Why (if anything) is a planet clearing its neighborhood important?

[LibraryOgre]

Here's a thought:

If we use lambda, are there "rogue planets"... planet-sized bodies that are independent of a solar-system... by any definition?

[Jay R]

Here's a thought:

If we use lambda, are there "rogue planets"... planet-sized bodies that are independent of a solar-system... by any definition?

Yes. Since the difference between a planet and a dwarf planet is defined by its effect on its solar system, rogue planets would not be either one, but the category clearly exists. Not much is known about them yet.

Here's the Wikipedia article on one such.

[LibraryOgre]

Yes. Since the difference between a planet and a dwarf planet is defined by its effect on its solar system, rogue planets would not be either one, but the category clearly exists. Not much is known about them yet.

Here's the Wikipedia article on one such.

But, as the article says, it's a planetary mass object. If we're defining planets by lambda, not mass, what constitutes a planetary mass? The geological definition relies just on rounding.

[Radar]

But, as the article says, it's a planetary mass object. If we're defining planets by lambda, not mass, what constitutes a planetary mass? The geological definition relies just on rounding.

This is why the definition has three separate points as I quoted above and mass requirement is also there. So planetary mass would mean that it did reach hydrostatic equilibrium as far as I understand.

[halfeye]

This is why the definition has three separate points as I quoted above and mass requirement is also there. So planetary mass would mean that it did reach hydrostatic equilibrium as far as I understand.

This is Bennu, half a kilometre in diameter, it seems to me it's in hydrostatic equilibrium:



Much more so than e.g. Vesta at 525 kilometres diameter:



Raindrops are also in hydrostatic equilibrium.

So it seems to me, that "planetary mass" is a meaningless term.

[factotum]

Something randomly being round "just because" does not mean it's in hydrostatic equilibrium. In fact, neither body in your photos is massive enough to be in that state--you need to get up to the size of Ceres before you get an asteroid which is big enough to be pulled into a sphere by its own gravity.

[Jay R]

This is Bennu, half a kilometre in diameter, it seems to me it's in hydrostatic equilibrium:

...

Raindrops are also in hydrostatic equilibrium.

So it seems to me, that "planetary mass" is a meaningless term.

OK, first of all, let me deal with the red herring: a raindrop is roundish from surface tension, not from a hydrostatic equilibrium of its internal gravity and pressure. This is similar to why the air in a balloon is round -- because it has a thin surface coating holding it in. Without the balloon or the surface tension, the body's gravity is nowhere near enough to balance the internal pressure. If it were in equilibrium, then it would stay round after you puncture the balloon.

Hydrostatic equilibrium doesn't mean "sort of round". It means an equilibrium of gravity and pressure, as shown here:


On 101955 Bennu, I'll bite. Why do you think that this rock is in hydrostatic equilibrium? Where are your fluid dynamics equations?

One of the indicators of hydrostasis (hydrostatic equilibrium)is being an ellipsoid, which 101955 Bennu is not. But even if it were, being an ellipsoid isn't proof. Here's one form of the Equation of Hydrostatic Equilibrium:

dP/dr = - G M(r) ρ(r) / r²

If you haven't done this math, then you haven't even started to determine if it's in hydrostatic equilibrium.

You really can't show that a scientific term is "meaningless" without learning the science and math involved.

[Mechalich]

Something randomly being round "just because" does not mean it's in hydrostatic equilibrium. In fact, neither body in your photos is massive enough to be in that state--you need to get up to the size of Ceres before you get an asteroid which is big enough to be pulled into a sphere by its own gravity.

It depends somewhat on composition. Ceres is fairly rocky, and rock has greater ability to resist compression compared to the 'ices' that appear to be the primary component of various outer system objects. The 396 km diameter Mimas - much, much smaller than Ceres - appears to have previously been in hydrostatic equilibrium prior to taking Herschel to the face, which suggests the lower bound for the trait is somewhere in that range.

[halfeye]

OK, first of all, let me deal with the red herring: a raindrop is roundish from surface tension, not from a hydrostatic equilibrium of its internal gravity and pressure. This is similar to why the air in a balloon is round -- because it has a thin surface coating holding it in. Without the balloon or the surface tension, the body's gravity is nowhere near enough to balance the internal pressure. If it were in equilibrium, then it would stay round after you puncture the balloon.

