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Scientists have been looking for an explanation for the unusual orbits of extreme trans-neptunian objects. One of the possible explanations some astrophysicists have come up with was the possibility of Planet X. However, no one has been able to find planet X. What is the possibility that planet X is a spherical glass planet (not a typo, I actually mean SiO2), thus the reason behind why optical telescopes have not been able to find it.

What are the possible implications if we were to find it, to our current knowledge of astronomy and planet formation?

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    $\begingroup$ Note that a transparent ball bends light, so it's a type of lens. edmundoptics.com.au/knowledge-center/application-notes/optics/… $\endgroup$
    – PM 2Ring
    Oct 30, 2022 at 23:26
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    $\begingroup$ If it was made of glass, it would be almost completely opaque. Even a mile depth of the purest glass imaginable would block every single photon coming from the other side. $\endgroup$
    – Valorum
    Oct 31, 2022 at 7:56
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    $\begingroup$ @PM2Ring: That would require a reasonably smooth surface. But the hypothetical surface would be riddled with surface defects; space is a rather hostile environment. Not disagreeing with Valorum here, but you probably can't even see the inside. $\endgroup$
    – MSalters
    Oct 31, 2022 at 11:49
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    $\begingroup$ @MSalters Sure, the surface would probably be more like frosted glass than clear glass. And whatever light did manage to pass through would be highly attenuated (as I mentioned in a comment on Rob's answer). Although light can travel for miles through long optic fibres, those fibres utilise total internal reflection. $\endgroup$
    – PM 2Ring
    Oct 31, 2022 at 12:33
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    $\begingroup$ @PM2Ring total internal reflection doesn't preclude absorption by the bulk of the fiber. The reason why fibers can pass light for so large distances is the utilization of the spectral transparency windows of the materials: frequency of light being passed is chosen carefully to fall into this window. $\endgroup$
    – Ruslan
    Nov 1, 2022 at 1:02

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Zero chance.

Even if the planet was made of glass, and was transparent, it would still radiate heat, as all warm objects do. Thus, we could find it with an infrared telescope. In fact, this is likely how we would find it even if was a "normal" planet. Furthermore, at the distance it is likely to be, there is very little light to reflect. As such, it wouldn't matter if it's transparent or not as there is not much visible light to worry about.

I also doubt $\rm{SiO_2}$ remains transparent when subjected to the high temperatures and densities in the cores of planets. The Earth's core is ~6000 degrees Celsius. Glass would have melted at this point.

Then, you get into formation scenarios. How do you suppose it was made only of glass without being contaminated with all the other stuff that is floating in space and makes up planets? Add in 4.5 billion years of dust accumulation and material from asteroid impacts and the surface definitely won't be clean glass anymore. If it's not clean then you've lost the transparency.

If it was, then we get to throw out the book on planet formation and will need to start again.

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    $\begingroup$ Even high quality glass or silica attenuates light to some extent. There's some info & a graph at fowiki.com/b/understand-fiber-attenuation $\endgroup$
    – PM 2Ring
    Oct 30, 2022 at 23:22
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    $\begingroup$ +1 and related in Physics SE: Is the visible light spectrum from "red-hot glass" at least close to Blackbody Radiation? $\endgroup$
    – uhoh
    Oct 31, 2022 at 0:11
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    $\begingroup$ If we're hypothesizing, why not Cosmic Windex? $\endgroup$
    – Barmar
    Oct 31, 2022 at 13:59
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    $\begingroup$ I still low key wanna know if it's feasible to encounter a "glass planet". I don't care about its transparency. Hmm. Presumably a gas giant with a whole lot of silicon and oxygen, smashes it all together, forming a "glass planet", then has the atmosphere blown away by some cosmic event? I dunno, I'm spitballin here! $\endgroup$
    – JamieB
    Oct 31, 2022 at 21:30
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    $\begingroup$ @JamieB If a 100% glass planet was encountered, it would also prove the existence of wildly advanced intelligent life. It could not form naturally. It's possible (but unlikely) that a mostly SiO2 planet could form, but no matter how enough SiO2 to form the bulk of a planet was isolated (large body collision, emission from planetary volcanic source) would mean "contaminates". Additionally core and solar heat would prevent "glass", resulting in another state of SiO2. $\endgroup$
    – jon.bray.eth
    Nov 2, 2022 at 4:49
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If you cool silicates quickly, you get glass. But planet-size objects cool slowly, making crystalline rock. Natural glasses like obsidian and Pele's hair only form from cooling magma at the surface.

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  • $\begingroup$ +1 It should also be noted that neither obsidian nor Pele's hair are very transparent. $\endgroup$
    – WaterMolecule
    Nov 2, 2022 at 13:39
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Even a very high purity glass is not transparent at planetary sizes. No glass is fully transparent and the amount of scattered light increases exponentially with the optical path length (the thickness of the material).

Even if a planet sized object could naturally form out of window-grade glass (which is probably impossible by itself), it would scatter light sufficiently well to look no different than a gas planet.

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A planetary mass black hole orbiting a star would absorb all light which hit it, and it would have an extremely tiny diameter. Thus it would be effectively invisible and could be detected only by its gravity on other objects.

But even though the interplanetary medium in our solar system is a hard vacuum, it does contain a very thin scattering of subatomic particles, atoms, molecules, and dust particles. Thus some matter would fall into the planetary mass black hole. And possibly someone might care to calculate how much electromagnetic radiation at various wavelengths the infalling particles might emit as they are rapidly accelerated into the black hole and how detectable it would be.

A black hole with the mass of planet couldn't form by the collapse of a star which would have many times the mass of even the most massive possible Planet Nine.

So if planet Nine is a planetary mass black hole, it would have to be a primordial black hole, formed in the Big Bang which could have formed black holes with a wide range of masses. There are no known primordial black holes.

No doubt astronomers who searched for signs of them have calculated how a upper limit to how common they could be, and thus might be able to say how astronomically large the odds against Planet Nine being a primordial black hole are.

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    $\begingroup$ The question isn't about black holes... Did you post this answer on the wrong question? $\endgroup$
    – James K
    Nov 2, 2022 at 20:21
  • $\begingroup$ The question was about one hypothetical way for Planet Nine to be invisible. So I mentioned another way that Planet Nine might conceivabley be invisitible. $\endgroup$
    – M. A. Golding
    Nov 6, 2022 at 17:00
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Even perfect glass will reflect radiation depending on the angle of incidence according to Fresnel's equations. For grazing incidence, there will even be total reflection. So if you observe in the right position, you would see a 'hot spot' of reflected sunlight from the glass sphere.

You should rather be looking at the opposite scenario, that is a material that absorbs all radiation.

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