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Posted by: Myran Jul 30 2005, 05:51 AM

NASA sponsored astronomer Michael Brown of CIT just announced he have found a 10'th planet.
The claim was made for one object based on its brightness alone and that for one object at 3 times the distance of Pluto. In short its smack in the middle of the belt of other KBO's.
The guy claim he was forced to reveal his data since a hacker had threatened to release information about the object.
Im strongly unconvinced about that story.
Why then claim it to be a planet rightoff. More likely think this guy must be looking for the fame of Clyde Tombaugh.
Without any infrared measurements or to establish the albedo properly which would have given a guesstimate of the objects size he singlehandedly claim a KBO planet, this should get the Bad astronomy 1'st price prize for preposterous extrapolation from a single unconvincing piece of scientific data.

Posted by: CosmicRocker Aug 2 2005, 04:58 AM

Myran:

I really don't have a lot of background information on this discovery, but I suspect you are correct about the decision to "go for the glory." But who wouldn't try for that if they had some strong data and the data somehow escaped to the Internet? Especially, consider the fact that there is no clear definition of the word "planet." It is certainly time for the scientific community to define that imprecise word.

I think there is some IR data from the Spitzer Space Telescope.

I think his thought that his data was hacked was a misunderstanding, if this explanation is accurate. http://groups.yahoo.com/group/mpml/message/15283

Perhaps the most significant outcome of this event will be the IAU finally coming to grips with the definition of a planet. This article claims we will have a definition this week!

http://www.nature.com/news/2005/050801/full/050801-2.html

I think I know how I would define a planet. It will be interesting to see how the IAU does. I now see that there is a lot of discussion on these topics elsewhere in this forum, but I'll have to explore them tomorow...

Posted by: volcanopele Aug 2 2005, 04:32 PM

Even near 100%, this object would still be about the size of Pluto.

Posted by: Myran Aug 2 2005, 08:02 PM

CosmicRocker: When I posted this, we didnt have much information either.
But thank you for the info and links.
It turned out that this discovery got a lot of attention in another thread whereas my post here got left in the cold until now. But it will be interesting to learn what
IAU thinks.

volcanopele: If its somewhat larger or smaller than Pluto doesnt matter.
If we could timetravel and tell Clyde Tombaugh and the astronomical community back then the true size of the object they had found -Pluto-, im certain they would he have to think not only once or twice before using the term 'planet' for one object of the diminutive size it turned out to have. Size alone, and now we know so much more and have put Pluto into a context, if the discovery of Pluto had taken until recent years, im personally certain we wouldnt have this discussion at all!

Viewing the other discussion of this same subject here, it obvious than many have gotten so used to the notion that Pluto is a planet that we might have to live with that idea.
But one mistake made 70 years ago, does in no way neccesitate that we make the same mistake again!
-"-

(Some background that might or might not be correct written off the top of my head from what I remember reading in many old astronomical textbooks:
Pluto was named a planet at its dicovery for several reasons, one of them that it was thought to be much larger and massive than what it eventually turned out to be.
Pluto was thought to be so massive that it affected the orbits of Uranus and Neptune, when astronomers couldnt match the values they had gotten for Plutos albedo, magnitude and the resulting size there were some wild suggestions that Pluto would about 5000 km and be incredibly dense. Another suggestion was that we did only see Pluto as a starlike object since we did only see the reflection of the sun in one ocean of liquid gas of a larger world.

Plutos status as a planet have been questioned before, then in connection with speculations that it might be one escaped moon of Neptune - turned out to be wrong, Neptune have brought Pluto into a resonance but they might not have had any common past at all, with the exception of Triton that could turn out to be Plutos twin.)

Posted by: David Aug 3 2005, 12:10 AM

I'm not sure size should be the only, or even principal, criterion for determining "planetary" status; that is, if we are going to retain the term "planet" at all.

Remember that "planet" was originally used as a word for any celestial body that had visible, regular motion against the background of "fixed" stars: not just Mercury, Venus, Mars, Jupiter, and Saturn, but also the Sun and Moon. Comets, whose periods were so long that they could not be identified as the same objects when they returned, were viewed as transient (perhaps atmospheric) phenomena, not similar to planets; other objects were invisible from Earth without apparatus that did not yet exist.

Following the Copernican revolution, the Sun ceased to be "a planet", as (in terms of the entire system), it did not "move". As motion, as seen from earth was no longer a criterion, then the Earth itself could be "a planet".

The telescopic discoveries of Galileo, Huygens and Cassini showed that there were many objects moving around Jupiter and Saturn just as the Moon moved around the Earth; as their motions, viewed from Earth, were not independent of their primaries, they were given a secondary status and not classed as "planets", though by earlier criteria they might well have been. By analogy with these satellites, the Moon also
was demoted, and "a planet" was now defined as "an object moving around the Sun".