Hydrostatic equilibrium doesn't mean "sort of round". It means an equilibrium of gravity and pressure, as shown here:

It is about that diagram yes (though the force arrows should all be identical in length), but the force inward does not have to be gravity. In the case of a balloon it is tension in the skin of the balloon which keeps the air compressed, and in the case of the raindrop it is the surface tension of the water, which does tend to make these things spherical. Balloons can be non-spherical due to the shape of the skin which causes different tensions in different parts of the balloon, but the principle is still hydrostatic.

In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force.[

https://en.wikipedia.org/wiki/Hydrostatic_equilibrium

On 101955 Bennu, I'll bite. Why do you think that this rock is in hydrostatic equilibrium? Where are your fluid dynamics equations?

It is not 'a' rock, it's a whole heap of rocks, varying on the surface from dust up to ten metre boulders, there's barely enough gravity to hold it together, there's almost no friction, it's not a single solid, of course it's in hydrostatic equilibrium.

One of the indicators of hydrostasis (hydrostatic equilibrium)is being an ellipsoid, which 101955 Bennu is not. But even if it were, being an ellipsoid isn't proof. Here's one form of the Equation of Hydrostatic Equilibrium:

dP/dr = - G M(r) ρ(r) / r²

If you haven't done this math, then you haven't even started to determine if it's in hydrostatic equilibrium.

You really can't show that a scientific term is "meaningless" without learning the science and math involved.

The science and the maths are not identical, you need to understand what you're talking about, but the maths is just a language for talking about things you understand.

Bennu is rotating, so it's an oblate spheroid. There's no telling what's inside it, but apparently it is rotating faster over time, and bits have been seen migrating over the surface toward the equator as it speeds up.

[georgie_leech]

It is about that diagram yes (though the force arrows should all be identical in length), but the force inward does not have to be gravity. In the case of a balloon it is tension in the skin of the balloon which keeps the air compressed, and in the case of the raindrop it is the surface tension of the water, which does tend to make these things spherical. Balloons can be non-spherical due to the shape of the skin which causes different tensions in different parts of the balloon, but the principle is still hydrostatic.



https://en.wikipedia.org/wiki/Hydrostatic_equilibrium




It is not 'a' rock, it's a whole heap of rocks, varying on the surface from dust up to ten metre boulders, there's barely enough gravity to hold it together, there's almost no friction, it's not a single solid, of course it's in hydrostatic equilibrium.


The science and the maths are not identical, you need to understand what you're talking about, but the maths is just a language for talking about things you understand.

Bennu is rotating, so it's an oblate spheroid. There's no telling what's inside it, but apparently it is rotating faster over time, and bits have been seen migrating over the surface toward the equator as it speeds up.

You might want to scroll a bit further down in your link and read what it says in context of planetary geology. No, a bunch of rocks stuck together in a mostly rounded shape is not the same as being in hydrostatic equilibrium. Else...

Spoiler: Behold, hydrostatic equilibrium!

Show

[halfeye]

You might want to scroll a bit further down in your link and read what it says in context of planetary geology. No, a bunch of rocks stuck together in a mostly rounded shape is not the same as being in hydrostatic equilibrium. Else...

Spoiler: Behold, hydrostatic equilibrium!

Show

That's not hydrostatic equilibrium because the snowball is produced by compressing the snow into a more or less spherical shape, whereupon the structural strength of the snow holds it together.

The problem for astronomy with Bennu is that it isn't being held together by it's own structural strength, it is only being held together by gravity, and it's shape is due to surface particles moving under gravity. Bennu is in hydrostatic equilibrium, gravity, cetripetal force and friction between the many rocks it is made of are the only forces acting on Bennu.

I disagree that Ceres is rocky, I suspect it is largely (maybe as little as 30%, maybe much more) water, and at some time that water was liquid which is why it is spherical, I suspect a solid rock of the same size that was never melted would be much lumpier.

[Scarlet Knight]

...
What I don't really understand is why so many people still seem to be put out over a decision about technical definitions that happened 16 years ago.

I can only speak for myself.

