surely?

If you have enough hydrogen, even at low pressure and temperature, some fusion ought to occur, even if it's on the order of a few reactions per cubic km per year.
Significant doesn't mean a lot in my mind, but significant in comparison to the heat acquired during accretion, and the power received from outside.


but you need some sort of limit and saying "any" makes more sense than some arbitrary number of reactions per whatever.

perhaps just say if it has a positive net energy output?

Then isn't Jupiter already a star by that definition? I'm pretty sure I heard it gives off more energy than it receives annually. Of course the difficulty is separating leftover heat of formation (i.e. gpe dissipating as heat) and radioactive decay output from fusion output, if any.

In fact I'm also pretty sure you could come up with a situation for a rocky earth-sized planet to qualify as a star based (solely) on net energy output if you caught it early enough in its formation/far enough away from its primary, since the dominant energy input is almost always going to be the local star. So energy balance qualifiers are tricky to generalise satisfactorily.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby gmalivuk » Sat Dec 17, 2011 9:46 pm UTC

I assumed AvatarIII meant positive energy balance from fusion.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby Copper Bezel » Sat Dec 17, 2011 10:56 pm UTC

So, you could drop net energy output from the definition completely, then, couldn't you? Phrase it to say that a star produces from fusion (as opposed to radiates or outputs) more energy than it receives in radiation (which excludes leftover heat as well as tidal forces, etc.)?
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby Mr_Rose » Sat Dec 17, 2011 11:04 pm UTC

Copper Bezel wrote:So, you could drop net energy output from the definition completely, then, couldn't you? Phrase it to say that a star produces from fusion (as opposed to radiates or outputs) more energy than it receives in radiation (which excludes leftover heat as well as tidal forces, etc.)?

But how do you distinguish fusion from the rest? Neutrino production I guess?
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby idobox » Sun Dec 18, 2011 4:55 pm UTC

Also, a planet expelled from its star would receive almost no energy.
I'd prefer to compare total energy produced over the life of the object by fusion to, for example, gravitational binding energy (the threshold doesn't need to be one, we could consider a star needs to produces 1000 times as much energy). This definition works in the early universe as well as the in the far future, for many different kinds of objects.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby mfb » Sun Dec 18, 2011 8:05 pm UTC

Gravitational binding energy can become really large for neutron stars or even black holes (with an additional definition problem there for free).

I think fusion is a good approach for a star, maybe measured as power per volume (core volume?) or total power. If it ever exceeds threshold x for at least some thousand years, it is a star. I don't think this timespan matters, I just want to avoid a shock wave in jupiter-sized planets from supernovae or other strange stuff. If it qualified as a star, but fusion is below x now, it is the remainder of a star.


>> I assumed AvatarIII meant positive energy balance from fusion.
Even the earth has that (and I assumed that you ignore fusion experiments ;)). Induced by cosmic rays (maybe partially with muons) or just by chance. Very unlikely per atom, but the earth has a lot of atoms, so it can happen.
If you want to use fusion, set some limit to it. It can be low, but just "fusion is present" does not help.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby starslayer » Mon Dec 19, 2011 1:47 am UTC

A star is an object that supports hydrogen (yeah, deuterium is technically hydrogen, but it's excluded here) fusion in its core for most of its life, and is in HSE due to the balance between the radiation pressure from fusion and gravity during that time. That excludes everything below .08 solar masses; if these low-mass objects are heavier than 13 Jupiter masses, they can support deuterium fusion for a time, and are thus brown dwarfs.

Jupiter does release more energy than it receives from the Sun, true, but that is entirely due to gravitational contraction, not fusion.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby yurell » Mon Dec 19, 2011 1:56 am UTC

starslayer wrote:A star is an object that supports hydrogen (yeah, deuterium is technically hydrogen, but it's excluded here) fusion in its core for most of its life, and is in HSE due to the balance between the radiation pressure from fusion and gravity during that time. That excludes everything below .08 solar masses; if these low-mass objects are heavier than 13 Jupiter masses, they can support deuterium fusion for a time, and are thus brown dwarfs.

Jupiter does release more energy than it receives from the Sun, true, but that is entirely due to gravitational contraction, not fusion.


That means a white dwarf isn't a star, since they're supported by degeneracy pressure and not fusion, and should last a lot longer than the stars ever did in the main sequence.
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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby starslayer » Mon Dec 19, 2011 2:39 am UTC

True; I suppose you could amend that to remove the "most of its life" to "a portion of its life." Personally, I would consider white dwarfs and neutron stars, etc., to properly be stellar remnants and not stars, but they're usually called stars, so whatever.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby ian » Wed Dec 21, 2011 7:20 am UTC

If we got rid of the sun, the moon would just carry on orbiting the Earth, but what would happen if we got rid of the Earth? Does anyone know if the Moon would likely settle in a orbit around the Sun, or be flung off? I suppose it would depend loads on the Moon's trajectory at the time of Earth-End, but likely scenarios?

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby Tass » Wed Dec 21, 2011 9:56 am UTC

ian wrote:If we got rid of the sun, the moon would just carry on orbiting the Earth, but what would happen if we got rid of the Earth? Does anyone know if the Moon would likely settle in a orbit around the Sun, or be flung off? I suppose it would depend loads on the Moon's trajectory at the time of Earth-End, but likely scenarios?


It would definitely keep orbiting the sun. No matter where in its trajectory it was.

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Re: "The Definition of Planet" (or, "The IAU is Dumb")

Postby King Author » Wed Dec 21, 2011 10:53 am UTC

I guess I should've made it clear at the outset that I'm neither pro-Pluto nor anti-Pluto. The fact that I think the IAU definition is dumb is based solely on its "clears its neighborhood" nonsense; they chose that definition specifically to strong-arm Pluto out of planethood, and with most of the IAU members not present. That's no way to conduct science; forming a definition just out of spite for an exception (and doing so underhandedly, by making sure to hold a swift vote while those who'd oppose you weren't in attendance). Seriously, the IAU just acted like a buncha eight year olds that day; whether Pluto is or isn't a planet should be based on whether it's actually a planet or not, not IAU politics.

