"The Dark Knight" is a nickname given by astronomers to the exoplanet TrES-2b, a gas giant with an albedo of one percent, blacker than paint, blacker than coal—in fact, blacker than anything on Earth! There are several reasons it is so dark. Its atmosphere is full of materials that absorb light, and it orbits its star so closely that it roasts at 2000 degrees Fahrenheit (1100 degrees Celsius), which is hot enough to melt gold.

In my story, while exploring multiple alternate Earths, our space telescopes have discovered one that is even just as dark as TrES-2b. But it wasn't orbiting nearby, so it couldn't have been hot enough to absorb the heat of its sun. And atmospheric gases are similar to what our Earth's may have been like prior to the Great Oxygenation Event 2.4Ga, not at all compatible with the supposed chemistry of The Dark Knight's atmosphere.

How else can this alternate Earth be blacker than black?

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    $\begingroup$ Can you be more specific about what qualifies as an alternate Earth for the sake of your question? Aside from composition, what about mass or radius? Does it need to have a water cycle and Earth-like temperatures? $\endgroup$ – Zxyrra Jan 28 '20 at 1:26
  • $\begingroup$ You want an Earth-like world (pre Oxygen) with an absurdly low reflectivity? $\endgroup$ – SRM Jan 28 '20 at 1:31
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    $\begingroup$ @JohnWDailey Would you mind editing those clarifications into the question? I think people generally talk about radii instead of diameter because astrophysics is radius-centric; you calculate orbits and gravity from the radius instead of the diameter. $\endgroup$ – Zxyrra Jan 28 '20 at 3:10
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    $\begingroup$ HD 149026b has a outer atmosphere consist of vanadium and titanium oxide. $\endgroup$ – user6760 Jan 28 '20 at 3:14
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    $\begingroup$ @Astrid_Redfern The former, yes. $\endgroup$ – JohnWDailey Feb 1 '20 at 17:37

If we start with a somewhat strange planet (mostly quartz, Iron, nickel and aluminum) and give it an atmosphere of almost pure carbon monoxide and dioxide. This is highly improbable to begin with but if we give it a very strong electric field and then hit with the mother of all solar flares you just might get optically aligned carbon nanotube growth as the planet surface approaches 1100c. Optically aligned carbon nanotubes, sometimes called Vantablack, are exceptionally dark, absorbing 99.6% of light. It's a bit of a stretch though.

Another alternative would be a world blanketing bed of fungi or mold with a velvet like texture. Just plain black velvet has a surprisingly low albedo, absorbing over 99% of light. This could be a fun kind of biosphere apocalypse scenario. A planet with a slowly dying star and eventually the only thing or things that can survive is a world spanning mat of knee deep, velvety fungus that's using carbon or some other incredibly dark material in an incredibly fine, dense tangle to utilize every last photon to survive.

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    $\begingroup$ I was considering how a Carbon Planet could be used to answer this, and I think this is more or less how. The problem is that it's highly unlikely to get a Carbon Planet without hydrocarbons, and those will tend to mess with the color. If there's some way to keep hydrocarbons out of sight? Maybe there was a great oxygenation event that removed most atmospheric hydrocarbons, but then any life responsible died, and the planet was too geologically dead / sterilized by radiation or somesuch, so most hydrocarbons are buried, or sequestered in dark material? Temperatures throughout all that, though… $\endgroup$ – CAE Jones Jan 28 '20 at 12:25

You say this small albedo is most probably closely connected to high temperatures. "1100 degrees Celsius hot enough to melt gold" is a large underestimation. It is hot enough to melt some rocks and most metals! And together with high sun radiation and magnetic fields it is hot enough to form plasma.

Plasma is a key here - it can both be a perfect mirror or a perfect absorber depending on condition and frequency of electromagnetic radiation. For example our Sun, having "plasma envelope" is also an "almost black body".

And being a black body doesn't mean "pitch black". It's quite the reverse: it means - shine brightly (at least at some specter - IR, visible, X-ray, so on.) with our Sun being good example.

So if we put that "Dark Knight" planet away from its star it would rise its albedo dramatically.

To have such a low albedo without plasma is impossible - there is just no such material. But using some special "metamaterials" we can create the illusion of super low albedo. The metamaterials I have in mind are small glass balls (which are closely connected to cooling down plasma). They act as perfect retroreflectors - all light which is coming to that planet is reflected to its source. So sand made from these glass balls would look black if looked at from the side, but would shine brightly if we looked from the light source.

Btw, we already have such situation before our own eyes: the Moon. Its surface shines brightly when the Moon is full. But its albedo drops greatly (lower than black earth) when it's a half moon.

The only thing that can spoil everything - is atmosphere. For our Earth atmosphere plays a great role in its albedo.

So, to summarize, you need the following to create "The Cold Dark Knight":

  1. No atmosphere at all
  2. Planet is made mostly of silicates (to form glass)
  3. Planet is covered with a sand/gravel mix made from small (0.1 - 10 mm) nearly perfect glass balls

This can be achived if the planet quite recently got away from being close from a star:

  1. All its "normal" atmosphere was blown away with solar winds
  2. Cooling plasma would form that "glass ball" sand (it's actually one of the technological processes to form fine glass balls for retroreflectors)
  3. Asteroids and tectonics would not destroy that glass-field yet.

Btw, for the last - asteroid craters would form very bright circles on that black surface. And do not look at that planet from starside: you may blind your eyes and devices.

  • $\begingroup$ You say "it's actually one of the technological processes to form fine glass balls for retroreflectors" - Is there any chance you could edit your answer to link to a source for that? Preferably one containing some more information on the process. $\endgroup$ – Astrid_Redfern Feb 17 '20 at 8:23
  • $\begingroup$ One of my own answers (worldbuilding.stackexchange.com/a/167939/61035) describes a scenario in which Earth's moon becomes very like this Cold Dark Knight. The bit about the silicates forming into small glass balls is new to me, though. I'd had them as a crystalline surface. This is why I'm interested in knowing about their formation into spheres. $\endgroup$ – Astrid_Redfern Feb 17 '20 at 8:28

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