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#1
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planet schmanet, it's probably a brown dwarf!
I think this new "planet" detected in a 13 billion year old
star system is most likely really a brown dwarf. Ahh, but you say that 2.5 MJup. is too low to be a brown dwarf! Firstly, no, we don't really know that, not yet. Secondly, let's assume for the moment that the mass limit assumption for brown dwarfs holds, imagine this scenario. You've got 3 stars in a system, a really massive star (a few solar masses), a kinda low mass star (just under one solar mass, or thereabouts), and a dinky brown dwarf around a dozen MJ. Now, the big star goes off the main sequence really soon, turns into a red giant then supernovas. It's just a hunch, but I'm betting that the blast from a type-II supernova is gonna brush off a teensy weensy bit of the outer atmosphere of a friend the brown dwarf. Then you've got the other star, which spends several billion years on the main sequence, but eventually it too goes into a red giant phase. First it heats up and its outer atmosphere balloons up, and then it blows off that outer atmosphere in the creation of enormously powerful stellar winds (which create a temporary planetary nebula). This is just a hunch too, but I bet both the red giant phase and the strong stellar winds phase might, just might, strip off a wee bit of the brown dwarfs mass. Ahh, but wait, there's more! There was almost certainly a scuffle involving the stars when they first got together and the poor little brown dwarf was tossed around quite a bit, there's plenty of opportunity in that, I think, for a fair amount of mass loss. So you've got three events that are likely to remove a fair amount of mass from a brown dwarf, are they powerful enough to remove 80% of the mass of a brown dwarf? I don't think that question has been answered satisfactorily yet. Nor has the question of the true mass distribution of objects which coalesce directly from stellar nebulae (i.e. stars, including brown dwarfs. I think there is a chance this object is a planet (i.e. was formed from a proto-planetary disc), but so far I don't think enough evidence has been presented to dismiss the possibility that it might be a brown dwarf which has lost mass. |
#2
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planet schmanet, it's probably a brown dwarf!
"Christopher M. Jones" wrote:
I think this new "planet" detected in a 13 billion year old star system is most likely really a brown dwarf. Ahh, but you say that 2.5 MJup. is too low to be a brown dwarf! Firstly, no, we don't really know that, not yet. These are just terms that we use but haven't defined yet, so saying, "It's not an X, it's a Y" is at this point pure semantics. I'm not familiar with any widespread acceptance of your use of the term _brown dwarf_ to mean "very large gas giant that evolved as the center of a protoplanetary disk, as opposed to on the periphery of one," and as I pointed out in the thread we were both involved in when this last came up, even that distinction is arbitrary and unclear -- all one has to do is consider multiple star systems formed from the same protostellar cloud to see that such a hard dividing line can't be all that significant in a classification system. Reduce the mass of the smaller star, and at some point it becomes "less than a star" and at some other point it becomes "just a planet." Where you draw those lines are arbitrary (the second much more so!), but it's a continuous spectrum. I think there is a chance this object is a planet (i.e. was formed from a proto-planetary disc), but so far I don't think enough evidence has been presented to dismiss the possibility that it might be a brown dwarf which has lost mass. I don't know, this sounds like an astonishingly large amount of sheer speculation based on a system we know so very little about. -- Erik Max Francis && && http://www.alcyone.com/max/ __ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE / \ You are inspiration to my life / You are the reason why I smile \__/ India Arie |
#3
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planet schmanet, it's probably a brown dwarf!
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#4
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planet schmanet, it's probably a brown dwarf!
