John Dobson's View on Defining Gas Giants as Stars or Planets.

[Suzy]

I'm interested to know what your views are on John Dobson's interesting theory of what defines a star/planet.

He insists Jupiter and Saturn are actually stars and explains why.
Take a look at this video, forward 10.36 mins into the interview for the segment I'm talking about.
http://www.youtube.com/watch?v=NnIf2...eature=related


What he's said, combined with the latest findings of "The star that shouldn't exist" (ESO press release), I'm wondering if Dobson may be on the right track?

...and to take things further, could this new star then be another Jupiter? As we seem to be finding lone planets (without stars).
I'm confused.

Aye.. the definition of stars and planets is getting more and more complicated each day,
especially as I'm trying to learn more about stars.

[bojan]

Suzy,
The star that "shouldn't exist" is a proper, giant star, nothing like Jupiter (by size, surface temperature and power output). The problem/issue with it is a lack of "metals" from it's chemical composition - it should have have more of them, but it doesn't.

Jupiter may be called a "failed" star, because it didn't have a chance to acquire enough mass to start thermonuclear fusion in it's core.
Apart from that, we can define planets in very different terms.. but this will not change the current definition imposed by IAU

[renormalised (Carl)]

For a gas body to be a star, it has to reach a mass so that the density of the core regions becomes high enough for the nuclei of the atoms to overcome their mutual repulsion and begin to fuse. For hydrogen, that starts at around 75-80 Jupiter masses, where the internal temperature has reached around 7-8 million K. That is when the object can be called a star, in the proper sense. Brown dwarfs, which are below that mass limit, cannot fuse hydrogen in any sustained fashion and so are not stars. They have been called failed stars, which they are in a sense. But in strictest terms, they are not stars but nor are they planets. Some of the heavier brown dwarfs can fuse lithium and deuterium for a short while...deuterium burning starts once a body passes 13 Jupiter masses (at around 1-2 million K for the core temp) and lithium burning occurs in bodies over 45 Jupiter masses (core temps around 4-5 million K). These episodes don't last very long as there's not that much of either inside stars to begin with, plus they fuse rather quickly.

Jupiter and Saturn are nowhere near heavy enough to be called brown dwarfs, let alone stars. They never gathered enough mass to become either of them.

[Poita (Peter)]

I've always thought that the gas giants should be classified as cold stars/pre-stars/failed stars, just gas balls or something else altogether, but not planets. I'll take a listen to that link, thanks.

[renormalised (Carl)]

The planet/brown dwarf cutoff point is at around 13 Jupiter masses, where deuterium burning in the core can commence (as I mentioned previously). Brown dwarfs and giant planets can form in very similar ways, either via bottom up (core accretion) or top down (disk instability). Brown dwarfs appear to favour the top down approach more so than the bottom up way and giant planets appear to follow the opposite, although it's less clear than with brown dwarfs. So long as the gas body doesn't begin to fuse deuterium, even if it's close to the mass limit and forms via disk instability, it's a planet. If it begins to fuse all its deuterium, it's a brown dwarf. Even if it formed outside of a solar system, it's still a planet, so long as it obeys the deuterium fusing and mass criteria. It's just a solitary planet, in other words. Or, it could've originally formed in a solar system and got flung out via gravitational interactions early on in its life. That can happen to both brown dwarfs and planets.

[renormalised (Carl)]

Originally Posted by Poita

I've always thought that the gas giants should be classified as cold stars/pre-stars/failed stars, just gas balls or something else altogether, but not planets. I'll take a listen to that link, thanks.

Nope, that's a popular misconception that has unfortunately been promulgated by many scientists. It was only done to explain the similarities and differences between gas giant planets and stars. The differences are more profound than you've been led to believe.

[Allan_L (Allan)]

Originally Posted by Suzy

I'm interested to know what your views are on John Dobson's interesting theory of what defines a star/planet.