Anyway, everyone's moved on to the delineation of a binary system now, but there were some good points made on the definition of a planet.

yurell wrote:
King Author wrote:I think a proper scientific definition of planet should be based upon the way celestial objects form.

Okay, and what are the consequences of this? What happens if I form a planet by your definition, and it gets worn down to the size of a pebble -- is that pebble still a planet? I think it's ludicrous to assume that it is. And what about artificially constructed planet? From your definition they wouldn't be considered planets, despite being identical to one in every conceivable way.

Pluto's identical to any other planet in every conceivable way, and the IAU definition excludes it, so why aren't you complaining about that?

But good points. Outlandish, but good -- what about a planet that gets whittled down, and what about artificial planets.

Has the former ever actually occured? Obviously, in the early solar system, things we'd call planets probably smashed into each other and were completely obliterated, but I've never heard of a planet being whittled down. And the latter is actually pretty simple -- what do we call a man-made lake? We call it a man-made lake. We'd call a man-made planet a man-made planet; in every appearance, a planet, but it was artificially created, so we note as much.

I'll definitely have to re-do my definition, which is exactly why I made this topic (I knew I couldn't think of every consideration on my own), so thanks ^_^

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LaserGuy wrote:
King Author wrote:Here's what I think is a much better, more scientific definition of "planet." A celestial body that...
1. was formed in the accretion disc of a star and
2. has sufficient mass to assume hydrostatic equilibrium.


Just gonna leave this here. From Wikipedia:

Also many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. There are 19 moons in the Solar System that have achieved hydrostatic equilibrium and would be considered planets if only the physical parameters are considered. Both Jupiter's moon Ganymede and Saturn's moon Titan are larger than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede has a magnetic field. Just as stars in orbit around other stars are still referred to as stars, so some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets.[66][67][68] Indeed Mike Brown makes just such a claim in his dissection of the issue, saying:[59]

It is hard to make a consistent argument that a 400 km iceball should count as a planet because it might have interesting geology, while a 5000 km satellite with a massive atmosphere, methane lakes, and dramatic storms (Titan) shouldn't be put into the same category, whatever you call it.


And indeed, based solely on hydrostatic equilibrium:

The qualifying feature of dwarf planets is that they must "have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical shape)."[1][2][3] Those dwarf planets lying beyond the orbit of Neptune are termed "plutoids", after Pluto. Except for Pluto and Ceres, observations are insufficient for direct classification. However, based on present knowledge of how icy bodies gravitationally relax into equilibrium shapes, there are currently about 73 potential candidates amongst the population of trans-Neptunian objects (TNOs).[4] It is estimated that there are around 200 dwarf planets in the Kuiper belt and up to 2000 in the region beyond.[4]


It's worth pointing out that most, if not all Kuiper belt objects (as well as the aforementioned moons) are almost certainly formed from the accretion disk of the Sun, and would therefore certainly quality. It's not exactly clear to me which objects in the Solar system wouldn't in fact meet this criterion, in fact.

...which is why I stipulated the two criteria together are necessary for an object to be a planet, not solely one or the other. Those moons have hydrostatic equillibrium, but they didn't form from the sun, they formed from planets. The Kuiper belt objects formed from the sun, but they don't have hydrostatic equillibrium. By my definition, neither would be planets.

LaserGuy wrote:Incidentally, under the barycenter definition, the Moon will, in some 4 million of years, presumably get a status upgrade to a planet or dwarf planet, since the Earth and Moon are receding from each other at a rate of about 4 cm per year* and after about that time, the barycenter will then be outside of the Earth.

I read that the sun is going to blow up to red giant state and destroy the Earth before the moon escapes Earth's orbit.

Mr_Rose wrote:
AvatarIII wrote:
idobox wrote:
AvatarIII wrote:if it undergoes any fusion at all, it's a star. if it doesn't, it's a planet. Surely?

If you have enough hydrogen, even at low pressure and temperature, some fusion ought to occur, even if it's on the order of a few reactions per cubic km per year.
Significant doesn't mean a lot in my mind, but significant in comparison to the heat acquired during accretion, and the power received from outside.


but you need some sort of limit and saying "any" makes more sense than some arbitrary number of reactions per whatever.

perhaps just say if it has a positive net energy output?

Then isn't Jupiter already a star by that definition? I'm pretty sure I heard it gives off more energy than it receives annually. Of course the difficulty is separating leftover heat of formation (i.e. gpe dissipating as heat) and radioactive decay output from fusion output, if any.

In fact I'm also pretty sure you could come up with a situation for a rocky earth-sized planet to qualify as a star based (solely) on net energy output if you caught it early enough in its formation/far enough away from its primary, since the dominant energy input is almost always going to be the local star. So energy balance qualifiers are tricky to generalise satisfactorily.

I can't remember where, but I heard that Jupiter is a failed star -- that if there had been more junk in its area of the solar system, it would've become massive enough for fusion.

Hmm...I wonder if we could turn Jupiter into a star...? Not with current technology, of course, but when the sun's about to go red giant, maybe our technology would be great enough for us to destroy it, and at the same time, turn Jupiter into a star. With the sun gone, Jupiter would be the next-most massive thing, so maybe after some craziness, the orbits of everything else in the solar system would come to rest around it, and we'd effectively do the solar system variant of that thing you'd do in grade school, with everyone turning their desks 180 degrees to make the old front of the classroom the new back, and the old back the new front.
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