One problem with this general definition came with the realization that comets were also satellites of the Sun -- but they were eccentric enough in other ways to merit their own classification. A more serious problem arose with the discovery of the asteroids. Indeed, the questions regarding Pluto and 2003 UB313 were foreshadowed in Piazzi's discovery of Ceres in 1801. Initially Ceres was celebrated as a new planet, just like Herschel's discovery of Uranus in 1781. By the criteria then used -- an object revolving around the Sun with a not-too-elongated orbit -- Ceres was a planet. The trouble came as astronomers over the next few years began to find more and more "planets" in the Main Belt. By the 1850s, the list was becoming unmanageable and it was clear that the asteroids were much smaller than the other planets -- by now eight (with the addition of Neptune in 1846). The asteroids were therefore packed off into their own section of the almanacs. Only at this point does size become an issue in terms of planetary definition. At this point (and up to the present day) the criteria include at least the following:

1) A planet is a natural satellite of the Sun
2) A planet has a not-too-elongated orbit (i.e., it is not a comet)
3) A planet is at least bigger than Ceres (a criterion to some extent redundant with 2), as no comet is bigger than Ceres)

Using these criteria, Pluto was correctly labelled "a planet" in 1930; the only cause for concern being the fact that its orbit is both elongated (somewhat) and tilted (a lot), and I recall that before we ever started talking about KBOs there were proposals to reclassify Pluto as "a comet" -- which was, indeed, the only alternative at the time, as it was obviously neither a Main Belt asteroid nor a moon. On the other hand, Pluto was believed to be, and is, far larger than any comet, which would make its presence in a list of comets far more anomalous than in a list of planets. After all, in the context of all the thousands of objects in the Solar System, Pluto is not that small; besides the other eight planets, only seven moons are larger (the four Galilean satellites, Titan, Triton, and our Moon).

Now of course we have a new list of additional objects (e.g. Sedna, Orcus, Quaoar, Varuna) that fit criteria 1) and 2). Whether they fit 3) is the present dilemma. They are (probably) larger than Ceres; but criterion 3) only says "at least bigger", but does not tell us how much larger than Ceres "a planet" has to be. As these objects are currently not being labelled "planets", one supposes that the real answer is "at least as large as Pluto". And indeed, if we want to avoid the planetary crunch occasioned by the accumulation of asteroids in the first half of the 19th century, that will have to be the answer.

Now we have 2003 UB313. And if our criteria for "a planet" are now: 1) A natural satellite of the Sun whose orbit is not 2) too elongated and is 3) at least as big as Pluto, then 2003 UB313 is a planet. And that is a conclusion that is not, in itself, unacceptable; there is no special reason to limit the number of planets to nine or fewer, and we would be in an even worse terminological muddle if, in 1781, tradition-minded astronomers had sought ways to avoid calling Uranus "a planet".

The problem, of course, will arise if a lot of objects bigger than Pluto are discovered out in the Kuiper Belt or beyond. Once we get beyond half-a-dozen (and so up to fifteen planets) the situation, if it gets to that (and there's no saying it will) will probably be felt to be intolerable. At that point, I would expect Pluto and his transneptunian siblings to be demoted to some new status, somewhere between "planet" and "asteroid". However, that situation has not yet been reached, and may never be reached; and there is plenty of time to think about possible consequences. At present, however, it seems to me that the safest thing to do is to accept 2003 UB313 as a planet, with the proviso that the term can be revoked if it becomes inconvenient.

However, if Pluto is pre-emptively demoted, simply in order to exclude 2003 UB313, I have to wonder what will be done if a distant transneptunian object is discovered that is larger than Mercury? Will Mercury also lose its status as "a planet"? Or will we add a fourth criterion to the three above, "A planet must not orbit beyond the orbit of Neptune"?

Posted by: CosmicRocker Aug 3 2005, 05:39 AM

David: That was an insightful description of some of the historical perspectives the IAU must be considering, and I learned a lot reading it. Thanks. I would have to guess that they must also be considering recent discoveries and our better understanding of the origin and evolution of the solar system.

To your list of parameters, I would also suggest adding orbital inclination and some other measure of orbital stability. If an object's future is uncertain, I don't think it should be classified as a planet.

What I find most amazing is that we have the opportunity to be at least peripherally involved in the discussion of the definition of a word that is thousands of years old. It has been reported that the IAU has been working on this definition for the past year and that they expect to finalize a definition this week.

That should be quite interesting.

Posted by: Myran Aug 3 2005, 03:11 PM

Um hold a second here David , I didnt question the status of Pluto in my reply, only had a lookback on some of the reasons that world ended up with the label.
And again, since people (including many astronomers) have gotten used to view Pluto as a planet quoting myself 'I think we have to live with the idea'.
So I didnt talk about demotion of Pluto, Mercury or any other world that have been declared a planet.

