Forum: Author Hangout

Would you care to share your answer?

AJ

I don't know the physics well enough to have any idea what the minimum density for a rocky planet would be.

It would depend on it's composition (i.e. what type of rocky surface it has (most planets in a 'system' have molten core, as the competing gravitations generate substantial internal heat.

Then there's the question of new or old star systems. Luckily for us, our galaxy is incredibly old, so complex substances like gold, silver, cobolt and other elements are fairly common. However, ALL those heavier elements are the results of dying older stars, which continually compress elements. In the case of black holes, they routinely 'burp' those heavy elements out, and they eventually reach other planets across the galaxy.

Newer galaxy's, however, don't yet have enough mass, so the sun never reach the necessary density to form black holes, or even to generate very heavy elements. In such a system, even the 'rocks' would be considerably lighter than ours would--and a planet composed of sand or 'space dust' would be even lighter. If there are large underground 'pockets' of some sort (ex: methane), it would make the planets gravity lighter and highly variable.

That's a tough question, as a given planet could just as easily be destroyed by another nearby planet, as it'll grow due to collisions. There's evidence that many of our early planets were effectively destroyed, allowing our other 8 planets to increase in size (Neptune sitting on it's side, the space between us and Mars, and our own moon being prime examples of said evidence).

The biggest factor, though, is the composition of the early solar system. If there's a LOT of 'gas' (i.e. small debris) and a few rocky comets big enough to survive collisions, they'll each grow bigger as they 'compete' with one another in a galactic game of roller derby. But, if there isn't that amount of free floating matter, then few, if any planets will ever form in the first place.

Thus, it's really not a matter of a planet's 'density' which determines how long it'll survive, it's more a matter of it's relative size, surroundings and proximity to the solar center of the system. Remember, the biggest bodies in our system area all Gas Giants, which are mostly composed of cosmic 'dust' which has fallen on them over hundreds of millions of years. (Hint: They now believe that the planet's closest to a 'sun' are usually the first to be destroyed, due to a wide variety of reasons.)

Which is about right. Close enough for fictional work, anyway. Earth isn't actually a sphere, which is much of the margin of error.

Actually, given that gravity affects every element in the universe equally, the vast majority of stellar bodies are basically circular, for the same reason that every ocean curves (i.e. gravity pulls everything towards the center of gravity). Those that aren't are either the pieces broken off by collisions, or those where asteroids loose enough volume due to heating as they near nearby suns that the larger pieces break off, leaving only large clumps of iron, in often irregular configurations.

As far as Earth not being a pure sphere, that boils down to gravitational fluctuations, which is why NASA (and others) are now busy measuring gravitity fluctuations across the globe, as they're often indicators of heavier collections of heavy elements deep underground (i.e. if one region of a planet is most loose dust compressed over time into rocks, and another is composed of large masses of iron, guess which side is going to be heavier?).

The density does matter here, at least for terrestrial planets. If it's too low, the gravity will be weak and if the gravity is too weak it won't collect as much debris as quickly in order to grow.

Although ALL planetary bodies grow over time as 'cosmic' dust rains down, accumulating as layers of dirt, the MAJOR growth of planets is NOT from this gradual accumulation. In fact, if it was, then few planets would ever reach planetary size.

The major growth of planetary bodies is from planetary collisions, as two bodies collide with each other, part of each being thrown into space, and the resulting mass creates enough gravity to gradually draw the ejected debris back to itself.

As far as preventing planets from having been 'mined out', that's a twofold problem. If the planet is not is a 'lively' system, there will be NO (i.e. minimal amounts of) heavy metals (i.e anything worth mining). Those elements, along with an molten iron core, are what produces most of a planet's density and its gravity.

However, if it orbits an immature star, there might not be enough cast off heavy elements to provide much iron in the first place.

A better excuse for a planet not having been previously mined is simply: no one ever bothered. If it's a small system, then getting to it would keep anyone from bothering with it. Remember, not all planetary bodies are equal, even if they are a similar volume/size.

I believe in aviation terms, that's known as a helicopter.

A collection of loose parts roughly flying in formation.

I believe in aviation terms, that's known as a helicopter.

A collection of loose parts roughly flying in formation.

Helicopter bodies? Might be a useful term to throw into a sci-fi book, as it readily conveys an otherwise complex, hard to explain topic.

Remus, I agree with your summary, but once again, all systems are not equal. Younger galaxies do NOT have the same abundant heavier elements, since they originate from debris and gasses cast off from either exploding supernovas, or the 'burping' of nearby black holes, as they consume other bodies and some parts of them escape before reaching the vanishing point.

It's also important to note that these elements are not inherently 'clumped' together. Take gold for instance, how it formed wasn't known until recently, when it was determined that it's generated by earthquakes. That's right, earthquakes. There are millions of microscopic amounts of gold and silver in any body of water (again, assuming there are sufficient heavy elements in the system). Whenever earthquakes start stressing the bodies, the heat generated evaporates virtually ALL the water in underground streams, leaving nothing but these heavier elements, the most common of which are lead, gold and silver (in varying degrees, of course). That's why both god and silver are seeming always found in 'veins' (aside from those found as clumps, broken off of a bigger collection by, you guessed it, other earthquakes.

Knowing i'm just out and out wrong on occasison, I stopped by here and asked. shortly after the clue bat landed on the back of my hand and gravity worked correctly again in my make believe universe.

Yeah, recognizing the difference between "more" and "less" is a problem which frequently trips many of us up. While we might not make that specific one, there are enough similar to that, that it's a common experience. (ex: "What kind of gun? Hell, I don't know. It's a damn gun. I needed one, so my character just grabbed the first one he could find!")