He insists Jupiter and Saturn are actually stars and explains why.
Take a look at this video, forward 10.36 mins into the interview for the segment I'm talking about.
http://www.youtube.com/watch?v=NnIf2...eature=related


What he's said, combined with the latest findings of "The star that shouldn't exist" (ESO press release), I'm wondering if Dobson may be on the right track?

...and to take things further, could this new star then be another Jupiter? As we seem to be finding lone planets (without stars).
I'm confused.

Aye.. the definition of stars and planets is getting more and more complicated each day,
especially as I'm trying to learn more about stars.

Thanks for posting the link Suzy.
I found the interview very interesting, and "down to earth". so to speak.
A lot of interesting stories in there, and an impressive speaker to listen to.
Not to mention controversial on a number of issues.

[CraigS]

What's he on about at 11:40(ish), when he says that spin down rates are caused by electrical fields interacting with charged particles surrounding the Sun, Jupiter and Saturn? He makes the claim that Jupiter and Saturn have no electrically charged particles surrounding them ??? Where did he get this from?

Cheers

[Poita (Peter)]

Originally Posted by renormalised

Nope, that's a popular misconception that has unfortunately been promulgated by many scientists. It was only done to explain the similarities and differences between gas giant planets and stars. The differences are more profound than you've been led to believe.

I know the differences, and that a Gas giant is in no way a 'star', I just think that the differences are just as profound between a nice hard crusty planet like earth, and a gas giant like Jupiter, having them both named planets is a pretty wide definition.

[renormalised (Carl)]

Originally Posted by Poita

I know the differences, and that a Gas giant is in no way a 'star', I just think that the differences are just as profound between a nice hard crusty planet like earth, and a gas giant like Jupiter, having them both named planets is a pretty wide definition.

Not really....add a bit more mass to the Earth and you'd get a giant planet. 10 Earth masses and the body starts to accrete hydrogen and helium in large enough quantities, plus its gravity is such it can hold onto the gases. They both formed from very similar processes....Jupiter just formed a lot faster.

Internally, their structures are basically similar, with several consecutive layers of material, only there's no solid surface to a gas giant. Where a gas giant has a solid core, it's pretty much like a large super earth in its structure.

[CraigS]

Originally Posted by Poita

I know the differences, and that a Gas giant is in no way a 'star', I just think that the differences are just as profound between a nice hard crusty planet like earth, and a gas giant like Jupiter, having them both named planets is a pretty wide definition.

The thing at the core of this issue/discussion is the diversity within the categories of 'Star' and 'Planet'.

We are only just beginning to scratch the surface and the more we discover outside of our own Solar System, the more the definitions will be stretched.

The categories have worked well up until now and I suspect it is our naivety about what exists beyond our measurement technologies that has gotten us this far.

Cheers

[renormalised (Carl)]

Originally Posted by CraigS

What's he on about at 11:40(ish), when he says that spin down rates are caused by electrical fields interacting with charged particles surrounding the Sun, Jupiter and Saturn? He makes the claim that Jupiter and Saturn have no electrically charged particles surrounding them ??? Where did he get this from?

Cheers

The poor boy is getting his science awfully confused The transfer of angular momentum from the Sun to Jupiter was a lot more involved than just magnetohydrodynamic breaking, although that played a major part in the loss of the Sun's angular momentum, very early on. It's also a lot more involved than the Sun stripping electrons off its gases and expelling the ionised gas as solar wind, though that is also a part of the process. It has very much to do with the way these two planets formed and interacted with one another, and the Sun's magnetic field and other interactions as well. Just because Jupiter has most of the Solar System's angular momentum doesn't, by definition, make it a star or anything else other than a planet.

This is precisely a case of where people with an incomplete and inaccurate knowledge of what they're talking about can get things quite wrong and end up misinforming and confusing other people, who don't know.

[CraigS]

Originally Posted by renormalised

Just because Jupiter has most of the Solar System's angular momentum doesn't, by definition, make it a star or anything else other than a planet.