But as said, we have found many more pieces of the puzzle that makes up the solar system. And one far from unimportant one is this Kupier belt.
UB313 is just one more addition to a list that have become rather long already.

And yes, I do look ahead, since im personally convinced theres not only a dozen larger objects still to be found, but far more perhaps several hundreds.

You used the terms "planet" and "asteroid" and there have been a suggestion some years back thats based on both; 'planetoid' where the name would hint that its the building block that planets are made from.

So yes, I have no problem using the term planetoid for UB313 keeping both sides of the fence happy.

Posted by: Jeff7 Aug 3 2005, 05:23 PM

I figure this is going to turn out like Saturn. How many moons do they say it has? It keeps increasing. And what exactly is a moon? Chunks only a few miles wide are being found and called moons. "Big boulder" or "debris" seem more appropriate terms.

In the coming decades, as we send more probes out into the Kupier Belt, and as better telescopes are built, we'll no doubt find a great many objects orbiting out far, as Myran also believes.



Hm, new idea for declaring something a planet - an orbiting body must be a specific percentage of either the mass or diameter (or a combination of both) of the body that it orbits.
Example - Mars has two moons, and they're both fairly small. But it seems ok because they are the only ones there, and because it's a small planet. Now put those moons around Saturn. There are lots of things there that are that size, and they are puny compared to the big gas ball they orbit. Are they still moons?

Posted by: Myran Aug 3 2005, 06:22 PM

Very good example, and there you happened upon the core of my thinking.
One of those boulders that are inside the ring system should be considered to be part of the same rings.
But those outside the rings might be considered a moon, unless if very minor in size.
So lets apply this line of thinking to the Kupier belt: Pluto are at the inner edge of same belt and could be viewed as being outside it, so Pluto can keep its planetary status. Yet UB313 are firmly placed among many other Kupier belt objects with orbits both inside and further out from the sun and so it belong to the group and can in no way be seen as a member of the planetary family.

Posted by: David Aug 3 2005, 07:24 PM

Forty-seven at last count. But there's a way to go before it matches Jupiter's sixty-three.



I've often thought that a distinction should be made between "moons" (objects similar in size and shape to our Moon) and other natural planetary satellites, which - on the model of the coinage "planetoids" - might be called "selenoids". If the former category were restricted to objects at least as big as Mimas, no one planet would have more than seven "moons".



Well - let's try that scheme. Ranking objects in order of relative size in relation to their primary, we have:

.49 Charon
.27 Moon
.1 Jupiter
.087 Saturn
.055 Triton
.043 Titan
.037 Uranus
.036 Ganymede
.035 Neptune
.034 Callisto
.031 Titania
.030 Oberon
.025 Io
.023 Umbriel
.023 Ariel
.022 Europa
.013 Rhea
.012 Iapetus
.0093 Dione
.0092 Miranda
.0092 Earth
.0088 Tethys
.0087 Venus
.0084 Proteus
.0069 Nereid
.0065 Phobos
.0049 Mars
.0041 Enceladus
.0039 Larissa
.0037 Deimos
.0035 Mercury
.0033 Mimas
.0032 Galatea
.0032 Puck
.0030 Despina
.0029 Sycorax
.0026 Portia
.0022 Hyperion
.0018 Juliet
.0017 Pluto
.0012 Amalthea
.0010 Phoebe
.0007 Ceres

I think the list is complete for relative sizes larger than Pluto/Sun, but I have only listed a few that are smaller for use as signposts. I guess Jeff7's definition works as well as others, but it provides no obvious answer to the question: "Where do you draw the line?"

The major problem that I see with this approach is that it makes it harder to draw parallels between bodies in the Solar System that may be similar overall, but happen to be in orbit around a different primary; e.g., if a particular moon happens to be a captured asteroid or comet, it would fall in a very different place in this list than its closest relatives would, because its primary is so much smaller than the Sun.

Posted by: ilbasso Aug 8 2005, 09:43 PM

Then you also get into the asteroids with moons, like Ida/Dactyl. And what about the asteroids that are essentially two bodies in contact with each other?

Posted by: ljk4-1 Jun 9 2006, 07:41 PM

Definition of 'Planet' Expected in September

http://www.space.com/scienceastronomy/060608_planet_definition.html

Historians and educators have joined astronomers in an effort to break a
deadlock on contentious discussions over a definition for the word planet.

Posted by: BruceMoomaw Jun 10 2006, 02:10 AM

Wonderful! Think of all the fun they can have next trying to officially define "moon", "asteroid" and "comet". The debates in Laputa won't be in it by comparison.

