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parallaxicality
2006-Mar-11, 10:14 AM
Before Xena made her bow, planetary astronomers said it was only a matter of time before we found a TNO larger than Pluto, and they were proven right sooner than expected. So I would also like to place my money on the table for the opposite end of the spectrum and claim it's only a matter of time before we find a brown dwarf the size of Jupiter or smaller. We've already found one of only eight Jovian masses and I think all that is limiting our search is the capability of our telescopes.

Here's hoping that when it is the IAU's frustratingly arbitrary distinction between "brown dwarf" and "planet" will be addressed.

Fr. Wayne
2006-Mar-11, 02:03 PM
agreed. If you can land on it, then its a p_______. If it's jumpin' jack flash, it's a gas, gas, gas. Please clear this up o great and wonderful IAU!

baric
2006-Mar-11, 04:57 PM
The distinctions beween stars, brown dwarves and planets are based on different criteria.

Obviously, stars require enough mass to start fusion. Brown dwarves are big enough to be convective throughout, meaning there is internal differentiation. Planets are what's left.

But the really key distinction between this and the planet debate is that no one outside of the astronomical community cares about these distinctions, so the IAU can make their definition without worrying about a huge public outcry.

antoniseb
2006-Mar-11, 06:51 PM
One thing that is missing from the dominant object description is what to call objects that are not orbiting stars. We are accustomed through science fiction to calling sub-stellar objects of this sort planets or rogue planets, but the IAU may have a different term in mind.

Ara Pacis
2006-Mar-11, 08:12 PM
One thing that is missing from the dominant object description is what to call objects that are not orbiting stars. We are accustomed through science fiction to calling sub-stellar objects of this sort planets or rogue planets, but the IAU may have a different term in mind.

Well, if a substellar mass passes a star, chances are it's orbit will be deflected somewhat. This would result in a hyperbolic or parabolic trajectory, which is still considered an orbit, IIRC. I'm not sure if this clarifies things, but it may be useful for definitional purposes.

Romanus
2006-Mar-11, 10:39 PM
If we're talking size rather than mass, than most brown dwarves--except for the very young--are roughly Jupiter-size. In fact, brown dwarves get smaller with increasing mass, right up to "ignition", around 75-80 Jupiter masses.

parallaxicality
2006-Mar-11, 11:21 PM
I meant mass :)

And I'm a bit confused; Earth is internally diffrentiated and convective. Do we know if Jupiter has a silicate/ice core or is hydrogen all the way down?

baric
2006-Mar-11, 11:45 PM
I meant mass :)

And I'm a bit confused; Earth is internally diffrentiated and convective. Do we know if Jupiter has a silicate/ice core or is hydrogen all the way down?

We can see from its surface that Jupiter is differentiated, therefore it is not fully convective

parallaxicality
2006-Mar-12, 12:57 AM
Could you provide me with some links to explain in more detail? I'm trying to write a wikipedia article on the definition of a planet and the difference between a planet and a brown dwarf is the one bit I've had to fudge a bit over. Thanks

baric
2006-Mar-12, 01:36 AM
Could you provide me with some links to explain in more detail? I'm trying to write a wikipedia article on the definition of a planet and the difference between a planet and a brown dwarf is the one bit I've had to fudge a bit over. Thanks

LOL. Here's a link to a wikipedia article on Brown dwarves:

http://en.wikipedia.org/wiki/Brown_dwarf

Ara Pacis
2006-Mar-12, 01:54 AM
Could you provide me with some links to explain in more detail? I'm trying to write a wikipedia article on the definition of a planet and the difference between a planet and a brown dwarf is the one bit I've had to fudge a bit over. Thanks

http://en.wikipedia.org/wiki/Definition_of_planet
http://en.wikipedia.org/wiki/Planet

Are you editing one of these?

parallaxicality
2006-Mar-12, 02:03 AM
The first one

baric
2006-Mar-12, 03:11 AM
http://en.wikipedia.org/wiki/Definition_of_planet
http://en.wikipedia.org/wiki/Planet

Are you editing one of these?

Hey, thanks for the "Definition" link. :) On it, you linked to a Nasa page that has a useful quote from Mike Brown about planetary status requiring a majority of mass in an orbital region. It seems low to me, but Brown's opinion carries far more weight than mine, so I'm including it in my post about orbital dominance.

Ara Pacis
2006-Mar-12, 04:16 AM
Hey, thanks for the "Definition" link. :) On it, you linked to a Nasa page that has a useful quote from Mike Brown about planetary status requiring a majority of mass in an orbital region. It seems low to me, but Brown's opinion carries far more weight than mine, so I'm including it in my post about orbital dominance.

Hehehe, I know. Just remember, that a majority is 50% +1, a level of dominance that Ceres appears to have attained (57%) via your new calculations in your definition thread. Gotcha!

baric
2006-Mar-12, 06:00 AM
Hehehe, I know. Just remember, that a majority is 50% +1, a level of dominance that Ceres appears to have attained (57%) via your new calculations in your definition thread. Gotcha!