"Erik Max Francis" wrote:
"Christopher M. Jones" wrote: I think this new "planet" detected in a 13 billion year old star system is most likely really a brown dwarf. Ahh, but you say that 2.5 MJup. is too low to be a brown dwarf! Firstly, no, we don't really know that, not yet. These are just terms that we use but haven't defined yet, so saying, "It's not an X, it's a Y" is at this point pure semantics. I beg to differ. My own definitions of X and Y are quite clear (formation via condensing nebula or via a proto-planetary disc). I think that my definitions are the only ones that really make sense in regard to planets vs. stars. I also think that they are the same basic definitions as the most popular or at least the "leading" definitions (i.e. the definitions used by those who's work is the most important in the field). Furthermore, I am very confident that eventually they will be the official and standard and majority accepted definitions. Nevertheless, I will admit that this definitional area of star vs. planet is one which is rather craptistic at the moment, and there's a lot of "accepted definitions and conventions" which are not helpful and some which are in fact detrimental. And, I'll also admit that this whole topic (definitions and all) is very short on observational evidence and especially statistics at the moment. Given all that, I'll still stick by my definitions. In the future I'll try to be more careful to explain what I mean more precisely rather than using shorthand, non-standard definitions. For now, just assume that when I say "brown dwarf" or "star" I mean an object which condensed directly from a nebula, either alone or in parallel with the formation of other stars, and when I say "planet" I mean an object which condensed from a proto- planetary disc, which itself was formed after and in parallel with the condensation of its parent star. I'm not familiar with any widespread acceptance of your use of the term _brown dwarf_ to mean "very large gas giant that evolved as the center of a protoplanetary disk, as opposed to on the periphery of one," and as I pointed out in the thread we were both involved in when this last came up, even that distinction is arbitrary and unclear -- all one has to do is consider multiple star systems formed from the same protostellar cloud to see that such a hard dividing line can't be all that significant in a classification system. Reduce the mass of the smaller star, and at some point it becomes "less than a star" and at some other point it becomes "just a planet." Where you draw those lines are arbitrary (the second much more so!), but it's a continuous spectrum. Proto-planetary disc formation is a long process in comparison to proto-stellar nebula collapse. Of special interest is the process by which the material in the disc becomes enriched in ices and dust and depleted in gases, as well as the formation of the disc shape itself. Of secondary interest is the difference between the process of gravitational collapse of a gas cloud vs. accretion. Now, it looks like there are formation processes for gas rich planets from proto-planetary discs which do not necessarily rely on accretion and which blur the line somewhat between star formation and planet formation, but not completely. There are many distinguishing characteristics between the two formation processes and their results, but I don't really feel like delving into that discussion right at the moment, as it's a long one, perhaps sometime later. For the mean time, note that the major difference lies in the composition of the materials going into the formation of the object. Proto-planetary discs are enriched to a high degree in dusts and ices (i.e. "metals"). Even for Jovian planet formations the increase in abundances of dusts and ices in the planetary makeup is enormous (even though they might still be very minor constituents in comparison to H and He). Also, as I've said before, but perhaps not forcefully enough, I think the distinction between planets and brown dwarfs based on mass is bogus in the extreme. Obviously, pulsars, white dwarfs, black holes, and main sequence stars of 1 solar mass are *not* the same objects nor necessarily have the same origins, but by the brown dwarf / planet system they would be presented as equivalent objects. That's just stupid. It's a more extreme case of the man with a hammer problem, not only does everything look like a nail but the world is divided into only two important categories, nails and non-nails. I think there is a chance this object is a planet (i.e. was formed from a proto-planetary disc), but so far I don't think enough evidence has been presented to dismiss the possibility that it might be a brown dwarf which has lost mass. I don't know, this sounds like an astonishingly large amount of sheer speculation based on a system we know so very little about. Precisely. That's what I take issue with. That's why I think it's critically important to consider alternatives to the narrow mule-headed view that if it's under 13 MJ it's a "planet". Even more so since the significance put on this discovery has been very, very clearly due to the assumed formation route of the object. Which, if it were a planet by *my* definition would mean it was formed from a metal enriched proto-planetary disc. And, obviously, if this system dates back to the dawn of the Universe or thereabouts there was very little chance of the possibility of such a metal enriched *anything*. Getting back to alternate hypothesis for the formation of this 2.5 MJ object, I thought of another scenario for substantial mass loss. The theory in the paper is that the stellar system was assembled after the fact due to stellar encounters near the center of a dense globular cluster (specifically, the posit that the pulsar nee massive star entered the system of the white dwarf nee red dwarf plus "planet" and caused a reorginazation where the planet ended up in orbit of the binary stars). Their very own scenario has the "planet" only a few AU from the white dwarf star during its earlier lifetime. By that it's enormously plausible that the object was well within the stellar atmosphere or very near it during the star's red giant phase for a considerable period of time and consiquently burned off a lot of mass. |
#5
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planet schmanet, it's probably a brown dwarf!
"Brian Davis" wrote:
Upshot: in all likelyhood, it really is a 2.5 MJup object. For clarity's sake I'll make this short. There is no good evidence at all that a 2.5 MJup object is necessarily only a "planet" and never a "brown dwarf". |
#6
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planet schmanet, it's probably a brown dwarf!