Posted by: Richard Trigaux Jun 10 2006, 07:22 AM

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)...

Posted by: ljk4-1 Jun 10 2006, 02:46 PM

Note scientists' attempts to introduce two new terms to strange new
worlds: Pegasids and Planemos.


Inside Exoplanets: Motley Crew of Worlds Share Common Thread

http://www.space.com/scienceastronomy/060605_mm_pegasids.html

Scientists have discovered a correlation between the amount of heavy elements in
giant Jupiter-like extrasolar planets known as "Pegasids" and their parent
stars.


* Strange New Worlds Could Make Miniature Solar Systems

http://www.space.com/scienceastronomy/060605_planemos.html

Planet-like objects floating alone through space harbor disks of material that
could make other planets or moons, something like miniature versions of our
solar system, astronomers said today.

Posted by: ngunn Jun 12 2006, 12:25 PM

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.

Posted by: Rob Pinnegar Jun 12 2006, 12:44 PM

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.

Posted by: ngunn Jun 12 2006, 01:10 PM

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.

Posted by: BruceMoomaw Jun 12 2006, 01:24 PM

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.

Posted by: ljk4-1 Jun 12 2006, 01:53 PM

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

We will have to call them Ptolemetoids.

Posted by: ngunn Jun 12 2006, 02:10 PM

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

Posted by: ilbasso Jun 12 2006, 02:36 PM

Would there be Moomoons in the Cowper Belt?

Posted by: paxdan Jun 12 2006, 02:45 PM

I've always liked the fact that the Sun-Jupiter http://en.wikipedia.org/wiki/Barycentre is above the surface of the sun

Posted by: Rob Pinnegar Jun 13 2006, 12:24 AM

Only if they're in the Milky Way!

Posted by: Richard Trigaux Jun 13 2006, 06:14 AM

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.

Posted by: David Jun 13 2006, 06:43 AM

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%.

Posted by: dvandorn Jun 13 2006, 07:41 AM

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

Posted by: ngunn Jun 13 2006, 07:52 AM

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

Posted by: Bob Shaw Jun 13 2006, 08:55 AM

Even I would fear to have made these atrocious puns.

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

Bob Shaw

Posted by: edstrick Jun 13 2006, 10:09 AM

".... 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.

Posted by: Richard Trigaux Jun 13 2006, 12:24 PM

I hope so, as the curves I already seen were increasing sharply with lower masses. If so, there would be such an abundance of brown dwarves that it could explain the dark matter. But searches of MACHOS (objects like brown, red or white dwarves) found essentially objects with half of the sun masses, probably red or white dwarves, and in much less quantity than expected to explain the dark matter. So objects like lone brown dwarves or lone planets (free in space) must be not so common than stars. But the theory predicted too that the IMF could go at such low masses than Earth. Also lonely planets could result from interaction between forming stars, or later of star encounters (especially in close clusters).

So we can still envision systems with small planets as primaries.

Posted by: ngunn Jun 13 2006, 04:21 PM

Even if the number of low mass systems is much less than indicated by simple extrapolation that doesn't mean that the number actually decreases with decreasing mass. As we have several orders of magnitude of mass range to consider that still leaves the possibility of relatively abundant low-mass systems, even without considering the secondary formation processes such as ejection or collision that Richard mentions.

Posted by: Myran Jun 13 2006, 05:48 PM

No there doesnt seem to be any decrease in number and I dont think it was edstrick's idea to suggest anything such either.
But there appear to be somewhat like a cutoff where the curve of even increasing numbers for lower mass objects flatten out. I say 'appear' since brown dwarfs are very hard to find. Even so, with out current knowledge it appears that brown dwarfs might not be as plentiful as some theoreticans first proposed.

Posted by: ngunn Jun 14 2006, 09:58 AM

O.K. let's make the very conservative assumption that the distribution is completely flat - in other words for each decreasing order of magnitude there are the same number of systems. All the nearby visible stars fall within just 2 orders of magnitude, 0.1 to 10 Msun approx. From there downward we have 5 orders of magnitude to get to terrestrial planet sizes, so on the above assumption low mass systems of terrestrial mass or greater would still outnumber visible stars by 5:2. My guess is that this is a gross underestimate and I predict that over the next couple of decades we will start finding numerous low mass systems closer than Alpha Centauri - if we look for them.

Posted by: edstrick Jun 14 2006, 11:21 AM

My *VAGUE* recollections of abstracts I've skimmed and PDF papers I've downloaded and glanced contains a phrase something like the "brown dwarf desert", where we're seeing very few BD's as planet-like companions of stars. I just don't know if the total abundance of BD's is less than the equivalent mass range of red dwarfs or what.