Yep .. seems like he's trying to use Angular Momentum (AM), as the primary basis for distinguishing a star from a planet (?) ..
That's weird …

Sort of sounds like he's saying that a system, like the 'Solar System', distinguishes the intrinsic nature of its components .. which means the system came first, and then caused the formation of the components ..
… Just downright weird !

Originally Posted by renormalised

This is precisely a case of where people with an incomplete and inaccurate knowledge of what they're talking about can get things quite wrong and end up misinforming and confusing other people, who don't know.

I notice he's an advocate of Steady State Cosmology, refutes the Big Bang and maintains that entropy remains constant, life is without a beginning, is anti-Evolution, etc ..
Obviously missing a few of the fundamentals, eh ?

I don't think I'd call him exactly 'controversial' ..


Hey Suzy;

I'd recommend giving his ideas on Cosmology and Astrophysics a very wide berth !

Cheers

[Suzy]

Originally Posted by renormalised

This is precisely a case of where people with an incomplete and inaccurate knowledge of what they're talking about can get things quite wrong and end up misinforming and confusing other people, who don't know.

That's people like me. Couldn't agree more, Carl.

As I said before... hard enough trying to learn as it is without being confused with theories such as this.
But as Craig said, I think I'd better give this theory of his a wide berth.

I am just so glad, and so incredibly grateful to you guys that when something confuses me, I'm able to bring it up with you'll for discussion and help me in a very respectful manner.
I thank you all for taking the time out.

I have a huge respect for Mr John Dobson regarding the hard work he did with the famous "Sidewalk Astronomers" in showing the night sky to the masses, including the famous dobsonian telescope which again, brought telescope affordability to the masses. I myself, am very thankful to him as I own a 10" dobsonian.

The link that I supplied (you may have noticed) is part of a 5 part series which runs for an hour. I highly recommend it being watched- it focuses mainly on his life, how he brought astronomy to the masses, and of course telescope making. A very interesting and enjoyable interview. He certainly has a bit of attitude tho doesn't he, but I still find him quite likable. The interview is conducted by Mike Simmonds from "Astronomers Without Borders".

P.S.
Oh I nearly forgot, thank you also for making it clearer to me regarding the star just found.

[astroron (Ron)]

Good thread Suzy still have to watch the video, after midnight
As Craig said
I'd recommend giving his ideas on Cosmology and Astrophysics a very wide berth !
After hearing the man in person and meeting him I concure with the above advice
Cheers

[ngcles]

Hi Suzy & All,

Originally Posted by astroron

Good thread Suzy still have to watch the video, after midnight
As Craig said
I'd recommend giving his ideas on Cosmology and Astrophysics a very wide berth !
After hearing the man in person and meeting him I concure with the above advice
Cheers

I have also met John personally. While you can only admire the extraordinary way he has evangelized astronomy over the years and has given so much to the hobby and brought so many people into it, he has some seriously unorthodox theories on things in astrophysics and cosmology that he will insist on inflicting on others as part of his talks.

Despite the exceptional work he has done in promoting observing and telescope making to the masses, I find it hard to like him much personally, though most people find him quite engaging. I asked a simple, straightforward question at the conclusion of a talk he gave: "When you use a telescope, what sort of things do you most like to look at?" Perfectly sensible valid question I thought. His response (word for word) "The Sky. Next question". Undeterred about 5 minutes later I re-cast the question and put it in a different form: "What are your favorite objects to look at with a telescope or binoculars?" His response: "I told you, the sky. Next ..."

That really turned me off. He answered a couple of other perfectly legitimate questions from others in a very similar manner, while others were answered fully in the way one would expect. All I wanted to know was what he personally most enjoyed in the eyepiece, something I'm sure most people would be interested in. Really ridiculous answer and enough to put me off for life (along with his odd theories on quite a few things that detracted from his credibility on other subjects.

