[X] My Assistant

[ngunn]

Just as small stars are much more common than massive ones I'd expect these brown dwarf systems to greatly outnumber all the visible stars. This is just the kind of system that could have brought us Sedna.

[Rob Pinnegar]

I've said this before, but: Assigning names to groups of objects this early in the game just seems awfully premature to me. Obviously we have to come up with classifications at some point, but when *so much* more data is likely to be available within the next 10 years...

"Pegasids" are probably a safe bet, since even now it's fairly evident that these things are pretty common. But, for example, arguing about what group Sedna belongs to is a waste of time... we need to find *at least* another dozen Sednas before there will be any point to classifying objects of that type.

[ngunn]

I agree with you about assigning names prematurely. However I don't agree that it's a waste of time trying to think of a plausible dynamical history for Sedna (even while we have still only found one of them!) As I argued on another thread capture from a passing loosely-bound multiple system is one such plausible history. My suggestion was criticised there on the basis that loosely bound (low mass) binary or multiple systems were not likely to be common, if they existed at all. Well, now we have them.

[Guest_BruceMoomaw_*]

Not speaking on going "up" is size. Jupiter, a gaz giant, orbiting round the Sun, is a planet... But larger gaz giants can still do this. When a gaz giant is much larger than Jupiter, it can remain luminous like a star for many hundred millions of years. When it is large enough to burn its deuterium, it becomes a brown dwarf, and still larger a red dwarf. Star or planet? where to set the limit? If it has nuclear reactions, it is a star, but if it turns around a star, it is a planet. Most stellar companions would rank after Charon, and even after Jupiter, into David's classification above.

Throw this stone into the debate, and look what happens (while keeping at safe distance)...

Still, at least in this case we do have an actual physical phenomenon which can serve as a divider: nuclear fusion, of different types.

Alan Stern would claim that we have a similar physical principle that can provide a lower-level threshold for distinguishing planets from nonplanets -- namely, a gravitational field strong enough to make the object spherical -- but I will repeat that it seems to me that in that case, there are far too many seriously doubtful cases here which make the borderline hopelessly fuzzy. Consider the serious irregularities in the shapes of some quite large moons -- are Iapetus and Proteus "spherical" or not? -- and the fact that there are also important differences in the triaxial dimensions of even the three biggest asteroids, which Alan wants to officially list as "planets". And among the KBOs, we already have the horrendous case of 2003 EL61, as long as Pluto on one axis but only half as wide on another. Who knows how many other similar little surprises await us in the Kuiper Belt?

We could try defining a planet as something above a certain average diameter (or mass) whose difference between its longest and shortest diameters is below some (small fraction), making it "near-spherical". But in that case, we're really not that far from the same kind of arbitrariness that would be involved in simply defining a planet as some object whose largest (or smallest) diameter is above a certain size. And I honestly don't think that astronomers can continue to wrangle endlessly over this question without confusing schoolkids and nonscientist adults even more than they're already confused. Besides, they're starting to make themselves look like fools. Just admit that, given what we now know about the Solar System, the question is hopelessly ambiguous, and set some arbitrary planet/nonplanet dividing line that openly recognizes that fact.

As far as I'm concerned, the very discovery which has made the planet/nonplanet distinction impossible to decide any more -- the Kuiper Belt, an entire new major division of the Solar System which was totally undiscovered until the 1990s -- is remarkable and facinating in itself, and trying to maintain some artificial "planet/nonplanet" distinction simply obscures that marvelous new discovery from the general public.

[ljk4-1]

I am waiting for the discovery of a star orbiting a planet.

We will have to call them Ptolemetoids.

[ngunn]

I propose 'Moomoon' for a moonlet orbiting a moon.

[ilbasso]

Would there be Moomoons in the Cowper Belt?

[paxdan]

I am waiting for the discovery of a star orbiting a planet.
We will have to call them Ptolemetoids.

I've always liked the fact that the Sun-Jupiter barycentre is above the surface of the sun

[Rob Pinnegar]

Would there be Moomoons in the Cowper Belt?

Only if they're in the Milky Way!

[Guest_Richard Trigaux_*]

I think this debate is not by itself a astronomy/science debate: what is important is to study the objects, giving them a name is a lesser issue. However there was such an issue into biology, when scientists were to name the different animal/vegetal species and classify them (a much more complex problem than in astronomy). In the beginning it seemed an impossible task, as there was so many different species, apparent families and deep structures hidden under apparend diversity. It took centuries to disentangle this (an important stake, as the classification of species was the key to understand the history of life) and there are still some small moves taking place today.


I shall anyway try some classification, keeping with a "naturalistic mind".

-is a star something which has permanent nuclear reactions.

-a brown dwarf is a more ambiguous object, which has transitory nuclear reactions, or which at least shines like a star for a great span of time, one billion years or more. (In practice it is difficult to guess if an object has nuclear reactions or not. A clue is that deuterium is burned between 20 an 22 Jupiter mass. But smaller bodies without reaction can shine a long time like a small star).