In article ,
"Christopher M. Jones" writes: ... imagine this scenario. You've got 3 stars in a system, a really massive star (a few solar masses), a kinda low mass star (just under one solar mass, or thereabouts), and a dinky brown dwarf around a dozen MJ. Now, the big star goes off the main sequence really soon, turns into a red giant then supernovas. It's just a hunch, but I'm betting that the blast from a type-II supernova is gonna brush off a teensy weensy bit of the outer atmosphere of a friend the brown dwarf. "Teensy weensy" sounds about right. Why not try calculating the momentum in the supernova ejecta and what fraction intercepts the brown dwarf? How much atmosphere can that strip away? I suspect direct heating might be more of an issue but haven't done the calculation. (It doesn't look simple. While the heating is intense, a large fraction of the heat will probably be reradiated. Now that I think about it, it isn't obvious to me that neutrino heating is negligible.) Then you've got the other star, which spends several billion years on the main sequence, but eventually it too goes into a red giant phase. First it heats up and its outer atmosphere balloons up, and then it blows off that outer atmosphere in the creation of enormously powerful stellar winds (which create a temporary planetary nebula). This is just a hunch too, but I bet both the red giant phase and the strong stellar winds phase might, just might, strip off a wee bit of the brown dwarfs mass. I think you'll lose that bet, but let's see the calculation. Ahh, but wait, there's more! There was almost certainly a scuffle involving the stars when they first got together and the poor little brown dwarf was tossed around quite a bit, there's plenty of opportunity in that, I think, for a fair amount of mass loss. As others have said, there is no obvious mechanism for mass loss via gravitational interactions. You are right, I think, to be skeptical of any proposed formation scenario. We simply have too few examples and know too little about planet formation to be sure of anything in this subject. Still, it's an intriguing observation. -- Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA (Please email your reply if you want to be sure I see it; include a valid Reply-To address to receive an acknowledgement. Commercial email may be sent to your ISP.) |
#7
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planet schmanet, it's probably a brown dwarf!
"Christopher M. Jones" wrote:
For clarity's sake I'll make this short. There is no good evidence at all that a 2.5 MJup object is necessarily only a "planet" and never a "brown dwarf". Even if we accept your definitions (based on a distinction that as far as I know the astronomical community does not weigh in strongly on), there's no reason to think that it isn't, either. In your response, we saw you speculate _quite_ wildly in order to try to get a body with the right mass that once had a greater mass, in order to back up your hypothesis. This sounds more like pure guesswork and handwaving than astrophysics. Maybe it was once a Real Honest to God Brown Dwarf (whatever one might mean by that) but had most of its mass stripped away by aliens who wanted to use it for fusion fuel? We're literally in that same realm of speculation here. -- Erik Max Francis && && http://www.alcyone.com/max/ __ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE / \ It is much safer to obey than to rule. \__/ Thomas a Kempis |
#9
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planet schmanet, it's probably a brown dwarf!
Erik Max Francis writes:
"Christopher M. Jones" wrote: Precisely. That's what I take issue with. That's why I think it's critically important to consider alternatives to the narrow mule-headed view that if it's under 13 MJ it's a "planet". The terminology isn't even _that_ well nailed down; the 13 Jupiter mass limit is a common convention, but as far as I know it has no official endorsement as a classification criterion by the IAU -- which has _no_ objective classification criteria for practically any of the terms that are used to identify substellar objects. It _does_, however, have a semi-objective basis in that the 13 M_Jupiter dividing line is the point at which the object becomes massive enough to start fusing deuterium in its core. So, it may not be a "star" in the usual sense of the word, but it's not exactly a "planet," either... It should be noted that one of the reasons why the IAU has not yet managed to decide on and "official" definition of terms like "planet" is that nearly every definition of "planet" that anyone has been able to come up with is falsified by some obeject within our own solar-system... :-/ -- Gordon D. Pusch perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;' |
#10
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planet schmanet, it's probably a brown dwarf!
"Gordon D. Pusch" wrote:
It _does_, however, have a semi-objective basis in that the 13 M_Jupiter dividing line is the point at which the object becomes massive enough to start fusing deuterium in its core. So, it may not be a "star" in the usual sense of the word, but it's not exactly a "planet," either... The 13 Jupiter mass dividing line is "arbitrary" in the sense that you could have picked some other dividing line, but it definitely is specific and indicative of a characteristic somewhere between planets and stars, as you say. It should be noted that one of the reasons why the IAU has not yet managed to decide on and "official" definition of terms like "planet" is that nearly every definition of "planet" that anyone has been able to come up with is falsified by some obeject within our own solar-system... :-/ Indeed. A planet is what the IAU says is a planet; there is no, and has never been, any objective definition. The same is certainly true of brown dwarfs, which I'm not even sure the IAU has ever used as an official designation (though they may have). Enthusiasts tend to severely overestimate the usefulness of objective criteria for defining asteroids, comets, planets, and so on. What matters is the object and its properties, not which classification name we attach to it. -- Erik Max Francis && && http://www.alcyone.com/max/ __ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE / \ Of war men ask the outcome, not the cause. \__/ Seneca |
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