I listened to a lecture by Dr, Neil Degrasse Tyson who basically told everyone to "get over" Pluto. He took only 5 questions so I never got to speak with him.

What I wanted to say to him was: For most of my lifetime, it was YOUR planetarium that told me that Pluto was a planet. Not only that, but that I should care, saying how lonely and cold it was, like it's mythological namesake.

Plus you're only famous to the public because of Pluto. You were not going on The Big Bang Theory or Steven Colbert's show if it wasn't for Pluto. You made us care, took our donations, and now say "Get over it"?

Show a little gratitude, man.

[Mechalich]

I can only speak for myself.

I listened to a lecture by Dr, Neil Degrasse Tyson who basically told everyone to "get over" Pluto. He took only 5 questions so I never got to speak with him.

What I wanted to say to him was: For most of my lifetime, it was YOUR planetarium that told me that Pluto was a planet. Not only that, but that I should care, saying how lonely and cold it was, like it's mythological namesake.

Plus you're only famous to the public because of Pluto. You were not going on The Big Bang Theory or Steven Colbert's show if it wasn't for Pluto. You made us care, took our donations, and now say "Get over it"?

Show a little gratitude, man.

Neil Degrasse Tyson is not the person responsible for the reclassification of Pluto, however much he's been associated with the issue publicly. That honor goes to Dr. Mike Brown, the astronomer primarily responsible for locating the population of large Kuiper Belt objects similar to Pluto. In his book, How I killed Pluto and Why it had it Coming he explains that Pluto's original discovery and subsequent classification as a planet was a massive historical accident.

Clyde Tombaugh got super lucky with his blink comparator finding the evidence of motion of a tiny dot manually as opposed to having computers run thousands or tens of thousands of similar comparisons for each set of plates. It was a freak accident of a discovery. There's a reason that no one else found anything else in the Kuiper Belt for almost seventy years.

Had Pluto been discovered in the 1990s alongside the rest its Kuiper Belt kindred, it would never have been classified as a planet in the first place, but the discovery of large numbers of outer system objects of significant size coupled with the exoplanet discovery era would mean that we'd still be arguing over how to define a planet today.

[Jay R]

It is about that diagram yes (though the force arrows should all be identical in length), but the force inward does not have to be gravity. In the case of a balloon it is tension in the skin of the balloon which keeps the air compressed, and in the case of the raindrop it is the surface tension of the water, which does tend to make these things spherical. Balloons can be non-spherical due to the shape of the skin which causes different tensions in different parts of the balloon, but the principle is still hydrostatic.

https://en.wikipedia.org/wiki/Hydrostatic_equilibrium

Red herring duly noted. This is technically true and irrelevant to the subject of this thread. The IAU’s 2006 resolution states (in relevant part):
A "planet" [1] is a celestial body inside the Solar System that … (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, …”

It’s not just hydrostatic tension; it’s hydrostatic tension caused by its self-gravity.

So, yes, you caught me out in a mistake. Good for you. Getting back to planetary definitions, ...

It is not 'a' rock, it's a whole heap of rocks, varying on the surface from dust up to ten metre boulders, there's barely enough gravity to hold it together, there's almost no friction, it's not a single solid, of course it's in hydrostatic equilibrium.

That’s a very strong statement, and I’m going to ask you to cite your source. I’ve spent awhile looking and can’t find one. [Admittedly this is not my field; my Ph.D is in Operations Research.]

[I'm not asking for a citation about it being a heap of rocks, or varying size, or low gravity, or low friction. But none of these imply hydrostatic equilibrium, so I'd like to see your source for maintaining that it is.

Of course, when I ask for your sources, I should provide mine. Here are a couple.

In the paper “Shape of (101955) Bennu indicative of a rubble pile with internal stiffness” by O.S. Barnouin et al., published in Nature Geoscience, VOL 12, APRIL 2019, the only relevant statement I can find is this:

“Nevertheless, Bennu’s shape and surface features imply that the asteroid has some structural rigidity, despite being a rubble pile. Evidence for structural strength includes Bennu’s non-hydrostatic shape, the high-standing longitudinal ridges, the long linear grooves, and apparent mass wasting.” [Emphasis added]

In the presentation “DEVIATION OF THE SHAPE OF BENNU FROM ROTATIONAL FIGURES OF STABILITY”, which was presented at the Lunar and Planetary Science Conference in March 2019, one bullet point says, “Shape deviates from hydrostatic surface”. [Again, emphasis added] I found this on the NASA Technical Reports Server. This report cites the previous one, by the way.

