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We have a sun that is like ours, does not matter I think because of scaling. That sun is completely encased in large rock formations like that of a planet. Basically if you replace the core of a planet with a sun.

Obviously I want it to be naturally formed planet but the focus on the theoretical aspect of it is more important since the universe is a big place it's enough to know it could happen.

I understand there are complications regarding other aspects but I'm only focusing on one thing here and will ask later about other problems. I want it later to orbit a sun and have an atmosphere...etc But again I just wanna focus on one aspect:

Is such a thing even theoretically possible?

And what type of real or theoretical material would be used? to achieve that idea?

I understand that those are two things, do they need to be broken? But I'm worried that it would get closed if the question has a simple yes or no answer.

Extra fluff!

NASA says a planet must

It must orbit a star (in our cosmic neighborhood, the Sun).
It must be big enough to have enough gravity to force it into a spherical shape.
It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

However I'm only interested in figuring out this first step now, could it be real?

I'm well aware that the gravity of the sun is huge. Even one like ours has a lot of gravity. So. This is part of the question. I mean that what type of material could withstand such gravity

Obviously, the sun also outputs a lot of energy. So the planet will have to find a way to handle that if possible.

I'm content if it not even theoretically possible. But I'd just love to know more on why.

I will ask about an atmosphere later, as well as if it can orbit a star or not...etc Just want it to be focused.

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    $\begingroup$ Are you talking about a Dyson Sphere? en.wikipedia.org/wiki/Dyson_sphere They would definitely not occur naturally. Engineering would be improbable but not outside of possible. $\endgroup$ – DWKraus Jan 6 at 22:34
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    $\begingroup$ Can you clarify the "scaling" part? Do you want to see a planet-sized white dwarf encased in rocky shell, or a Sun-sized star with the same shell? $\endgroup$ – Alexander Jan 6 at 23:33
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    $\begingroup$ If I read it correctly this is like fitting a watermelon into a grape... $\endgroup$ – user6760 Jan 7 at 2:53
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    $\begingroup$ @user6760 I think that would be easier. $\endgroup$ – DKNguyen Jan 7 at 4:25
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    $\begingroup$ OF COURSE you can.. Use the same technique you would use to stuff an Elephant inside a Thimble. $\endgroup$ – PcMan Jan 7 at 11:38
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No this is not even remotely possible. The gravitational attraction of a star would be great enough to suck any surrounding planetary surface sphere into itself. Making the planet or crust deeper would not help either as it would only serve to increase the gravitational field centred on the star. The energy output of the star would also rapidly vaporise any surrounding material.

Note your NASA quote number 2 "It must be big enough to have enough gravity to force it into a spherical shape" any object with enough gravity to do this would collapse any hollow sphere.

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  • $\begingroup$ And a star would vapourize any solid matter that surrounded it. although, it a dyson sphere an edge case of a "planet", albeit artificial? $\endgroup$ – Bohemian Jan 7 at 6:26
  • $\begingroup$ Maybe remotely possible, if you stretch the definitions of star and planet. You can have a neutron star, or perhaps a white dwarf, which has cooled to the point where it can have a solid crust. This was the premise of Robert Forward's SF nover "Dragon's Egg": en.wikipedia.org/wiki/Dragon%27s_Egg $\endgroup$ – jamesqf Jan 7 at 7:01
  • $\begingroup$ Best not confuse fact with fiction. Dragon's egg is a very imaginative work of science fiction that takes place in a gravity field field 67 billion times that on Earth. $\endgroup$ – Slarty Jan 7 at 10:35
  • $\begingroup$ What if the planet crust was made of some special material that was strong enough to withstand the gravity? $\endgroup$ – dwjohnston Jan 8 at 0:43
  • $\begingroup$ one of the many problems with the "hollow earth theory" $\endgroup$ – Michael Jan 8 at 6:50
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What is "Star"?

Technically this would just be a star, something along the lines of a binary star. A sun, in some sense, is a nuclear-powered "planet" shaped object that can orbit another larger star. Replacing the core of a planet with a star would cause the planet to collapse because of gravity, as well as causing the surface to radiate with the same total heat output as the interior star, thus making it...a star again. It's stars all the way down, I'm afraid.

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  • $\begingroup$ Yes. Even if make planetary shell of an indestructible unobtanium which would not crash or melt, it would still heat up proportionally to the star's energy output and shine just as bright as a star. $\endgroup$ – Alexander Jan 7 at 18:08
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Nope

The universe is a big place it's enough to know it could happen.

We expect the laws of physics to be the same anywhere in the Universe, so no. If it is not feasible here, it is not feasible anywhere else. Maybe in another universe with different constants.

The smallest stars possible are brown dwarves, with a lower bound for their mass at around 13 jovian masses. Giant planets do not have a solid surface. So...

Size comparison between Earth, Jupiter, and a small brown dwarf.

Source: see video in the link.

I could stop here, but I'll give you a different idea: neutron stars have an atmosphere and a solid crust.

The components of a neutron star: atmosphere, crust and core.

Source: see video in the link.

