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Old 09-18-2016, 02:09 PM
Yersenia Pestis Yersenia Pestis is offline
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Are planets and stars things on the same "spectrum"?

Are stars and planets essentially the same things in the same spectrum? Meaning planets, and it probably sounds crazy, given sufficient additional mass could potentially "convert" into a star like object?

Are there any examples of "almost stars" but still sort of a planet that we know of?
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Old 09-18-2016, 02:21 PM
watchwolf49 watchwolf49 is offline
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Jupiter does not have enough mass, and therefore not enough pressure at it's center, to start nuclear fusion. As far as I know, that's the only reason she doesn't burn like a second sun in our solar system.
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Old 09-18-2016, 02:31 PM
Colibri Colibri is online now
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There is essentially a continuum between the largest planets and smallest stars, with stars being defined as being massive enough to support nuclear fusion of hydrogen. The largest "almost-stars" are known as brown dwarfs. Below them are sub-brown dwarfs.
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Old 09-18-2016, 02:36 PM
Chronos Chronos is online now
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Note a key word there: Between the largest planets and stars. Jupiter is basically the same sort of thing as the Sun is, just with less mass. But Earth is not the same sort of thing, nor is Pluto the same sort of thing as either. I've long said that we should abandon using the single term "planet" for all of these objects, and instead refer to "rockballs" (like Mercury, Venus, Earth, Luna, Mars, and Ceres), "gasballs" (like Jupiter, Saturn, Uranus, and Neptune), and "iceballs" (like Pluto, Eris, Quaoar, and Sedna).
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Old 09-18-2016, 02:44 PM
HMS Irruncible HMS Irruncible is offline
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Originally Posted by Yersenia Pestis View Post
Are stars and planets essentially the same things in the same spectrum? Meaning planets, and it probably sounds crazy, given sufficient additional mass could potentially "convert" into a star like object?
Yes, with one caveat. The additional mass must composed of elements lighter than iron. Otherwise there'll be no fusion and no star.
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Old 09-18-2016, 02:48 PM
levdrakon levdrakon is offline
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Right. Gas giants and brown dwarfs are mostly hydrogen and helium. Earth is mostly iron, oxygen, silicon and magnesium. If you kept adding more earth, to Earth, you wouldn't eventually get a star. Not sure what would happen, but it wouldn't be a star I don't think.
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Old 09-18-2016, 02:54 PM
Colibri Colibri is online now
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Quote:
Originally Posted by Chronos View Post
Note a key word there: Between the largest planets and stars. Jupiter is basically the same sort of thing as the Sun is, just with less mass. But Earth is not the same sort of thing, nor is Pluto the same sort of thing as either. I've long said that we should abandon using the single term "planet" for all of these objects, and instead refer to "rockballs" (like Mercury, Venus, Earth, Luna, Mars, and Ceres), "gasballs" (like Jupiter, Saturn, Uranus, and Neptune), and "iceballs" (like Pluto, Eris, Quaoar, and Sedna).
But that's only true for our own system. Exoplanets show a continuum in the distribution of their masses, with many being intermediate between Earth and Uranus. One might expect that these planets would show a continuum in physical characteristics as well. Is there any reason to suppose a priori that all planets in the Universe would break down into a few completely distinct classes?
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Old 09-18-2016, 03:02 PM
Chronos Chronos is online now
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Well, nothing in nature ever falls completely into a few distinct classes. But most of the planets we've found so far are either basically rocky or basically gassy. We know that Earth isn't the largest possible rocky planet and Uranus isn't the smallest possible gassy planet, but there is still some maximum size for a rocky planet, and some minimum size for a gassy one.
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Old 09-18-2016, 03:05 PM
Francis Vaughan Francis Vaughan is offline
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One might argue about the early history of a planet. That might get you two categories. Those that were formed as gas-balls, and those that never had any atmosphere worth worrying about, and were, and are, just accretions of dust. Some gas-balls it seems get turned into rock-balls. A very active sun can literally blow the deep atmosphere off, and leave just the rocky bit inside. Later, a thin atmosphere might form from a mix of outgassing of the rocks and accretion of stuff from collisions with comets or other bodies.

