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This is just a simple question, and I haven't thought all that well on it, but I think in terms of sci-fi and fantasy less tied to probable scenarios this could be quite possible.

Imagine a free-floating planet, one that is not tied to any particular solar system, and therefore floats around in space. Where did it come from? Nobody knows. The real question is how it could have originally formed. Could it have been created from the remnants of a star?

Of course, this could in theory be applied to planets restricted in movement by stars that they permanently orbit. However, is it possible that all the materials from a once-existing solar system were dragged together around its star to form a crust, leading to the formation of a giant planet?

The planet would probably follow the same structure as the terrestrial planets of our solar system, with a core surrounded by mantle and the crust. In order to form this hypothetical planet, it would have to be quite large to incorporate the materials of the old solar system's star as its core, as well as all the planets that orbited it. I don't know how the planets were dragged into the star (perhaps it has a large gravitational force due to its mass, or the planets' elliptical orbits drew them into the star's vicinity?)

So, I guess the question I want to ask you all is if and how a planet like this would and could form, and would it be possible at all?

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  • $\begingroup$ This isn't an answer, but based on what L.Dutch said, an observation. Galactic and intergalactic gravity could be roughly modeled in 3D in the same way terrestrial ocean currents are modeled in 2D. Consequently, those currents will form channels of influence and eddies. It's the eddies that are interesting. As mass from an exploding star (and the consequences on its planetary system) expand into the universe, it will travel until Newton's 1st law kicks in and those currents and eddies allow it to settle - and form a rogue planet (mass too great for the eddy to restrain it). Just a thought. $\endgroup$
    – JBH
    Apr 6, 2021 at 20:21

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Our Sun, and the solar system with it, is a third generation star, meaning that it formed with the remains of one or more exploded stars which formed with the remains of one or more stars formed at the beginning of the Universe. This has been determined by the type of elements contained in the Sun and requires at least two generations of star to be synthesized.

However, is it possible that all the materials from a once-existing solar system were dragged together around its star to form a crust, leading to the formation of a giant planet?

When a star explodes any body orbiting it would first of all be vaporized by the enormous amount of energy released in the process, and if by any chance there should be anything remaining of significant size, it won't move coherently in a unique direction: lacking the centripetal force granted by the star, everything would continue to move along the tangent of the orbit it had when the star vanished, so it would be flung around.

Therefore your planet is either a single remnant of a once way more massive body, or the aggregation of the residuals of some star explosion which was then expelled by a planetary system because of gravitational interactions.

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  • $\begingroup$ Thanks so much for this answer! It has made me rethink the way in which I may portray and use the planet in my worldbuilding. I hadn't - and couldn't have - thought of this myself. :) $\endgroup$
    – P C I A
    Apr 6, 2021 at 19:10
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A star never just 'disappears'. From what we understand as of now, dead stars usually end up as neutron stars or black holes or explode into a supernova.

When they end up in a neutron star or a black hole, they will just swallow the planets up.

When they can end up in a supernova, they can (depending on their mass and radius) end up as supernovas and ultimately turn into planet-forming nebulae. Although it's highly unlikely for all the mass in a nebula to ends up forming one giant planet, I'm unaware of any research or theory that marks this hypothesis as theoretically impossible.

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There are believed to be hundreds of billions of free floating planets in our galaxy, if not trillions. You don’t need to invoke any special or unusual circumstances for them to be formed; the ejection of one or more planets from a new solar system due to their gravitational interactions is perfectly normal

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First to answer your stated question:

Yes. But not as you envision it, with the planetary matter settling on the star remnant to form a crust.
If enough matter stayed nearby, and coalesced, it would just form a new smaller star. But typically when a star dies in glory, it sweeps any and all planetary system it used to have before the winds of its demise.

The resultant nebula of an exploded star can, and most certainly have many times, join with whatever it encounters and form new stars. Our own sun is a third generation example of this. This means there was a star(or actually several stars) that lived, and dies in glory. Their remnants coalesced with other stellar gasses and dust, and reformed into a second-generation star. And that star also died in glory, and seeded its corpse over space. And from this came together the nebula which became our sun.

But if your actual question is this snippet from your posting:

a free-floating planet, one that is not tied to any particular solar system, and therefore floats around in space. Where did it come from?

That is easy.
This rogue planet was formed around a star, and then was gravitationally ejected from it. A simple unlucky encounter with a suitably large planet like Jupiter can eject a smaller planet from the system, forever after to drift through dark space.

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Stars are big. Really big.

But leaving that aside, let's suppose you have enough of the right material from somewhere and want a planet with a crust, built on/from the same body that was previously a star, without going through "make something new out of gas clouds".

It wont happen.

A brown dwarf is the smallest main-sequence star category, and is more like Jupiter than Earth. Add enough mass to a gas giant, and fusion will start in the core. This gets worse as you go up the main sequence. So no hard rocky surface.

So, what if we use a stellar remnant like you asked for?

White dwarf: This is where gravity is so strong that conventional concepts like "an atmosphere" break down. A white dwarf is composed of gas so pressurised that it can't squish any more because there's nowhere for the electrons to go. It is the atmosphere. Trying to add rock to it will just add to the gravity, and squeeze a little more tightly. Yes, adding more matter makes it smaller. Eventually you'll pass the Chandrasekhar limit, and then you lose the idea of atoms. Everything collapses into one really big atom, or tries to ... and that means fusion, which blows it up in a couple of seconds. (Do not touch.) If you added your material slowly, you might still have a remnant of that, which will be ... another white dwarf.

Neutron star: A white dwarf loses the idea of molecules and chemistry. A neutron star dispenses with the idea of atoms. You end up with one big atomic nucleus, which combines electrons into its protons to make neutrons.

Black hole: When a neutron star is so heavy it gives up the fight against gravity. No rocky surface here either.

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