I'm hoping that the following scenario is one of those "you'll never actually see that so don't bother looking for it" things Because Reasons™ but I want to look at whether, and how, it is physically possible:

The Larry Niven essay Bigger Than Worlds has a lot of ideas for making artificial habitable environments in outer space. In the section titled Inside Outside Niven suggests that you can create one such environment with a chunk of iron/nickel asteroid shaped like a mile long cigar. One drills down the centre of it, fills it with water, reseals the ends, and then heats it by spinning it under the glare of a parabolic solar mirror. The body will be semi molten when the heat reaches the water and turns it to steam puffing the whole thing up into an ovoid bubble that can then be moved into. There are serious issues with the proposed technique but the form of the final product is sound.

What I want to know is: How could it happen naturally?

Not a smooth artificial bubble being spun for pseudo-gravity obviously but a rocky-iron asteroidal shell around a void with enough volatiles to create some sort of atmosphere, when it is warm enough, contained in it. Sorry let me be clearer I'm not asking for the blown spheroid of Niven's suggestion, I'm pretty sure it won't work as written anyway, I just want an asteroid with a bubble of internal atmosphere contained in it.

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    $\begingroup$ How can a low entropy configuration happen naturally? Hard, with a hint of impossible. $\endgroup$ Sep 27, 2021 at 7:42
  • $\begingroup$ @AdrianColomitchi Yes kind of what I was going with when I said you'd never go looking for one because you wouldn't expect to ever find such an object. $\endgroup$
    – Ash
    Sep 27, 2021 at 7:45
  • $\begingroup$ As in, how could an asteroid naturally form with a water (liquid or solid) core and a basically solid iron or nickle shell, and then find its way into a solar system with a close-enough-to-the-star orbit that it explodes into a more-or-less spherical shape on its own? That sounds like really long odds to me ... but some pretty strange things have been found in Mother Nature's universe. $\endgroup$
    – JBH
    Sep 27, 2021 at 11:16
  • $\begingroup$ I am not sure you have covered exactly how the body would be so absolutely homogenous that there would be no weak spots whatsoever, such that the expanding super-pressurized super-heated steam would not vent through a localized weak channel and then burst out like a pimple being squeezed, thus releasing all of the pressure. $\endgroup$ Sep 28, 2021 at 1:03
  • $\begingroup$ @JoinJBHonCodidact I was thinking more a rock that looks like any of hundreds in orbit around a given star but it happens to have a pocket of gas/water inside it. $\endgroup$
    – Ash
    Sep 28, 2021 at 5:17

3 Answers 3


a rocky-iron asteroidal shell around a void with enough volatiles to create some sort of atmosphere, when it is warm enough, contained in it.

It seems unlikely: once a body starts to have significant self gravity, so that it is bound by it and not by electrostatic forces, also buoyancy starts to be sensible, and that will result in denser materials sinking toward the center of mass, while the less dense materials will move toward the outer of the body, eventually being vented out.

What might happen is that, if the material is in a molten state, the outer shell can solidify first and then trap any degassing coming from the inside, which will then end up undergoing coalescence and forming localized bubbles.

  1. Take Ceres Europa out of Jupiter's influence and throw it into space until it freezes to interstellar space temperature by radiation loss
  2. send it for some billions of year wandering in an iron rich nebula - perhaps you can find a cluster of old supernovas somewhere
  3. when you have a thick enough iron shell on top of some form of metallic ice, send it at grazing incidence at high speed in the polar jets of a neutron star. With a bit of luck, the eddy currents will sinter the outer shell and give it a rotation high enough to create the inner void from the ice. You may be lucky enough to let some of the ice briefly breach the shell and be ejected outside
  4. place it around a nice star to keep it warm.

I would never bet even an atom of iron against a planet size diamond you'll be that lucky to have your hollow dwarf planet. Maybe because I just love iron atoms so much I'd never part with one.

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    $\begingroup$ Hollow Planet? Even if we tag dwarf on as a size qualifier how did you get from one mile long to a hollow planet? $\endgroup$
    – Ash
    Sep 27, 2021 at 8:09
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    $\begingroup$ @ash one mile long chunk of ice will stand no chances to evolve as described. Therefore, Ceres. Or a dwarf planet sized chunk of ice. Because you need thermal inertia to keep the core solid enough until the shell sinters. $\endgroup$ Sep 27, 2021 at 8:11


Well, you need a volatile (icy) core covered in a rock or metal shell. This is the opposite of what happens normally when a melted body differentiates.

There are bodies that are all ice. So let's start with that, and figure out how to make a hard crunchy shell like a cosmic M&M.

Planets normally grow by accretion anyway. You just need to move it from the region where it accumulated ice particles and into a region where it accumulates rock or other heavy material. These regions are normally separated in the protoplanetary disk, on either side of the frost line.

So, at first glance, it seems we just need to have a body that grows large as it tacks inward and eventually crosses the frost line. Planets do in fact move around as they influence each other and interact with the surrounding media.

The main issue is that accumulating material causes the body to heat up, both from the energy gained by consolidating the particles and from radioactive materials incorporated. Many asteroids show signs of separating out denser material, and Ceres and Vesta were warm enough to relax into spheres. This is exactly what we don't want to happen.

But... giant supernucules comets do exist; they grew to large size without boiling themselves off. So take that as a given.

So how about this: A "rubble pile" recently got blown apart from an impact, forming a cloud of debris that will mostly pull itself back together again. It formed well inside the frost line but was pushed outward. Meanwhile, out frozen chewy center is moving inward and they happen to cross orbits right when the rubble pile was existing as an extended cloud spread out around an arc of its orbit. The large mass of the center easily pulls it back together again, but the debris gathers around the solid frozen center.

Now these rubble piles normally get blown apart and re-aggregate and indeed stay as loose rubble, not melting from the heat of formation. Again, they exist, it happens. The pieces are mostly in the same orbit already and come back together reasonably gently, compared to random collisions of protoplanets.

Meanwhile, the ice serves as a heat sink: a crashing mountain will blow off a small part of the ice, rather than heating up the whole globe. It stays cold, though loses some mass.

Once it is covered in a protective shell of rock, it can continue migrating inward and collect additional material. The now thicker crust does melt were it is impacted by random arrivals (not part of the same co-moving debris), and starts to form a solid shell. We just have to add to it gradually enough that it doesn't melt through, and cools between collisions.

So, we can quite plausibly get our cosmic M&M!


Now, how do we get it to heat up?

Move it to the oven! It's tacking inward... towards the forming star. It seems most convenient to continue on this tack rather than introduce more bodies.

This region will naturally make rocky bodies, as ice melts and vaporizes, but rock does not. So, the crust will not melt into magma but the ices will melt and boil off.

Now we need to get the temperature just right so the shell becomes soft and plastic, but doesn't melt into magma and sink into the ice. Layering helps: the outer layers exposed to the sunlight will melt but under that is still solid and the solid part cracks and shifts but the melted surface plasters over any gaps that open up as it expands.

Eventually, the initial high-UV phase of the star settles down into normal main sequence, and the oven goes from broil to keep-warm.

You are left with a hollow shell filled with an ocean and a thick atmosphere. Magma from the shell dripped down in places, forming long spires that criss-cross through the interior. The ocean might touch once side of the shell due to density differences, but keeps clearance on the opposite side at the very least.


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