The other question night be to look at where in the generation history of the stars the sun of the system is. The further you go back, the less of the heavier elements there will be in the formation of the system. Very early stars likely had planets too, but the lack of heavy elements would mean that they were all pure gas-balls, right to the core.
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Old 09-18-2016, 03:52 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by Chronos View Post
Note a key word there: Between the largest planets and stars. Jupiter is basically the same sort of thing as the Sun is, just with less mass. But Earth is not the same sort of thing, nor is Pluto the same sort of thing as either. I've long said that we should abandon using the single term "planet" for all of these objects, and instead refer to "rockballs" (like Mercury, Venus, Earth, Luna, Mars, and Ceres), "gasballs" (like Jupiter, Saturn, Uranus, and Neptune), and "iceballs" (like Pluto, Eris, Quaoar, and Sedna).
Even those distinctions may be top general; despite the fact that they are all considered to be "gas giants" or "gasballs" in your numenclature, Uranus and Neptune are very different in construction than Jupiter and Saturn. The latter are dense enough that their cores are likely composed of hydrogen compressed to a metallic state around a small nugget of solid mass with a mantle of superheated liquid hydrogen, while Uranus and Neptune likely have a rock/ice core surrounded by a mantle of liquid water, ammonia, and methane. While Venus, Earth, and Mars are all superficially similar (Venus and Earth both have substantial tectonic activity but have much different atmospheres and rotate at vastly different rates, while Earth and Mars both have a relatively thin atmospheres with somewhat comparable weather events, rotate at almost the same rate, and display rocky surface features showing signs of liquid water flows) each planet is quite distinct in numerous ways, and depending on what characteristics you want to use for classification may be more or less similar to each other in comparison. Mercury, despite notionaly being a planet, is really little more than a glorified moon, and we've now recognized that Pluto is a "minor planet", a part of a coupled system with Charon, and part of a class of Kuiper belt objects (KBO) that have a very different development than the nominal major planets. The term "orbium copernicoid" (or just "copernicoid") has been discussed to describe naturally occuring bodies orbiting the Sun in a not-too eccentric orbit with enough mass to draw them into a sphereoid shape, although it has not been adopted by any body of astronomers as a recognized class.

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Originally Posted by Francis Vaughan View Post
The other question night be to look at where in the generation history of the stars the sun of the system is. The further you go back, the less of the heavier elements there will be in the formation of the system. Very early stars likely had planets too, but the lack of heavy elements would mean that they were all pure gas-balls, right to the core.
The earliest (Population III) stars likely did not have planets; they were so massive that they had very short lifetimes and had essentially no heavy elements around which for planets to form a core. Population II stars sill had very little metallicity and so it is speculated that most planets around such stars would be sparse and primarily large gas giants. Population I stars such as our own and everything close enough to see signs of planetary systems have formed a wide variety of planetary types that were previously unsuspected, from supermassive rocky worlds to "hot" supergiants orbiting very close to their parent stars.

Although there is a continuum of gravitationally-bound bodies, from small oblong moons and comets through supergiants and brown dwarfs, stars represent a distinct class insofar as they have enough mass to achieve the conditions necessary for nuclear fusion of hydrogen or heavier elements, and radiate away massive amounts of energy and charged particles which are many orders of magnitude greater than radiation from gravitational contraction or radioactive decay that drives internal planetary circulation and tectonics. Of course, such stars will eventually burn out or explode as they fall off the main sequence, and then become distinct objects (stellar remnants such as white dwarfs or hypothetical neuron stars or black holes), which is also phenomena that planetary objects will never display.

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Old 09-18-2016, 04:12 PM
Colibri Colibri is online now
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Quote:
Originally Posted by Chronos View Post
Well, nothing in nature ever falls completely into a few distinct classes. But most of the planets we've found so far are either basically rocky or basically gassy. We know that Earth isn't the largest possible rocky planet and Uranus isn't the smallest possible gassy planet, but there is still some maximum size for a rocky planet, and some minimum size for a gassy one.
But there would seem to be plenty of overlap in the possible sizes of rocky and gassy planets. It is apparently possible for a planet no more massive than the Earth to be a gas dwarf. It seems to me that the planetary classification scheme you propose is rather parochial and based more on the limited range of types in our own system rather than attempting to account for the full range of possible types.
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Old 09-18-2016, 04:40 PM
watchwolf49 watchwolf49 is offline
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How are we to say the proto-Earth wasn't gaseous? If we assume a fairly uniform mixture of Hydrogen, Helium and dust from whence our solar system condensed from, then the same materials that made up the proto-Jupiter also made up the proto-Earth.

While the gravitation collapse is going on, these proto-planets would begin to differentiate with the heavier denser materials sinking toward the center, like Iron, Cobalt, Nickel; and the lighter less dense materials floating up to the top, like Hydrogen, Helium. These proto-planets would have begun the process of sweeping out their orbits of dust, junk and small rocks. However the proto-solar system would still be pretty much a cloud of gases and dust.

Upon the ignition of the fusion reactions at the center, the radiative pressure outbound would blow off the Hydrogen/Helium envelope of the inner planets as well as any loose gas or dust. As these inner planets continued to out-gas their original Hydrogen/Helium the continuous radiative pressure would strip this all away leaving behind all the various oxides and heavy material that we see today.

Unfortunately, counter-examples to this abound ... many of the exoplanets we're finding are gaseous giants well within the orbit of Earth ... however, I don't think we know enough to say these very close gas giants have always been this close to their respective stars.

If we're to accept Chronos' three distinct classes of planets, I think we'll need three distinct methods of formation other than just simple distance from the central star.
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Old 09-18-2016, 05:43 PM
Chronos Chronos is online now
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Quote:
Quoth Colibri:

But there would seem to be plenty of overlap in the possible sizes of rocky and gassy planets. It is apparently possible for a planet no more massive than the Earth to be a gas dwarf.
Which is evidence that Earthlike planets are not on the same continuum as Jupiter and the Sun, like I said. My classification scheme has no problem with the notion that there can be some small gasballs.
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Old 09-18-2016, 05:56 PM
Colibri Colibri is online now
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Which is evidence that Earthlike planets are not on the same continuum as Jupiter and the Sun, like I said.
How so? It's proposed that some rocky planets can originate from the cores of gaseous planets from which most of the gas has been driven off.