The science and the maths are not identical, you need to understand what you're talking about, but the maths is just a language for talking about things you understand.

Bennu is rotating, so it's an oblate spheroid. There's no telling what's inside it, but apparently it is rotating faster over time, and bits have been seen migrating over the surface toward the equator as it speeds up.

Lots of things are rotating that are not oblate spheroids. Bennu is roughly spherical, but it is not an oblate spheroid from hydrostatic equilibrium. Some of the papers I looked at talk about that shape coming from its high spin. None of the papers talking about its shape suggest hydrostatic equilibrium as the cause.

So again – what are your sources? If you have access to more recent scientific papers that refute the ones I’ve cited, I’d like to see them. I know that this is a developing field.

But nothing I read this afternoon seems to agree with your contention that Bennu is in hydrostatic equilibrium.

[Radar]

Red herring duly noted. This is technically true and irrelevant to the subject of this thread. The IAU’s 2006 resolution states (in relevant part):
A "planet" [1] is a celestial body inside the Solar System that … (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, …”

I just wanted to add some focus on this part of the definition. Even if Bennu was an oblate spheroid (which it is not), it still would not be enough, as it does not have strong enough self-gravity to overcome rigid body forces. If its shape would equilibrate, it would be because it is a pile of loose rocks and not a rigid body.

[Lord Torath]

I just wanted to add some focus on this part of the definition. Even if Bennu was an oblate spheroid (which it is not), it still would not be enough, as it does not have strong enough self-gravity to overcome rigid body forces. If its shape would equilibrate, it would be because it is a pile of loose rocks and not a rigid body.

I was going to post this very thing. The fact that it's a rubble pile, that the individual boulders making it up are still individual boulders instead of being deformed by gravity into a single solid mass (with a few surface boulders), means it does not meet the astronomical definition of hydrostatic equilibrium.

[halfeye]

Red herring duly noted. This is technically true and irrelevant to the subject of this thread. The IAU’s 2006 resolution states (in relevant part):
A "planet" [1] is a celestial body inside the Solar System that … (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, …”

It’s not just hydrostatic tension; it’s hydrostatic tension caused by its self-gravity.

So, yes, you caught me out in a mistake. Good for you. Getting back to planetary definitions, ...

I am not interested in catching people out, I am interested in arguing honestly about interesting things. I was unaware that the definition of planet added the bit about overcoming rigid body forces, that does eliminate Bennu from the possible planets. I'm not sure that it actually applies to anything much short in mass of the moon, there is some scope for everything in the inner solar system to have been liquid at some point in that thing's history, and that would seem to disqualify everything from being a planet.

That’s a very strong statement, and I’m going to ask you to cite your source. I’ve spent awhile looking and can’t find one. [Admittedly this is not my field; my Ph.D is in Operations Research.]

Operational Research won the battle of the Atlantic in WW2, so that's a useful field to be in.

[I'm not asking for a citation about it being a heap of rocks, or varying size, or low gravity, or low friction. But none of these imply hydrostatic equilibrium, so I'd like to see your source for maintaining that it is.

Of course, when I ask for your sources, I should provide mine. Here are a couple.

In the paper “Shape of (101955) Bennu indicative of a rubble pile with internal stiffness” by O.S. Barnouin et al., published in Nature Geoscience, VOL 12, APRIL 2019, the only relevant statement I can find is this:

“Nevertheless, Bennu’s shape and surface features imply that the asteroid has some structural rigidity, despite being a rubble pile. Evidence for structural strength includes Bennu’s non-hydrostatic shape, the high-standing longitudinal ridges, the long linear grooves, and apparent mass wasting.” [Emphasis added]

In the presentation “DEVIATION OF THE SHAPE OF BENNU FROM ROTATIONAL FIGURES OF STABILITY”, which was presented at the Lunar and Planetary Science Conference in March 2019, one bullet point says, “Shape deviates from hydrostatic surface”. [Again, emphasis added] I found this on the NASA Technical Reports Server. This report cites the previous one, by the way.

