New Planets!

[ewmayer]

Regarding the captured-planet scenario brought up by Uncwilly, that is fine, because (analogously to the "frozen slushball way out in the Kuiper Belt" classification) one could clearly differentiate "captured" from "native" planets. We could envision various planetary and subplanetary species, say:

* "regular" planets - formed in the accretion disk of the star(s) they orbit, geologically differentiated, still in "regular" orbit - e.g. the inner 8 of our solar system;

* "dwarf" planets - e.g. asteroids (shattered remnants of planets, gelogically undifferentiated) and KBOs ("frozen slushballs", including Pluto)

* "eccentric" or "enigmatic" planetoids - not in any of the above categories, eccentric or bizarre orbits, either result of capture of "orphan" planets (ones ejected from their native system) or via near-ejection events from their own system which leave them in weird orbits.

Given that planetary dynamics is a chaotic many-body thing, we obviously need to be somewhat flexible, and there will always be objects that defy neat classification. But the whole purpose of a reasonable classification scheme is to define scientifically meaningful categories that capture the evolutionary history and physical properties of most planet-like objects. It's not dissimilar from biological taxonomy - it'll never be perfect, but it's OK as long as it's helpful.

[masser]

Maybe I'm an idiot, but I don't understand this "clears its neighborhood" language. A lot of the press is reporting that Pluto is being demoted from the list of planets because its orbit intersects Neptune's orbit.... but doesn't that mean Neptune's orbit intersects Pluto's, and should also be demoted?

[ewmayer]

Originally Posted by masser

Maybe I'm an idiot, but I don't understand this "clears its neighborhood" language. A lot of the press is reporting that Pluto is being demoted from the list of planets because its orbit intersects Neptune's orbit.... but doesn't that mean Neptune's orbit intersects Pluto's, and should also be demoted?

Seems like more silliness in place of fundamentals to me - if it's big enough and has a stable enough orbit, it will eventually clear said orbit. If it has a sufficiently eccentric orbit, that orbit may interset/cross/overlap that of other planets or planetoids.

But at least things seem to be trending in a somewhat more reasonable direction than that of the initial proposal - and no debates about gay planetary marriage yet.

[ixfd64]

Well, this will have quite an effect on education.

1. Will kids lose points on exams if they answer that Pluto is a planet, as they have been taught earlier?
2. What are the costs of rewriting all those textbooks?
3. What are the costs of remodeling those museums with large solar system models?
4. Will "traditional" solar system model kits become collectible, or maybe even rare, in the future? (eBay in 2100: "(rare) vintage solar system model kit with Pluto! $500)

[Uncwilly]

This is far less dramatic than when earth was pushed out of the centre of the SS.

[xilman]

Originally Posted by ewmayer

By way of an interesting illustration of this, the COM of the Earth/Moon system is actually not far from the Earth's surface (currently at roughly 0.8 Earth radii from Earth's center, so based on the 3.8 cm/year drift rate of the moon away from the earth due to tidal dissipation of gravitational energy, in around 2 billion years the moon will have increased its orbital distance by the roughly 20% needed to assume full planetary status. Someone should put together a "You may already be pre-approved as a planet!!!" mailing for our dear neighbor.

I personally regard the earth/moon system as a double planet. The moon is easily big enough to be approximately spherical (an observation that can be made with the naked-eye) and there is good evidence that significant differentialion has occurred during its history.

The killer criterion, in my opinion, is that the moon's orbit is everywhere concave to the Sun. That's why I account it double-planet status and not merely a satellite of planetary mass.


Paul

[garo]

Paul could you explain this a bit further:

The killer criterion, in my opinion, is that the moon's orbit is everywhere concave to the Sun.


Thanks

[alpertron]

Originally Posted by xilman

The killer criterion, in my opinion, is that the moon's orbit is everywhere concave to the Sun. That's why I account it double-planet status and not merely a satellite of planetary mass.


Paul

I think that this is related to the ratio between the translation period of the Moon around the Earth and the translation period of the Earth around the Sun. If I put a small satellite twice the distance Moon-Earth, the orbit around the Sun will be concave also.

[xilman]

Originally Posted by garo

Paul could you explain this a bit further:

The killer criterion, in my opinion, is that the moon's orbit is everywhere concave to the Sun.


Thanks

Simply put: if you sit somewhere well above the Sun's north pole (or south pole if you prefer) and look down on the path of the Moon as it goes around the Sun, that path is always concave. The same is true of the orbits of the major planets. It's also true of many other objects including coments and asteroids, which is why we need other criteria such as minimum mass, or minimum diameter, or geological differentiation, etc. It's very hard to set minima for those quantities which exclude the Moon from the list of planets without also excluding Pluto.

The other major satellites in the solar system, apart from Charon, have "lumpy" orbits as seen from the Sun. That is, there are portions of their orbits which are convex and other portions which are concave.

In my mind, this is justification for treating Pluto and Charon as a double planet.


There is also historical justification for counting the Moon as a planet, but here I'm just being mischievous.

Paul

[mfgoode]

Originally Posted by xilman

Simply put:
In my mind, this is justification for treating Pluto and Charon as a double planet.
There is also historical justification for counting the Moon as a planet, but here I'm just being mischievous.

Paul

One of the criteria if you have read my first URL I gave is that THE BARYCENTRE ( a more astronomical term than the 'Centre of mass' which is used in more disciplines like Appl. Maths.,Physics, chemistry etc.etc. with different meanings) should be in one of the bodies and not outside them.
In the case of the earth and the moon this is so (the BC is in the earth), whereas in the case of Pluto and Charon it is NOT and lies outside the mass of either of them. This is sufficient reason to rule them both out- more a binary body.

I will not dwell on the astrological and psychological aspects of the moon at length which plays a very important part, and much more than causing the tides on earth!.
To put it simply it causes 'tides' in the brain and psyche also which is evident to the the even casual observer of mental patients. If you doubt this pay a visit to a shrink's at Full moon time and then at New Moon time. You will see a vast different in the number attending. Then to be scientific you will have to make many more to justify whatever conclusions that can be formulated into A theory! In these experiments the observer must also be counted!
Mally

[jasong]

Originally Posted by mfgoode

jasong: Please take the pains and refer first to the URl's given in the previous
posts before making a comment. These criteria are all given therein.
Also please refrain from off the bat comments when you havent done the necessary homework.
Thank you,
Mally,

Since I was right, I'd like to ask why people can't do their own d**n searching.

http://en.wikipedia.org/wiki/Brown_dwarf