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Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
Defining Planethood
steeljaw354Date: Wednesday, 17.08.2016, 17:05 | Message # 211
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Classifying planets by location makes no sense. They should be classified by physical attributes, not by location. Would you think that this is a better definition? Math shouldn't be used to determine if the object is a planet or not, mine is simple, no math or arbitrary boundaries. Planemos are entirely different discussion as are moons also. Yes it might include alot of objects, but mnemonics don't really matter in science. The number of these objects by the rules below is probably around 30, but so what?

1) Must orbit a star or stellar remnant.
2) Must maintain hydrostatic equilibrium.
3) Must not be massive enough for fusion in the present, future or past.

Edited by steeljaw354 - Wednesday, 17.08.2016, 17:14
 
WatsisnameDate: Wednesday, 17.08.2016, 18:30 | Message # 212
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Your definition isn't bad. I told you that with the last one. It's a lot better than some of the others you've suggested. Personally, I prefer a system that captures the dynamics of planet formation, and explains how a few dominant bodies are separated from a lot of non-dominating ones. Which depends on location, distasteful as you find that for whatever reason. :)

I'm sorry you don't like math, but math is a tool which helps us understand nature. Without math we wouldn't have Space Engine.



 
steeljaw354Date: Wednesday, 17.08.2016, 18:39 | Message # 213
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It's not that I don't like math, it's that it shouldn't be included in a planet definition. If you cloned mercury and put where every planet is, Mercury-Pluto. And you erased all moons. All of the mercuries would be planets except for the one that is in Pluto's place by your standards. Exact same objects classified differently makes no sense what so ever.

Classifying objects by orbital distance, whether or not the orbit is clear isn't a good system to use. Again if Neptune and Jupiter had cleared there orbits there would be no objects in their orbits that isn't the case. The IAU definition says nothing about orbital domination.

Quote
is massive enough to be rounded by its own gravity,
is not massive enough to cause thermonuclear fusion, and
has cleared its neighbouring region of planetesimals
Quoted directly from wikipedia.

Edited by steeljaw354 - Wednesday, 17.08.2016, 18:44
 
WatsisnameDate: Thursday, 18.08.2016, 02:07 | Message # 214
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Quote steeljaw354 ()
It's not that I don't like math, it's that it shouldn't be included in a planet definition.


You don't have to include the math. You can recognize without math that the 8 currently defined planets in our system are very unique in their orbital space, whereas objects like Ceres and Pluto and Eris are not that unique -- they share the region with a lot of similar objects.

The math (easy math -- dividing one number by another number) shows that this difference isn't small. It's huge. Too huge for us to simply ignore.

The harder math shows why that happens. It captures the dynamics of planet formation. That's why it's so useful. That's why this definition works. And, much as you may not like it, this dynamics is orbital-distance dependent. Orbits are faster at smaller distances, so it takes a less massive object to dominate its orbit at smaller distances. This crazy-sounding location-dependent definition comes directly from nature. To dismiss it is to ignore nature.

Quote steeljaw354 ()
The IAU definition says nothing about orbital domination.


Orbital domination and orbital clearing are exactly the same idea expressed in different language. One naturally leads to the other.

Quote steeljaw354 ()
If you cloned mercury and put where every planet is, Mercury-Pluto. And you erased all moons. All of the mercuries would be planets except for the one that is in Pluto's place by your standards.


The planet formation process does not allow an isolated Mercury to form in Pluto's place. Precisely because it isn't massive enough to clear that orbit in solar system timescales. Either the disk at that location must form a more massive planet, or it remains a disk. The definition which you think makes no sense, works. smile



 
spacerDate: Thursday, 18.08.2016, 02:32 | Message # 215
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Watsisname, that what make mercury a planet if understand right.
if it was in pluto place. his orbit probably wont be clear and then mercury would not be a planet? smile
or you mean that there is no chance for a mercury sized object to even form there?



"we began as wanderers, and we are wanderers still"
-carl sagan

-space engine photographer
 
WatsisnameDate: Thursday, 18.08.2016, 04:54 | Message # 216
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A little bit of both. It's subtle and requires a bit of thinking. smile

If in Pluto's place, there had formed a Mercury-mass object, then it wouldn't quite be massive enough to clear out its orbit within 10 billion years. There would still be a belt of similar objects hanging around with it.

But the dynamics of accretion growth make it unlikely for objects to reach that mass and then stop -- either they stay well below the orbit clearing mass, or, upon reaching it, they quickly continue to grow to domination. This is why we don't see borderline cases, which we think is true in general from simulations. The process is so rapid, like a runaway snowball, that it's unlikely to stop right at that line.

So probably the most succinct way to put this is this:

Disks either form planets which clear out a path in the process, or they form belts of smaller, similar bodies, like the asteroid and Kuiper belts.

Margot's Pi is a measure of the minimum mass needed to achieve the clearing process within the timescale of the life of the star. Once an object reaches that mass, it tends to go on to dominate its space, accreting or clearing out other material from its orbit so that it's the most massive thing around by far. Soter's discriminant quantifies by how much it is the most massive thing.



 
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