planet schmanet, it's probably a brown dwarf!

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Christopher M. Jones - 16 Jul 2003 07:36 GMT

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.

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Erik Max Francis - 17 Jul 2003 06:36 GMT

> 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.

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Christopher M. Jones - 18 Jul 2003 06:44 GMT

> > I think this new "planet" detected in a 13 billion year old
> > star system is most likely really a brown dwarf. Ahh, but
[quoted text clipped - 3 lines]
> 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
[quoted text clipped - 8 lines]
> 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
[quoted text clipped - 4 lines]
> 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.

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Steve Willner - 18 Jul 2003 19:20 GMT

> ... imagine this scenario. You've got
> 3 stars in a system, a really massive star (a few solar
[quoted text clipped - 5 lines]
> 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
[quoted text clipped - 6 lines]
> 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.

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Space Forum / Space Flight / July 2003

planet schmanet, it's probably a brown dwarf!