-is a planet an unique object which orbits around one of the previous, more or less in a Titus-Bode step. A planet (with moons or not) formed from an unique original cloud or ringlet. On the countrary if this cloud or ringlet was unable to give a single object, but many objects which don't orbit around each other, then we have asteroids into an asteroid belt. With this meaning, Pluto is a planet, even if it don't have a correct positioninto the Titus Bode structure, while asteroids (Vesta) and KBOs are not, even if Vesta and its belt are in a correct Titus-Bode position.

-would also be a planet something free into space, but too small to fall into the star/brown dwarf categories. May somebody propose another name for this special category, something simpe and easily retainable for the general public.

-Is a moon an object which orbits around another which is not a star or brown dwarf.

-Is an asteroid an object which is wandering alone, or which is a part of a belt.


-For a star orbiting around a planet, I wait for observations of these. Please ljk1-4 quote your sources



We must also cope with a variety of structures for multiple systems. In multiple systems, we keep with the above classification of individual objects. So we have star-star systems, star-brown dwarf systems, with planets orbiting about any of their members. An important thing to understand is that the mass ration of a primary object in a system, over its secondary objects, are not much depending of the size of the primary. We could have for instance the solar system scaled up 30 times: the sun would be a blue giant, Jupiter would be a red dwarf, and Saturn a brown dwarf. If the solar system was scalled down, it could become a planet (or other name for this category) and Jupiter a moon.

It is also important to understand that multiple stars can also come into relatively equal sizes. For instance two stars of relatively equal mass are called double stars. With this in mind, the Earth-Moon system could be called a double planet, or Pluto-Charon also. But the recent discovery of other satellites makes the Pluto system a true planet-moons system, a true miniature of a double star system. So the concept of "double star" or "double planet" is not really relevant, unless the two objects are of relatively equal mass or have close relations like mass exchange. If they are real twins, in a sense. But there is no clear limit, so this classification is not very useful. Only prevails the habit of calling double stars, star systems which can be more unbalanced than the Sun- and Jupiter.

This classification don't remove any unambiguity, but it removes some mess. To make it complete, remains to name planet-like objects wandering alone in space (not bound to a star). Two categories:

-with moons
-without moons.

[David]

-is a planet an unique object which orbits around one of the previous, more or less in a Titus-Bode step. A planet (with moons or not) formed from an unique original cloud or ringlet. On the countrary if this cloud or ringlet was unable to give a single object, but many objects which don't orbit around each other, then we have asteroids into an asteroid belt. With this meaning, Pluto is a planet, even if it don't have a correct positioninto the Titus Bode structure, while asteroids (Vesta) and KBOs are not, even if Vesta and its belt are in a correct Titus-Bode position.

I don't quite follow this. Isn't Pluto a prominent member of the Kuiper Belt? And Ceres is much more prominent a member of the asteroid belt (depending on who you believe, she has anything from 25% to 40% of the total mass of the main belt -- much seems to depend on 1) what mass you attribute to Ceres, 2) what mass you attribute to the asteroids as a whole, 3) where the boundaries of the main belt are -- and I don't know what the latest thinking on the matter is) than Pluto is of the Kuiper Belt, I think. I wouldn't venture to say what percentage of the total mass of the Kuiper Belt is Pluto, but I'm guessing it's not 25%.

[dvandorn]

Only if they're in the Milky Way!

It's getting so deep in chocolate (and chewy nougat!) in here that I need something to scoop with... anybody got a Big Dipper?



-the other Doug

[ngunn]

Would there be Moomoons in the Cowper Belt?

Maybe. The best time to look would be during a grazing occultation.

[Bob Shaw]

Maybe. The best time to look would be during a grazing occultation.

Even I would fear to have made these atrocious puns.

Perhaps this subject is best put put out to grass...

Bob Shaw

[edstrick]

".... I'd expect these brown dwarf systems to greatly outnumber all the visible stars....."

The IMF (Initial Mass Function) or size distribution of new stars shows a rather sharp rolloff in the frequency of very low mass M stars and that appears to continue and get stronger into brown dwarfs. It's very hard to measure the IMF of mature stars in the solar vicinity as the faintest are so faint that catalogs become very incomplete at distances of very few light years.

However, It's much easier to study mass/abundance distributions of stars in young clusters, where low mass M stars and brown dwarfs and even rogue-planets still have very substantial thermal output from formation. In addition, they're studying faint object abundances all the way from star-forming regions like Orion, to young clusters like the Pleiades to older clusters like the Hyades.

There is also a *LOT* of theoretical work on variations in the IMF between different types of star forming regions, low-mass-leisurly-accreting nebula to super-star-forming-complexes with high supernova rates and lots of super-heavy stars.

I'm barely familiar with the literature, but it's out there. The main point is that low mass M and brown dwarf stars are much less abundant than simple extrapolation of brighter star abundances used to suggest.