New Planets!

[mfgoode]

Three new planets may join the solar system

There will be at least 12 planets in our solar system, and probably
many more, if a new definition of the word "planet" is adopted. Next week
the International Astronomical Union will vote on a draft definition of
what distinguishes a planet from lesser space rocks.

The new scheme would retain Pluto – previously threatened with ejection
from the club – but it would also admit several new members, including
the former asteroid Ceres, and even Charon, which until now has been
classed as Pluto's moon.

Read the full story here:
http://www.newscientistspace.com/article/dn9761
Mally

[cheesehead]

Continuation from thread "10th Planet Discovered" at http://www.mersenneforum.org/showthread.php?t=4429

Stay tuned for news from the August 23 meetings of the International Astronomical Union's Working Group Planetary System Nomenclature and Committee on Small Bodies Nomenclature.

[jasong]

If I'm not mistaken, I think there was a time when a brown dwarf could be classified as a planet, but I forget the definition. I think it specifically had to do with size and whether or not it can emit it's own light.

[mfgoode]

Originally Posted by jasong

If I'm not mistaken, I think there was a time when a brown dwarf could be classified as a planet, but I forget the definition. I think it specifically had to do with size and whether or not it can emit it's own light.

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,

[mfgoode]

Since I am ahead in time in my part of the world compared to Europe and the U.S. I am anxiously waiting for the final line up of the planets picked by IAU.
In the meantime I give this article which gives the various criteria in making the final choice.

http://www.space.com/scienceastronom..._proposal.html

Will Pluto make it? I believe I read somewhere that 'pluton' is an archaeological term too. Is this correct?

Mally

[ewmayer]

My biggest problems with the IAU committee proposals as originally announced:

(1) The criterion re. the center of mass (COM) of a 2-body system being outside vs. inside the larger body: The COM depends not only on the relative masses but also on the *distance* between the bodies, so a very small moon which happens to be orbiting very far from its parent would also qualify for planethood via this criterion. 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.

(2) I also dislike the fact that the IAU committee proposal completely ignored fundamentals of planetary orbital mechanics (frozen slushballs tend to have highly eccentric and/or inclined orbits w.r.to the inner 8 planets) and formational geology - why use some arbitrary "quasispherical" criterion when one could look at fundamental geological properties like "is it massive enough to have a differentiated core?" (This would be a planetary analog of the stellar criterion, "Is it massive enough to initiate and sustain hydrogen fusion?") There seems to be some hope that at least the first of these will be remedied in the final proposal, but at this point I'm not holding my breath.

[jinydu]

If I remember correctly, Brown, the discoverer of UB313 (I may have the name wrong there) said that the word "planet" has become a cultural word and should be left out of science altogether.

[xilman]

Originally Posted by ewmayer

"Is it massive enough to initiate and sustain hydrogen fusion?") There seems to be some hope that at least the first of these will be remedied in the final proposal, but at this point I'm not holding my breath.

The Gallilean satellites would count as planets under this criterion.

Solid silcate centres, liquid water mantles, solid water lithospheres.

Compare the earth's solid iron inner core, liquid iron outer core, plastic slicate mantle, solid silicate lithosphere.

I've nothing against your definition and I've nothing against the proposal that planets can orbit other planets. I'm just pointing out one of the ways in which it is difficult to satisfy everyone. Personally, I think the IAU's definition is by far the least bad.


Paul

[jinydu]

It looks like Pluto has been demoted:

http://news.yahoo.com/news?tmpl=stor...lanet_mutiny_9

[ewmayer]

Originally Posted by xilman

The Gallilean satellites would count as planets under this criterion.

Good point, but I had in mind that the differentiatedness criterion would only apply in an "and" sense, i.e. if the body in question doesn't flunk any of the others.

I neglected to add that by way of a replacement for the COM criterion I suggest a simple relative-mass criterion: any body in mutual orbit around another (or around others) which is less than (say) 5% the mass of the largest body in the lot is considered to be a moon of the larger. Thus Earth's moon remains a moon irrespective of its outward orbital drift. The Charon/Pluto system (in which Charon is slightly > 10% the mass of Pluto) would be co-planets, but flunks the orbital-eccentricity-and-inclination criterion. I believe Ceres is geologically undifferentiated, reflecting its evolutionary history as an asteroid.

So to be clear, let me summarize "Ernst a.k.a. random-dilettante-off-the-street's criteria for planethood:"

1) It is in orbit around a star or within a gravitationally bound group of stars, and is not itself a star; and

2) Has an orbital eccentricity and inclination which lie within reasonable bounds (to be defined) relative to the other planets of the system and/or the rotational plane of the central star, in single-star systems; and

3) Is sufficiently massive to have a geologically differentiated core, except in the case of giant gas planets or ones sufficiently close to a star to maintain them in molten state; and

4) If it is part of a gravitationally bound group of non-stellar bodies, has a mass at least 5% that of the most massive member of the group.

[Uncwilly]

Originally Posted by ewmayer

"Ernst a.k.a. random-dilettante-off-the-street's criteria for planethood:"2) Has an orbital eccentricity and inclination which lie within reasonable bounds (to be defined) relative to the other planets of the system and/or the rotational plane of the central star, in single-star systems;

Following this, if Mars had been ejected from the SS during the formative years:

1) While free floating it would not be a planet.
2) If it were captured by a different star system, but at a 90-degree angle to the rest of the native planets (think of the orbit of Ulysses, resonant with Jupiter), it would still not be a planet. Even if, it turns out to be the most massive member of the non-stellar bodies of the resultant system.

If Jupiter were ejected when a second stellar object enters the SS and is co-captured by the sun:

1) As 1 above.

What of a star born in a dense stellar nursery, where it has many close and more massive systems, but no native planets?

1) It captures an ejected planet (Neptune mass), but it's orbit is inclined to the rotation of the star by 45 degrees. Not a planet.
2) It captures a second body (Neptune mass) inclined 90 degrees to the other. Not a planet.
3) It captures a third body (Mercury mass) at 0 degrees to the rotation of the star. This is now a planet or no?

What of a system where 2 stellar objects have co-captured each other, but their mutual orbit is inclined to the orbital planes of their planets?

1) Intially their planets become declassified.
2) Once the planets orbits become aligned; they can't be reclassified until at least 50% of the new total have brought themselves in alignment to the stellar orbits.

What of a composite system? 2 Widely spaced co-captured stars, with small tightly bound planet systems. However during the capture the orbits of the planets gets warped. Do the most effected get declassified?