Xarchive.org has had many and various papers float by on relevent topics float by. Have at'm! (I currently just don't have enough time and "effort")

Posted by: ngunn Jun 14 2006, 12:32 PM

Thnks for that phrase edstrick:
http://www.aas.org/publications/baas/v36n4/dps2004/163.htm
I wish I could see that histogram, but I suspect the word desert is really a bit of an overstatement -it's only a relative thing.

Posted by: ljk4-1 Jun 14 2006, 01:12 PM

Here is the actual paper online:

http://arxiv.org/abs/astro-ph/0412356

Posted by: ngunn Jun 14 2006, 02:04 PM

EXCELLENT, thanks.

So: The 'brown dwarf desert' refers to objects in the range one tenth to one hundredth Msun, but applies only to the population of fairly close stellar companions with periods under 5 years. More generally the IMF (initial mass function) derived from cluster studies shows only a modest decrease in the continuing rise in numbers of objects at lower masses, and this decrease in the rate of increase could be due to observational selection anyhow. dN/d(log M) comes nowhere near to changing sign, as would be required to make the population of smaller objects lower than the population of visible stars. These observations do not constrain the number density of independent low-mass systems in the solar neighbourhood at all. In fact there could be thousands of them within 10 parsecs.

Posted by: Richard Trigaux Jun 14 2006, 05:19 PM

Googling "Initial Mass functions" I found several articles of interest (among many others)

curve:
http://webast.ast.obs-mip.fr/hyperz/hyperz_manual1/node7.html
theory:
http://www.astro.caltech.edu/~george/ay20/Ay20-Lec17x.pdf.

From these it appears that the IMF is a power law. So that the number of zero mass stars would be infinite... in reality ther would be a cutoff somewhere, but it is rather a matter of conjecture where this cutoff is. What happens in the 0.1-0.5 solar mass range is already uncertain (some flattening seems to appear), below it is purely speculative. See the curves on the second link, p16, according to various hypothesis.


So theoricians still don't know today where is the baryonic mass, in red dwarves or into brown dwarves sytems, which then would be very numerous, down to some Jupiter size, or even smaller (interstellar KBOs??

So we cannot rule out the discovery of some brown dwarves systems even closer than Proxima Centauri. But in the same time we cfannot yet state that this must happen.

Posted by: alan Jun 14 2006, 10:29 PM

http://www.aas.org/publications/baas/v34n4/aas201/679.htm

Gragh of log(N) vs log(M) in this paper
http://www.astro.psu.edu/~kluhman/BDreview.pdf

I've seen an estimate of brown dwarf population, based on 2MASS observations, as being roughly 2x the population of regular stars.

Posted by: edstrick Jun 15 2006, 04:36 AM

OK.... There's an inflection in the log-log curve, not a actual decrease in numbers per fractional delta-mass. That's what I couldn't remember.

The Japanese, wkith their current infrared whole-sky-survey mission in orbit, may be able to dectect near solar-system BD's. They've got much higher resolution than IRAS did except at the very longest wavelengths (70 micrometer is on the long-wavelength side of even primordial BD emission blackbody curves, anyway)

I've hoped and hoped that we do find some BDs or rogue planets nearer than Alpha/Proxima Centauri. The biggest problem in the search is the stellar background we have to search against. Until we can search against the milky way background for high proper motion BD like objects with sensativity and resolution to capture everything more massive than Jupiter out to maybe 15 or so light years, we won't have an essentially complete local census.

Posted by: Richard Trigaux Jun 15 2006, 06:39 AM

Thanks alan for your links, especially the second which is very interesting.

It is impressive to see how science advanced since my last post. From your second link, we can conclude:

-the Innitial Mass Function peaks into the red dwarves region, and then decreases into the brown dwarves region. The reason for this is still not clear, but observational evidences seem safe enough.

-The total number of brown dwarves may be great, but not much larger than of stars. The total mass of brown is not greater of that of normal stars.

-Brown dwarves have proven accretion disks qimilar to stars
-They may have planets too (not proven, but likely)


I should add:
-Brown dwarves are not numerous enough to be the dark matter

-brown dwarves are not so numerous that we may find many closer than Proxima Centauri. Discovering one is however far from impossible.

-Free planets lighter than Jupiter seem unlikely (unless they were ejected from a larger solar system. This may be rare in our region, but very common in dense star clusters)

At last some studies (see the thread on Pioneer effect) give narrow limits to the presence of massive objects near the solar system.

Posted by: ngunn Jun 15 2006, 12:46 PM

Sorry to disagree Richard but these conclusions are far from secure.
The last paragraph of the SUMMARY AND DISCUSSION section of Grether and Linewaver, May 27 2006 posted by ljk4-1 yesterday clearly refers to "The fact that there is a close orbiting brown dwarf desert but no free floating brown dwarf desert . . ." So at the very least the jury is still out on this. Furthermore, we have ABSOLUTELY NO IDEA what happens to the numbers below 0.01 Msun. I cannot believe that the mass function conveniently terminates just at the point where the objects cease to be visible. So I'm sticking with my hypothesis of many small objects closer than Alpha Centauri for now. However I am not suggesting that they outweigh the stars in terms of total mass.

