<p><strong>Zero chance.</strong></p>
<p>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 &quot;normal&quot; 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.</p>
<p>I also doubt <span class="math-container">$\rm{SiO_2}$</span> 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.</p>
<p>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.</p>
<p>If it was, then we get to throw out the book on planet formation and will need to start again.</p>

CC BY-SA 4.0
2022-10-30T23:22:13.453Z
Even high quality glass or silica attenuates light to some extent. There's some info & a graph at http://fowiki.com/b/understand-fiber-attenuation/
PM 2Ring
CC BY-SA 4.0
2022-10-31T00:11:59.180Z
+1 and related in Physics SE: [Is the visible light spectrum from "red-hot glass" at least close to Blackbody Radiation?](https://physics.stackexchange.com/q/254099/83380)
uhoh
CC BY-SA 4.0
2022-10-31T13:59:20.640Z
If we're hypothesizing, why not Cosmic Windex?
Barmar
CC BY-SA 4.0
2022-10-31T16:29:51.363Z
Glass is also somewhat opaque to infrared, so I imagine it would show up very easy from IR images, at least due to effects on what is behind it.
JMac
CC BY-SA 4.0
2022-10-31T21:30:44.813Z
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!
JamieB
CC BY-SA 4.0
2022-11-02T04:49:59.007Z
@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.
jon.bray.eth
CC BY-SA 4.0
2022-11-02T07:37:45.567Z
@JamieB There's a whole lot of silicon dioxide in the Earth's crust; silicon dioxide is one of the most common compounds in the universe. But it's not in the form of _glass_ - that would be _extremely_ odd. Even if you melted it all in a catastrophic collision, it would only form a relatively small amount of actual glass - and it would erode pretty quickly (on geological scales) into plain sand. If we ever find a planet with a glass surface, we can probably conclude someone did a lot of orbital bombardment recently :D (incidentally, this is what made Tatooine a desert world - it was glassed).
Luaan
CC BY-SA 4.0
2022-10-30T20:27:09.963Z
2022-11-03T23:52:42.960Z
2022-11-03T23:52:42.960Z
Rob
<p>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.</p>

CC BY-SA 4.0
2022-11-02T13:39:37.480Z
+1 It should also be noted that neither obsidian nor Pele's hair are very transparent.
WaterMolecule
CC BY-SA 4.0
2022-11-01T00:06:45.690Z
2022-11-01T00:06:45.690Z
John Doty
<p>Even a very high purity glass is not transparent at planetary sizes. No glass is fully transparent and the <a href="https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law" rel="nofollow noreferrer">amount of scattered light increases exponentially with the optical path length</a> (the thickness of the material).</p>
<p>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.</p>

CC BY-SA 4.0
2022-11-02T14:07:58.533Z
2022-11-02T14:07:58.533Z
WaterMolecule
<p>Even perfect glass will reflect radiation depending on the angle of incidence according to <a href="https://www.brown.edu/research/labs/mittleman/sites/brown.edu.research.labs.mittleman/files/uploads/lecture13_0.pdf" rel="nofollow noreferrer">Fresnel's equations</a>. 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.</p>
<p>You should rather be looking at the opposite scenario, that is a material that <strong>absorbs</strong> all radiation.</p>

CC BY-SA 4.0
2022-11-06T18:25:22.863Z
2022-11-06T18:25:22.863Z
Thomas
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2022-11-02T20:21:14.293Z
The question isn't about black holes... Did you post this answer on the wrong question?
James K
CC BY-SA 4.0
2022-11-06T17:00:00.263Z
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.
M. A. Golding
CC BY-SA 4.0
2022-11-02T20:11:04.583Z
2022-11-06T17:33:24.863Z
2022-11-06T17:33:24.863Z
M. A. Golding
<p>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 SiO<sub>2</sub>), thus the reason behind why optical telescopes have not been able to find it.</p>
<p>What are the possible implications if we were to find it, to our current knowledge of astronomy and planet formation?</p>

