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?
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.
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.
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).
Yes, with one caveat. The additional mass must composed of elements lighter than iron. Otherwise there’ll be no fusion and no star.
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.
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?
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.
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.
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.
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.
Stranger
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.
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.
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.
Do you have some independent cite that there are two distinct classes of planets, rocky and gaseous, when all planets are taken into consideration?
You left one out: “spaceballs” (like a Mel Brooks movie made several years too late). 
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.
Subatomic particles, perhaps?
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.
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.