But that's only a personal view.


Best,

Les D

[renormalised (Carl)]

The problem with that star, Suzy, is that it has such a low metals content that it's hard to see how such a star could've formed in the first place. Without going into the particulars of the mechanisms by which a gas cloud collapses to form a star, the dearth of metals within the cloud that formed this star would've normally made it very difficult for the cloud to collapse to form the star/stars in the first place....especially low mass stars like this one. With clouds that have such a low metals content, the clouds have a hard time trying to collapse because of the internal pressures generated by heating within the cloud. The presence of metals actually allows the cloud to radiate away that heat more efficiently than it would otherwise. The only way for metal deficient clouds to collapse is to build up mass, to a point that the instabilities within such clouds allows the gravity to collapse the clouds and form stars...which are usually much larger than they would normally otherwise be. That's why the early PopIII stars were so large because they formed from clouds of almost pure hydrogen and helium. Instead of many smaller stars forming from the clouds, the clouds collapsed and formed one or two very large stars...some of which may have weighed as much as 1000-2000 solar masses, possibly even more. Not only that, but nearly all of those stars were much hotter than today's stars. Because a cloud of almost pure hydrogen and helium is relatively transparent to radiation, in order for a star to be of a certain spectral class, it has to be hotter than it would be if it was like stars of today. Some of these giants in the beginning radiated at temps of around 250000-500000K and the largest of them were more than 1 billion times brighter than the Sun. They would've lasted only a few hundred thousand to a millions years or two before they all went hypernova and/or collapsed into black holes. The star they were talking about in the article isn't a PopIII star, despite its low metals content and age. It's more like what they call an extreme PopII star, much like the halo stars surrounding our galaxy, only with even less metals than what most of them have. It's more than likely one of the first stars that formed in the generation immediately proceeding the PopIII era, or very close to that time. There's bound to be more of these types of stars populating the halo and spread amongst the stars of the spiral. It's just a matter of time before than find them.

[CraigS]

At the moment, PopIII stars are still hypothetical.

From the paper about the "impossible" star (SDSS J102915+172927):

Stars similar to SDSS J102915+172927 are probably not so rare. Only 30% of the whole SDSS survey area was accessible to our VLT observations. We identified 2899 potentially extreme stars with metallicity less than Z ≤1.1×10*^5 in Data Release 725. Among those observable with the VLT we performed a subjective selection of the most promising candidates of which we observed six in our X*Shooter programme resulting in one detection. Depending on the subjective bias we attribute to the last selection step, we expect 5 to 50 stars of similar or even lower metallicity than SDSS J102915+172927 to be found among the candidates accessible from the VLT, and many more in the whole SDSS sample.

So, based on the current data, it looks like these PopII stars might be quite common, but the lower the metallicity, the fewer they are in number.

Lithium depletion seems to be a characteristic shared with blue stragglers and suggests that the stellar material has experienced temperatures about 2 million Kelvin. The origins may be the same, or they may share the same unusual formation conditions, in terms of the mixture of elements from the start.

Cheers

[renormalised (Carl)]

Lithium is one of the first things to go once a star ignites because it's "burnt" off very quickly. Most star that appear to be lithium enriched have swallowed giant planets to gain the lithium.

X*Shooter...sound like the latest FPS out on the market

[CraigS]

Hmm .. I was just reading this one ... where they've just decided that, as the abundance of precious metals (gold, platinum, etc) in the mantle of the Earth is tens to thousands of times more than 'anticipated', this now calls for invocation of the good-ol 'meteorite delivery method'. Actually, the study brings fresh evidence for this, also ... tungsten isotopic measurements …

.. Well .. we're still on topic with this one … the metallcity of planets also seems to be just as tricky a basis for categorisation, as the metallicity of stars!

… Which only goes to emphasise that one has to look at more than just one physical characteristic, to make a reasonable classification ..

Cheers