Do you have some independent cite that there are two distinct classes of planets, rocky and gaseous, when all planets are taken into consideration?

Last edited by Colibri; 09-18-2016 at 06:01 PM.
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Old 09-18-2016, 07:14 PM
Dendarii Dame Dendarii Dame is offline
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Originally Posted by Chronos View Post
I've long said that we should abandon using the single term "planet" for all of these objects, and instead refer to "rockballs" (like Mercury, Venus, Earth, Luna, Mars, and Ceres), "gasballs" (like Jupiter, Saturn, Uranus, and Neptune), and "iceballs" (like Pluto, Eris, Quaoar, and Sedna).
You left one out: "spaceballs" (like a Mel Brooks movie made several years too late).
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Old 09-18-2016, 08:40 PM
Lumpy Lumpy is offline
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Right. Gas giants and brown dwarfs are mostly hydrogen and helium. Earth is mostly iron, oxygen, silicon and magnesium. If you kept adding more earth, to Earth, you wouldn't eventually get a star. Not sure what would happen, but it wouldn't be a star I don't think.
Probably would resemble a white dwarf star, although I don't know if a stellar core remnant can contain silicon or iron without collapsing into a neutron star.
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Old 09-18-2016, 09:34 PM
Xema Xema is online now
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nothing in nature ever falls completely into a few distinct classes.
Subatomic particles, perhaps?
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Old 09-18-2016, 09:38 PM
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Wouldn't it depend on the process that more earth had been added?

A white dwarf is only white because of left over heat from when it had been a red giant, yes? If somehow you just accreted more earth to Earth and kept going and going you wouldn't have that, you'd directly go through a phase of looking like the hypothesized black dwarf, and then with enough extra earth added become a black hole.

Maybe some very faint white as the relatively few lighter elements of earth ignite and burn off producing more heavy ones.

I think.
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Old 09-18-2016, 10:57 PM
levdrakon levdrakon is offline
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Well, eventually we're going to end up with a very hot iron core. Does it get hot enough to initiate fusion of the lighter elements surrounding the core?

Does the iron core itself attempt fusion?

Either way it sounds explody.
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Old 09-18-2016, 11:49 PM
watchwolf49 watchwolf49 is offline
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Well, eventually we're going to end up with a very hot iron core. Does it get hot enough to initiate fusion of the lighter elements surrounding the core?

Does the iron core itself attempt fusion?

Either way it sounds explody.
Well, if the Earth absorbed all the mass of Jupiter, it still wouldn't be enough to start fusion, maybe all four outer planets ... but that would wreck land prices in Missouri.
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Old 09-19-2016, 12:22 AM
Francis Vaughan Francis Vaughan is offline
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As noted above, an iron core fusing isn't just a matter of pressure. You are trying to slam together nuclei that will only deign to do so if you add the energy deficit inherent in the mass change of the resultant nucleus. This is, as a good approximation, not dissimilar to all of the energy released when you took the lighter elements and fused them to get the iron in the first place. Or, is exactly the same energy released when you fission the new heavy elements back to iron. We are talking energies similar to the sum of the entire energy released by the star over many many millions of years, all concentrated into a tiny moment in time as the core of the supernova collapses in a freefall. The energy is utterly insane. Indeed "insane" doesn't seem to be even on the same planet as the word needed to describe the energy.

Last edited by Francis Vaughan; 09-19-2016 at 12:24 AM.
  #22  
Old 09-19-2016, 08:18 AM
Chronos Chronos is online now
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Quote:
Quoth Lumpy:

Probably would resemble a white dwarf star, although I don't know if a stellar core remnant can contain silicon or iron without collapsing into a neutron star.
White dwarfs are, in fact, largely iron. And neutron stars, of course, are mostly not made up of any chemical element at all, and what elements they do contain (in their outer layers) are mostly isotopes which would be impossible under less extreme conditions.

Quote:
Quoth watchwolf49:

Well, if the Earth absorbed all the mass of Jupiter, it still wouldn't be enough to start fusion, maybe all four outer planets ... but that would wreck land prices in Missouri.
To fuse even deuterium (which is significantly easier than ordinary hydrogen) requires 13 times the mass of Jupiter (this is usually used, nowadays, as the cutoff for the definition of a "brown dwarf"). Since there are fewer than 13 outer planets, and since Jupiter is the largest of them, it follows trivially that all of the outer planets combined would not be enough even for the simplest form of fusion.
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Old 09-19-2016, 09:42 AM
levdrakon levdrakon is offline
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According to wikipedia, white dwarfs are usually oxygen and carbon, Depending on the initial mass of the star you might get neon and magnesium or a low mass star that can't fuse helium could become a helium white dwarf.

As far as I know, our sun is never going to make iron.

https://en.wikipedia.org/wiki/White_dwarf
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