CC BY-SA 4.0
2022-10-30T23:26:56.353Z
Note that a transparent ball bends light, so it's a type of lens. https://www.edmundoptics.com.au/knowledge-center/application-notes/optics/understanding-ball-lenses/
PM 2Ring
CC BY-SA 4.0
2022-10-31T07:56:33.803Z
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.
Valorum
CC BY-SA 4.0
2022-10-31T11:49:47.693Z
@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.
MSalters
CC BY-SA 4.0
2022-10-31T12:33:16.603Z
@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.
PM 2Ring
CC BY-SA 4.0
2022-10-31T22:08:10.503Z
From the Help Center: *"What questions are off topic here? ... Questions that are purely hypothetical, for example a question such as 'Could a black hole destroy the universe' or 'What if our solar system had two suns'"*
Peter Mortensen
CC BY-SA 4.0
2022-10-31T22:08:40.927Z
Related: *[What exactly count as a "hypothetical question"?](https://astronomy.meta.stackexchange.com/questions/786/what-exactly-count-as-a-hypothetical-question)*
Peter Mortensen
CC BY-SA 4.0
2022-11-01T01:02:16.280Z
@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.
Ruslan
CC BY-SA 4.0
2022-11-02T17:41:45.360Z
Not all glass is made of SiO2; a glass is just an amorphous solid without a regular crystalline structure. I can't imagine a protoplanet cooling quickly enough under natural conditions to form a glass, so a glass planet would be strong evidence of artificial construction.
chepner
<planet><trans-neptunian-object>
What is the likelyhood that planet X is a glass (not a typo for gas) planet? What would it mean if it were for what we know of astronomy?
Dimitri Morvaine
# What is the likelyhood that planet X is a glass (not a typo for gas) planet? What would it mean if it were for what we know of astronomy?

(Asked by: username_0 on 2022-10-30)

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?

**Tags:** planet, trans-neptunian-object
> **Comments:**
> **username_1**: Note that a transparent ball bends light, so it's a type of lens. https://www.edmundoptics.com.au/knowledge-center/application-notes/optics/understanding-ball-lenses/  
> **username_2**: 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.  
> **username_3**: @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.  
> **username_1**: @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.  
> **username_4**: From the Help Center: *"What questions are off topic here? ... Questions that are purely hypothetical, for example a question such as 'Could a black hole destroy the universe' or 'What if our solar system had two suns'"*  
> **username_4**: Related: *[What exactly count as a "hypothetical question"?](https://astronomy.meta.stackexchange.com/questions/786/what-exactly-count-as-a-hypothetical-question)*  
> **username_5**: @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.  
> **username_6**: Not all glass is made of SiO2; a glass is just an amorphous solid without a regular crystalline structure. I can't imagine a protoplanet cooling quickly enough under natural conditions to form a glass, so a glass planet would be strong evidence of artificial construction.  

---

## 5 answers

### Answer by username_7 on 2022-10-30. Score: 43

**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.

> **Comments:**
> **username_1**: Even high quality glass or silica attenuates light to some extent. There's some info & a graph at http://fowiki.com/b/understand-fiber-attenuation/  
> **username_8**: +1 and related in Physics SE: [Is the visible light spectrum from "red-hot glass" at least close to Blackbody Radiation?](https://physics.stackexchange.com/q/254099/83380)  
> **username_9**: If we're hypothesizing, why not Cosmic Windex?  
> **username_10**: Glass is also somewhat opaque to infrared, so I imagine it would show up very easy from IR images, at least due to effects on what is behind it.  
> **username_11**: 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!  
> **username_12**: @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.  
> **username_13**: @JamieB There's a whole lot of silicon dioxide in the Earth's crust; silicon dioxide is one of the most common compounds in the universe. But it's not in the form of _glass_ - that would be _extremely_ odd. Even if you melted it all in a catastrophic collision, it would only form a relatively small amount of actual glass - and it would erode pretty quickly (on geological scales) into plain sand. If we ever find a planet with a glass surface, we can probably conclude someone did a lot of orbital bombardment recently :D (incidentally, this is what made Tatooine a desert world - it was glassed).  

---

### Answer by username_14 on 2022-11-01. Score: 8

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.

> **Comments:**
> **username_15**: +1 It should also be noted that neither obsidian nor Pele's hair are very transparent.  

---

### Answer by username_15 on 2022-11-02. Score: 3

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](https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law) (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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### Answer by username_16 on 2022-11-06. Score: 0

Even perfect glass will reflect radiation depending on the angle of incidence according to [Fresnel's equations](https://www.brown.edu/research/labs/mittleman/sites/brown.edu.research.labs.mittleman/files/uploads/lecture13_0.pdf). 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.


---

### Answer by username_17 on 2022-11-02. Score: 0

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.

> **Comments:**
> **username_18**: The question isn't about black holes... Did you post this answer on the wrong question?  
> **username_17**: 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.  

---