My case is : Don't rule them out or their continuing non-discovery could become a self-fulfilling prophecy over the next decade or two.

Posted by: Richard Trigaux Jun 15 2006, 04:29 PM

I (or rather the study I quote) never said that there are NONE, I (the study) just say that they are much less than we could predict with simply extrapolating the IMF. Progress was made into the determination of the IMF, and the cutoff and decreasing slope was found (thantks to studying recent clusters where Brown dwarves are much more luminous than in older ones). So the more numerous category is the red dwarves category, not the brown dwarves. And, about very low masses (3 Jup or less) we still don't know, we cannot exclude the IMF having a second peak for some reason. But it is not very likely.

To have several brown dwarves closer than Proxima Centauri would require that, statistically, the brown dwarves would be much more numerous than the red dwarves, which seems not the case. Of course we could be lucky and find one, and perhaps several. But the statistical average gives closest brown dwarf rather in the 10- 20 light years. Unless of course you disagree with the study.

Posted by: ngunn Jun 16 2006, 08:26 AM

I suppose what I'm saying is this:
1/ The different studies lead to very different conclusions about free floating brown dwarf numbers, although this is not the main focus of either study.
2/ Both agree that brown dwarfs are relatively more common among the free-floating population than they are as close orbiting companions to stars.
3/ Free-floating brown dwarfs are harder to find than brown dwarf companions, so the total observed sample is skewed in the direction of the (relatively scarce) brown dwarf companions.
4/ The smaller they are the harder they are to find so there is a very strong effect of observational selection at the low mass end which is difficult to quantify properly.
5/ There are at least some very loosely bound brown dwarf binaries (30-100 AU)
6/ All the observable brown dwarfs occupy only about one order of magnitude in mass.
7/ I am not saying brown dwarfs outnumber stars - only that low mass systems probably do, spanning at least 4 orders of magnitude below the presently detectable brown dwarfs.
8/ Low mass binary or multiple systems (30-100AU) would be dynamically the easiest places from which the sun could capture an object like Sedna.
9/ We need to be on the lookout, not only for more 'Sednas', but also for dark interstellar low-mass systems as soon as we have the means to search for these very difficult observational targets. If there are more objects in one category there are likely to be more in the other.
10/ We are still literally in the dark about all this but new developments are about to expand our observational powers enormously. That makes it an especially good time to keep an open mind while we look for what is there and what is not.

Posted by: edstrick Jun 16 2006, 09:48 AM

Middle-term, the really solid determination of the initial mass function for sub-stellar non-planets (BD's and rogue planets) will come from the James Webb Space Telescope. With high sensitivity and resolution in the near, middle and thermal infrared, it will be able to detect low-mass objects in reasonably nearby star forming regions down to very low masses, and get a decent observational (instead of theoretical) plot of luminosity-distribution vs. cluster age to test luminosity/spectral evolution models as a function of mass and age.

*eesh... long sentence warning!*

What we'll really need someday is an infrared version of the under-development Synoptic Survey telescope, to get an infrared sky survey down to something like 30'th magnitude. That would really nail down the "local" population of low mass objects.

Posted by: Richard Trigaux Jun 16 2006, 09:58 AM

The point is that, these objects are not only very weak, but also they dwelve in the infrared spectrum, see in radio.

Brown dwarves are in the 1000-2000°C, and as small as Jupiter (Jupiter is about the maximum diametre allowed for a cold body without nuclear reactions)
Large planets could be still colder: Jupiter, without sun warming, could be at something less than 50°K
Small planets, KBO-like, lone comets, fluffy balls, are in thermal equilibrium with the surrounding space. Thus they cannot be detected by their thermal radiation.


The IMF into an interplanetary cloud and around a parent body is likely not the same, as the presence of an accretion disk makes things much faster and at a much smaller scale. So we may expect that the IMF around a star is shifted toward smaller bodies with several orders of magnitude. But observational evidences are still far out of reach...

Posted by: ljk4-1 Jun 16 2006, 12:56 PM

I thought Jupiter radiated more heat than it receives from the Sun?

Posted by: ngunn Jun 16 2006, 01:37 PM

I think that just means it re-radiates what it receives from the Sun plus a little bit more. Richard's estimate would be for the case where that little bit was all it had.