:)

Well, that 57% was based on a hypothetical involving only the 9 largest asteroids (out of thousands). We can remove some more and get it up to 100% if you think that would be convincing ;)

parallaxicality
2006-Mar-12, 08:59 AM
Well, I should point out that ol' Mikey Brown, since he discovered the "tenth planet" has shifted his position rather radically into the "planet is a cultural definition. so make Pluto the cutoff" camp. I can understand why. Given the choice between being remembered as "they guy who discovered the tenth planet" and "the guy who turned Pluto into an asteroid", who would you want to be?

One other thing (I was thinking about this in bed as I woke up; best time for thoughts like this) surely the only thing keeping Jupiter from being fully convective is the amount of energy at its core. I assume that early in its history, when its internal heat was still great,it was convective. Given its small size, it's no surprise that its convection has slowly unwound from the serface to the centre. Brown dwarfs are likely to be as old as Jupiter or older. Given that, would not a brown dwarf the size of Jupiter also not have a convective surface?

Eroica
2006-Mar-12, 11:21 AM
I thought that deuterium-fusion was one of the defining characteristics of brown dwarfs? According to that wikipedia article (http://en.wikipedia.org/wiki/Brown_dwarf) this is not necessarily the case, though there is some debate about it:


Brown dwarfs are sub-stellar objects (~5 to 90 Jupiter masses) that do not fuse hydrogen-1 into helium and heavier elements in their cores, as do stars on the main sequence, but have fully convective surfaces and interiors, with no chemical differentiation by depth. There is some question as to whether brown dwarfs are required to have experienced fusion at some point in their history;

Doodler
2006-Mar-12, 01:26 PM
Before Xena made her bow, planetary astronomers said it was only a matter of time before we found a TNO larger than Pluto, and they were proven right sooner than expected. So I would also like to place my money on the table for the opposite end of the spectrum and claim it's only a matter of time before we find a brown dwarf the size of Jupiter or smaller. We've already found one of only eight Jovian masses and I think all that is limiting our search is the capability of our telescopes.

Here's hoping that when it is the IAU's frustratingly arbitrary distinction between "brown dwarf" and "planet" will be addressed.

Been there, done that, got the T-shirt. The physical size of at least one or two brownies has gotten down to about Jupiter's size, but they're MUCH denser. For brownie to be a brownie and be Jupiter's mass would make it much smaller than Jupiter.

parallaxicality
2006-Mar-12, 01:44 PM
If that's the case, and it's so easy to tell the difference between a brown dwarf and a planet, then why this equivocal definition from the IAU?


Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.

Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

If density were an issue, surely it would have made a useful differential tool for astronomers?

Ara Pacis
2006-Mar-12, 07:01 PM
If that's the case, and it's so easy to tell the difference between a brown dwarf and a planet, then why this equivocal definition from the IAU?

If density were an issue, surely it would have made a useful differential tool for astronomers?

So, Jupiter would be an almost-star if it weren't orbiting an already-star? What is their problem with the adjective "rogue"?

Kullat Nunu
2006-Mar-12, 08:51 PM
Jupiter has only about 1/80th of the mass needed for a star.

grant hutchison
2006-Mar-12, 10:23 PM
For brownie to be a brownie and be Jupiter's mass would make it much smaller than Jupiter.I'm not sure how that would work: it's the extra mass that makes these massive objects dense. A jupiter-mass object composed largely of hydrogen is going to be jupiter-sized, if it's cool enough. (Adam Burrows' review paper The theory of brown dwarfs and extrasolar giant planets (http://zenith.as.arizona.edu/~burrows/papers/rmp/RMP-final.pdf) gives a lot of useful information. Caution: it's a 1.6MB pdf.)
But I'm not sure why we're hearing reports of brown dwarfs with masses less than 13 jupiters: the IAU's deuterium-fusion cut-off is obviously not being followed, or the theory has changed to predict deuterium fusion in lower-mass objects. But it seems like some astronomers are now going down the route of labelling objects according to their mechanism of formation: whether they condense directly from a precursor cloud (stars, brown dwarfs) or form in the disc surrounding a previously condensed object (planets).

Grant Hutchison

parallaxicality
2006-Mar-13, 01:27 AM
Thanks. Added that article to my secondary sources section

Doodler
2006-Mar-14, 01:59 PM
I'm not sure how that would work: it's the extra mass that makes these massive objects dense.

Allow me to clarify by saying that I don't believe its possible. Larger amounts of mass in a more condensed configuration leading to a 13 Jupiter mass object being of a physically similar size to Jupiter, I can buy, heck, its been documented. I have not heard of any mechanism which would facilitate a Jupiter mass worth of hydrogen, or whatever else, to achieve sufficient density to trigger deuterium burning. Not saying there isn't a mechanism to achieve this, its just something that hasn't been documented.

In any event, all I was saying was that were such a mechanism to exist, a Jupiter mass brown dwarf would be substantially smaller than Jupiter itself.

grant hutchison
2006-Mar-14, 02:20 PM
Allow me to clarify by saying that I don't believe its possible.Ah, OK: sorry I misunderstood your point.
The fact that deuterium-burning seems to have been sidelined (at least by some astronomers) as a criterion for "brown-dwarfhood" was leading me off in a different direction.

Grant Hutchison