Is there an upper limit physical boundary on how big a solid matter planet can be?
Well at some point it would certainly collapse into a black hole. I’m not sure if it would collapse into a neutron star before that point.
Well, yeah, but long before that it would be considered a star. The problem here is that there is no clearcut dividing line between the various astronomical bodies. Below about 1/20th the mass of our Sun, there isn’t sufficient gravitational force to trigger fusion, so it’s reasonable to call this the maximum mass for a gas giant. Large bodies that coalesce from a protostellar planetary ring as our solar system once did are likely to have enough gravity to hold onto a thick hydrogen atmosophere the way the four gas giants Jupiter, Saturn, Uranus and Neptune did, so at some point in the size spectrum, solid planets giove way to gas giants. This is further compounded by the fact that planets closer to the sun are naturally hoter, and so volatiles, like hydrogen, become harder to hold onto. The result is that larger solid planets can exist closer to the sun. There will be a range of values which depend on proximity to the primary star, the energy output of the star and the mass of the planetary body.
Clear as mud?
That was what I was taught, and it certainly seems to make sense to me, but until a few years ago, that was entirely conjecture based on our solar system – a sample size of one. In the past several years, we have finally reached the point of being able to survey nearby stars, and the stars in our neighborhood DO NOT seem to bear out our earlier assumptions.
A glance at this catalog of extrasolar planets shows quite a few that are Jupiter-sized or larger, orbiting at small fractions of an AU – much closer than to their parent star than Earth – or any of our planets, including Mercury. Many have orbital periods of just a few days.
On one hand, this is not so surprising: extremely large planets very close to their primaries are the easiest to detect by stellar wobble, and easiest to confirm by photography. However only a relative handful of stars are close enough for even a crude planet detection and confirmation with our current technology, and the surprisingly large number of extremely large (gas giant size) planets orbiting very close (less than half of Mercury’s 0.387 AU) to their parent has cause consideravble consternation in some circles, and enthusiastic theorizing in others.
AFAIK, there have only been crude attempts at spectrophotometry to assess the outer composition of the “easiest” large planets, but those preliminary efforts have indicated that some of them may be gas giants. Don’t ask me how --it doesn’t fit my intuition, either-- but that seems to be the evidence.
The figure I’ve heard bandied about is that a planetary core will become a gas giants when it reaches something like 4-6 Earth masses (depending on its distance from its sun.) At that size, it starts accreting hydrogen from the solar nebula, though, so its final mass will be considerably larger.
So how come Gas giant’s cores are so different from rocky planet’s? Is it just that there’s so much Hydrogen that all the other elements get lost in the mix, or is there something else going on?
The OP asked about soild matter. I took it to mean adding more and more material as in the earth. This would not turn into a regular star since there would be too little hydrogen and helium. I suppose if you made it massive enough it woudl start burining the Oxygen and Silicon and everything up through Iron.
If it’s too close to its sun, like HD209458, the hydrogen will get hot enough to escape into space, even from a near Jupiter sized planet. Incredible shrinking planet:
I don’t know for sure if this is applicable, but if a pile of building stone having a compressive ultimate strength of about 13500 psi is stacked up up on the surface of the earth the stress on the bottom stone will equal its ultimate strength when the pile is about 12100 ft. high. The computation took into account the reduction in the force of gravity with height but not the reduction in the downward force resulting from rotation.
All that means is that the bottom stone will crumble but if it is surrounded on all sides by more stone I think all that would happen is that the pile might settle.
It looks to me like there will be a limit to how big a solid object can be. I can’t figure out what that limit is though.
True enough, but that doesn’t happen in nature. I was trying to keep within the framework of natural planetary formation. But, you’re correct, if you kept dumping rocks onto a planet it would eventually become too massive to support itself and would collapse first into a neutron “star” and then into a black hole.
KP, I suspect those giant close-orbiting extrasolar planets will turn out to be something in between rocky and gas giant planets. Maybe super-massive rocky planets with heavy methane and hydrogen atmospheres or something even weirder.
Whatever happened to the idea that there might be a core of metallic hydrogen? (We’ll leave Lucy and 2061 alone for now.)
My guess is that a gas giant’s core probably isn’t all that different from a rocky planet’s, it’s just that you’re not thinking correctly of the scale. Gas giants are huge.
Here’s how I see it: during formation, planets are generally hot. The heat is caused by friction as the planet condenses gravitationally and by impacts as it vaccuums up more stuff, especially larger objects. Because it is hot, molten, heavier elements sink to the core while lighter elements float above. Hence the iron core found at the center of the earth. If you’re on your way to becoming a gas giant, it means a) that you’ve got enough mass to start vaccuuming up and hanging on to gases and b) that you’re probably on the “outskirts” of your sun where the gas isn’t so hot that it’s available for you to vaccuum up. So on top of your solid stuff, you start adding layers of gas.
Eventually you get huge. Incredibly huge. So huge, that your solid stuff is pretty insignificant, even if it is 4 - 6 Earth masses. And all that hugeness causes lots of pressure, enough pressure that your inner layers of hydrogen and helium become this metallic liquid type stuff with its own special properties. But I’m guessing that your solid core is still down there, underneath it all. It’s just pretty small in comparison to your hugeness.
Hopefully someone else will be along with a correction.
If a solid-matter planet got big enough for fusion to take place at the core (which I’m assuming would happen before it got big enough to collapse into a black hole), wouldn’t it go kaboom? (that’s a technical term, dontcha know)
Actually, the technical term is “kablooie”, but we get the drift. But I’m not at all sure that fusion would occur before it became a black hole. The core of the Earth is largely iron, which will never fuse exothermically at all, no matter the temperature. It can fuse if enough energy is put into it, and I suppose this might happen in the core of a star-sized planet, but that’s not going to give you a kablooie. Even elements lighter than iron (silicon, say) are going to be a lot harder to ignite than the hydrogen burning in a typical star. And without fusion-generated heat, our object is going to be much denser than a burning star of the same mass. We know that a decent sized star (one large enough to burn “metals”) will collapse into a hole when fusion stops (and even then, the star is still mostly hydrogen), so I think it’s reasonable to presume that our planet would collapse before fusion even started.
Btw…when an astronomer uses the term “metals” they mean any element that isn’t hydrogen or helium.
Is there any way, say a massive asteroid strike or volcanic eruption, that could cause Jupiter to vent the majority of it’s atmosphere? I believe that Jupiter’s solid core is thought to be 10-20 times the mass of Earth- how thick would Jupiter’s atmosphere be if it had an Earth-like ratio as a “solid giant”? Could it rebuild an as thick atmosphere again?
How about this: When the Sun dies, would it either (a) burn off Jupiter’s atmosphere when the red giant edge got closer to Jupiter or (b) blow away Jupiter’s atmosphere as it ejects some of its own mass toward the end?
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