Lots of things are rotating that are not oblate spheroids. Bennu is roughly spherical, but it is not an oblate spheroid from hydrostatic equilibrium. Some of the papers I looked at talk about that shape coming from its high spin. None of the papers talking about its shape suggest hydrostatic equilibrium as the cause.

So again – what are your sources? If you have access to more recent scientific papers that refute the ones I’ve cited, I’d like to see them. I know that this is a developing field.

But nothing I read this afternoon seems to agree with your contention that Bennu is in hydrostatic equilibrium.

I don't have any citations for Bennu being subject to hydrostatic equilibrium other than the fact that Ryugu is more or less the same shape, which suggests to me that the shape is entirely caused by the forces acting upon them, which seems to me to be strongly suggestive that the shape is a case of hydrostatic equilibrium.

There are plenty of asteroids that are nothing like spherical, and the conclusion that they are more or less monolithic solids seems reasonable to me, I don't see why the idea that a more or less spherical pair should be in hydrostatic equilibrium should be so objectionable.

I just wanted to add some focus on this part of the definition. Even if Bennu was an oblate spheroid (which it is not), it still would not be enough, as it does not have strong enough self-gravity to overcome rigid body forces. If its shape would equilibrate, it would be because it is a pile of loose rocks and not a rigid body.

Yeah, that part of the definition about having to overcome it's own single body rigidity is a corker, I was unaware of that.

I was going to post this very thing. The fact that it's a rubble pile, that the individual boulders making it up are still individual boulders instead of being deformed by gravity into a single solid mass (with a few surface boulders), means it does not meet the astronomical definition of hydrostatic equilibrium.

Hydrostatic equilibrium is not something that astronomy as a field gets to define. Astronomy can and has defined planets, I don't like the way they've done it, but hydrostatic equilibrium is not within their gift.

[Lord Torath]

Hydrostatic equilibrium is not something that astronomy as a field gets to define. Astronomy can and has defined planets, I don't like the way they've done it, but hydrostatic equilibrium is not within their gift.

Are you sure? Because it seems very much like they've done just that. Many fields have very specific definitions of words that don't apply in general use or in other fields. Want an example?

Theory. In any scientific field of study, theory has a pretty well-defined definition. In general use, it means any idea someone comes up with, regardless of whether or not it is provable or falsifiable. There are countless other examples of words that mean completely different things to people in different fields. This is no different. You may not like it (like I don't like that literally means both literally and figuratively), but not liking it doesn't make it wrong.

[chaincomplex]

Math/phys researcher here. Actually the roundness of water packets in vacuum is due both to hydrostatic equilibrium (the surface evolves according to a process called Ricci flow) and surface tension (due to a separate Ricci flow). The metric Lagrangian for both has an action extrema that's spherical. The surface tension should be the dominant term unless the packet is supermassive, at which point the system becomes more complicated because the model as packets of water deforming in vacuum doesn't quite work.

Only on Earth or near some dominant gravitational field does hydrostatic equilibrium not contribute at all, but a similar non-grav variant of this force can be found in e.g. bubbles suspended in water.

[halfeye]

Are you sure? Because it seems very much like they've done just that. Many fields have very specific definitions of words that don't apply in general use or in other fields.

If they can redefine hydrostatic equilibrium, why do they have the extra clause about "overcome rigid body forces"?

Ornithology doesn't redefine "flight", ichthyology doesn't redefine "swim". They may well have definitions for things which relate to these, but in general there's a hierarchy, with physics as the most basic at the top.

[Lord Torath]

If they can redefine hydrostatic equilibrium, why do they have the extra clause about "overcome rigid body forces"?

Not being a member of the IAU, I can only speculate. It may have something to do with excluding 'rubble piles' from the definition.

[halfeye]

Not being a member of the IAU, I can only speculate. It may have something to do with excluding 'rubble piles' from the definition.

Nope, if their new definition of "hydrostatic equilibrium" (which they didn't actually make) included the bit about overcoming rigid body forces, they wouldn't need to include that in their definition of "planet"