Posted by: Myran Jun 16 2006, 07:57 PM

Yes thats correct, i've read that it is thought that the excess energy comes from gravitational contraction. Or in simple terms that Jupiter shrinks a very small bit each year.
(The powerhouse behind winds of Saturn on the other hand might, according to one theory, get the power from helium rain where the latent heat is released when the helium condenses.)

Posted by: ngunn Jun 19 2006, 12:08 PM

Since our recent discussion on brown dwarf numbers I've found this on Wikipedia:

"Since 1995, when the first brown dwarf was confirmed, hundreds have been identified. They are now believed to be the most numerous type of body in the Milky Way."

Can anyone help me track this statement to a source, authoritative or otherwise?

Posted by: Alan Stern Jul 10 2006, 01:07 AM

This is a laugh. There are more comets in our Oort Cloud alone (10^13 to 10^14) than there are
brown dwarfs in all the galaxy. If our OC is typical and OCs form around even 1% of the stars, then
there are >10^22 comets in the Milky Way (and that ignores the ones ejected to interestellar space
during OC formation-- probably another factor of 10-30x higher). Planets, comets, and asteroids
out number any and everything stellar/substellar.

-Alan Stern

Posted by: Greg Hullender Jul 10 2006, 03:33 PM

Forgive my presumption, but I had thought the Oort cloud was still a hypothesis. How confident are we of those numbers?

Posted by: Alan Stern Jul 10 2006, 06:19 PM

Quite confident. Long period comets are observed on OC orbitsall the time: over 800 now. These are the basis of the population estimates, which also agree to an orfder of magnitude with prediction of how much should bein the OC from giant planet clearing.

Posted by: The Messenger Jul 13 2006, 05:40 PM

Thanks, Alan.

There is an interesting, but EXTREMELY speculative alternative to the current Oort cloud scenario emerging. Listen at what our probes are telling us:

1) According to Voyager I, the solar wind slows more than expected, and the magnetic field associated with the solar wind increases in strength while decreasing in frequency. Solar wind particles must be piling up more than expected.

2) Both Wild 2 and Temple 1 are populated with fine dust, rich in iron and other minerals not expected to be found in the comets.

It is not difficult to add these observations together, and imagine what amounts to solar raindrops: solar wind particles condense onto Oort and/or Kuiper belt nuclei and rain back into the solar system as comets. Since the 'seed' nuclei is richer in water and other volatiles that condense only in the outer SS, mild heating percolates these volatiles through the dusty crust, and after just a few passes we would get the explosive venting we witness as comets.

For this to be true, the ratios of heavy elements in the solar wind, and what we have observed from comets must be roughly the same.

Posted by: Richard Trigaux Jul 13 2006, 06:14 PM

Messenger, I had an intuition of something like that from long ago. I was not convinced by the theory as what Ort cloud objects were in orbit around the Sun. If so, these orbits would be more or less circular, but not so excentric. If all the comets of the Oort cloud where in the very elliptic orbits we see, they would all be burned by the sun for long. If they needed to be disturbed from a more or less circular orbit, They would come in groups.

The idea you propose is much more appealing: the comets are not in orbit, they fall back on the Sun. In the process, they all more or less gather some lateral motion which makes them appear on very elliptic orbit (as a free fall trajectory is none else than a very elliptic orbit).

If the theory of the disturbance was true, only very few comets would have their orbital speed just canceled, and most would be on far orbits. There are too much comets falling right on the sun for this.

Posted by: ljk4-1 Jul 13 2006, 06:14 PM

Let us hope the Sol system and the rest of the galaxy's stars aren't like
the ones described in a David Brin story (I think it was called "The Great
Silence", but please correct me here).

All star systems are surrounded by a literal shell of comets that needs to
be penetrated by a fast starship in order to be destroyed - usually at the
cost of the ship and any crew and then the system has to endure a hail of
comets for a while.

The story also "explained" why SETI had not been successful until after
the first starship made the first crash through: Only a few other species
had also made the discovery, while others never even bothered to leave
their planets.

Posted by: Mongo Jul 14 2006, 09:39 PM

I believe that the story was called "The Crystal Spheres". Another important point in the story was that these spheres could only be broken from within itself, so only those civilizations that sent out a massive, high-velocity (meaning a substantial fraction of c) object -- such as a starship -- would have their enclosing shells broken. Oh, and the shells would absorb all artificial radio-frequency emissions from within themselves.

Bill

Posted by: Richard Trigaux Jul 14 2006, 09:52 PM

These stories are interesting, but fictional. It is curious that so many people explain ET behaviours or SETI results with their own behaviour or way of thinking...


The Oort cloud is a threat for spaceships going through it, but no more than our asteroid belt, which was traversed several times by spaceships, without any encounter. But, I remember, not so long ago it was seen as a terrible obstacle forbidding any exploration of planets beyond. I remember seeing a fiction story with asteroids as close together as trees in a forest. I remember it, it was in a story of... Donald Duck.