You could consider them a kind of planet with a stellar core, if you follow the IAU definition (which is what NASA is quoting in your link) and you find a neutron star orbiting another neutron star.

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Stars are made of hydrogen and helium. A star is a star and not a cloud of hydrogen and helium by virtue of its gravity which is due to its mass. The gravity being strong enough to cause fusion is what makes a star a star.

Planets (the rocky kind you are thinking of) are made of rock. Rock is much denser than hydrogen and helium, so a planet large enough to fit a star inside would be so massive that its gravity is so strong that it would collapse into a blackhole, neutron star, or something similar.

So no.

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As others have pointed out, a literal star isn't possible because of scaling issues. The minimum size of the star (not too much smaller than our sun) is still far too large to have an enclosing solid surface that wouldn't undergo gravitational collapse. There's a couple of alternatives I can suggest if that's okay.

One would be a Dyson sphere. This is a collection of structures that either orbit the sun (held up by centrifugal force) or are suspended via light sails. Get enough of them together and they can completely surround the sun. There's a ton of science fiction stories that feature these.

Another would be an artificial fusion powered source of electromagnetic energy that mimics the output of a sun but on a much smaller scale. That could be small enough to be enclosed in a rocky body such as a hollowed out asteroid. There's lots of stories that take this route as well.

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If it is possible, there are so many equilibrium points that must be balanced that I doubt such thing would exist for too long. These are the points I can think:

  • To prevent the rock crust surrounding the star to crumble under its own weight, the crust must be thin enough and the radius so big that the average density is low enough so that the material rigidity can keep it together. Didn't run the numbers, but may be possible.
  • With a star inside, the crust radius may have to be even bigger, so that the star's gravity does not to crumble the structure.
  • The entire energy output of the star would have to pass through the crust, so it must have enough heat conductivity and/or transparency to prevent the inner surface from melting and dipping into the star. Also, the radius must be so big that the energy input is not much higher than a couple of kW/m², otherwise you also have dipping lava robbing mass and structural integrity from your shell. Probably no smaller than the radius of a closely orbiting planet to the star. Must account for reflected/irradiated light from the structure itself, that will make the interior much hotter than just the direct sunlight.

Interestingly, the position might be stable. Such thing can not rely on rotation to maintain a stable orbit around the star, because the poles would crumble leaving only an equatorial ring: instead, it must be rigid enough to sustain itself. Considering it's gravity center will match the stars gravity center, star's pull will propagate through the structure, tending to always match their center of mass. Any delta would make the shell oscillate around the star over the course of days, but radiation absorbed from the interior might act as dampening of sorts, and oscillation should eventually dissipate as heat.

The star's radiated light will also exert an stabilizing pressure on the structure, pushing outwards in every direction like gas inside an inflated balloon.

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  • $\begingroup$ A planet, by definition, is shaped into a sphere by its own gravity. If the rock crust does not collapse with a massive star at the center of the planet, it wouldn't have collapsed beforehand when the far less massive core was there. If a body's own gravity is insufficient to reshape the surface, it is by definition not a planet. Essentially, a planet must "crumble under its own weight", or else it's not a planet. $\endgroup$ – Nuclear Hoagie Jan 7 at 15:12
  • $\begingroup$ Quoting OP: "However I'm only interested in figuring out this first step now, could it be real." $\endgroup$ – lvella Jan 7 at 16:06
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Sink and Melt.

The Sun's surface is about 6000C. Stone melts at about 1500C. So the stone shell will melt into Lava.

Even ignoring the heat, the stone will sink under the sun's surface. Most types of stone have density about 2.5 g / cm$^3$. The Sun is less dense at about $1.5 $g / cm$^3$. That means if you put a stone shell around the sun it will sink in the same way stone sinks in water.

If you put something less dense that can withstand the heat then it will remain on the surface. For example water is 1 g / cm$^3$ at room temperature. However being on the surface of the sun definitely changes things, due to massive heat and gravitational pressure.

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    $\begingroup$ That's the net density of the entire sun, the core being much denser than the "surface", which is just a region of plasma hot enough to be mostly opaque, but of such low density it's practically vacuum. Even aside from the temperature, nothing solid's floating on the surface of a star. $\endgroup$ – Christopher James Huff Jan 7 at 15:33
  • $\begingroup$ The erth's core is about the same temperature as the surface of the sun at some points. Pressure keeps it solid. $\endgroup$ – The Square-Cube Law Jan 7 at 18:41
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Well, you could have a running fusion reaction going in there, the size of a smaller moon, that powers some very energy intensive projects (like shooting a concentrated beam to several distant planets/solar systems).

That way you are producing the energy levels of a star using conversion of hydrogen to helium - if the energy you get out is bigger than the energy getting in your only limiting factor is hydrogen input/recycling to run "perpetually".

So you have a "star" inside a planet.

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  • $\begingroup$ By using some other mechanism (besides sheer amount of mass) to compress the matter enough to provoke a fusion reaction, this gets around the gravitational problems pointed out by the other answers. $\endgroup$ – Jeremy Friesner Jan 8 at 16:26

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