Posted by: Myran Jul 15 2006, 11:17 AM

I would like to voice the idea that the Oort cloud would be far less of a threat.
Even if the number of objects might be quite higher in the Oort
cloud. They are spread out of one far lerger orbit around the Sun, and then not even bound to the ecliptic but spread out in a spherial volume.

Posted by: Jyril Jul 15 2006, 08:46 PM

Although the Oort Cloud has most likely far larger number of objects than the Kuiper Belt, let's not forget how insanely large it is. It has been calculated (can't confirm, too lazy to check) that the average distance between Oort Cloud objects is about the same as the distance between the Earth and Saturn. So colliding with one would be really bad luck...

Posted by: David Jul 15 2006, 10:47 PM

If the average distance between Oort Cloud objects is 10 AU, and if the Oort Cloud is about 50,000 AU distant, then by my (doubtless very fallible) calculations, a patch of sky measuring 1x1 degree of arc ought to contain c. 7500 of them. They ought to be occulting stars like crazy. Why haven't we detected them already?

Posted by: helvick Jul 15 2006, 11:32 PM

Just taking a quick stab at this.

If we took a 100km diameter Oort cloud object that was at 50K AU then it would be ~3e-6 arc seconds across. A sun type star at an average distance away in the galaxy (50K light years) is about 7e-7arc seconds across. So the Oort object could occult the star in theory.

Assuming stars are fairly uniformly distributed to make things simple then that 1x1 degree box will cover about 750000 stars assuming there are a 100 billion stars in the galaxy.

The 1x1 degree block has about ~1.4e18 spots the size of a kuiper belt object so if your 7500 number is right then the chance of any one of those spots (and a star behind it) being occluded by any one of those objects is around 1 in 2e14. The chance of any one of those having a star behind it at any point in time is about 1 in 2.5e8.

Assume it takes about a second for a typical object to move sufficiently to be covering a different spot then it will take on average 7.9 years before one occultation occurs in that particular spot. Of course there are many spots and if it was all uniform then I reckon there might be 1 occultation every 30 minutes or so somewhere in the sky. The problem is that you'd have to watch the whole sky very carefully indeed to catch it.

Posted by: alan Jul 16 2006, 12:04 AM

The Taiwanese-American Occultation Survey will look for occultations causeed by Kuiper belt objects and Oort cloud objects. They expect the occultations to last ~ 1/2 second and produce diffraction patterns. Discription here
http://widefield.lbl.gov/2004/posters/occultation_lehner.pdf

Posted by: ngunn Jul 17 2006, 08:26 AM

I thought the Oort cloud objects were supposed to be early ejecta from the giant planet zone, not distant circularly-orbiting bodies that only occasionally get perturbed inwards. If in fact nearly all of them have perihelia less than 30 AU then there don't have to be so many of them to account for what we see. After initial scattering, history would have selected them in two ways. The shorter period ones visit the sun too often and sublime away, while those with hyperbolic velocities never return at all. After 4,500 million years we are left with mainly the ones that have aphelia at 'Oort cloud' distances. Since they travel most slowly at aphelion that's where most of them are at any given time, hence a spherically symmetric 'shell' of objects with relatively low angular momenta. For once(!) I can't see any problem with this conventional view.

Posted by: Richard Trigaux Jul 17 2006, 11:04 AM

Yes, this is the conventional theory. Or at least one of the conventional theories, as another would be that the Kuyper Belt and Oort clouds would rather be remnants of the accretion disk which never condensed into planets. In facts none of these theories is supported by firm evidences.






... except that comet dust was recently found to contain olivine and pyroxene, which are not meant to form at the distance of the Oort cloud, not into a accretion disk (in the hypothesis the Oort cloud is a remnant of such a disk) and even not at the distance of the giant planets (in the hypothesis it is the giant planets which scattered the oort cloud objects). This makes the formation and structure of the Oort cloud more difficult to understand.

If this olivine dust arrived at the time of the accretion disk, arises que question of how it was raised in this place. If it was formed more recently from the solar wind, it is difficult too to explain how it did. Perhaps into the shock wave with the interplanetary medium??

Posted by: ngunn Jul 18 2006, 07:53 AM

I don't think this is too serious a problem. In the early solar system there would have been scattering by the inner planets too, throwing some rocky material outward. Also there could have been quite large protoplanets in the giant zone capable of forming rocky minerals that were later blasted apart by collisions. Furthermore most of the comets we observe have made numerous previous visits to the inner solar system where they could have picked up rock dust. With orbits as eccentric as the long period comet orbits are they could have picked up material from almost anywhere, and their individual histories could be radically different. I think we'll have to study quite a large sample at close range before we can significantly improve on current guesses.