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I’m thinking about making a detailed world on Luhman 16 B (not around the star, right on it). This brown dwarf's mass is 28.6 times that of Jupiter and has a radius 104% that of Jupiter. It has a temperature of 1210 degrees K, with methane and iron rain in the atmosphere. What would the surface (or something similar) of such a world look like?

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    $\begingroup$ Would there be a surface? Luhman 16 B is still a star, so it's made of mostly H+He. $\endgroup$
    – Vesper
    May 18, 2023 at 13:21
  • $\begingroup$ It would have less of a surface than jupiter, and it wouldn't be nearly as hospitable. I don't think you'll really get iron rain either, but you might get a trace of vapor in what passes for an atmosphere. $\endgroup$ May 18, 2023 at 13:40
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    $\begingroup$ Any time you have a section of atmosphere that is hot enough to vaporize a material, with a cooler layer of atmosphere above it, you'll get rain of that material. The problem I've had with that statement is that, when you describe something as "rain," there should be a surface for it to rain upon. There is an ocean of compressed hydrogen somewhere deep inside of a gas giant, but the same forces that keep it liquid would keep iron from vaporizing. $\endgroup$ May 18, 2023 at 15:08
  • $\begingroup$ I'm going to add my voice to those doubting that you'd get any meaningful "rain" on a star. $\endgroup$
    – jdunlop
    May 18, 2023 at 18:24

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I'm not an astronomer, but I'll try to answer.

As far as I know, stars are made of mostly hydrogen and helium. This makes them just a large clouds of gas. Typically, stars's density is quite low: for example, our Sun has density almost like water's (1.4 vsd 1.0 g/cm^3). But you would not swim in the Sun, obviously, it's tad bit too hot.

'Brown dwarfs', or 'failed stars' are still stars, they are made of star stuff too. Their feature is that they are much, much more dense: 10 to 1000g/cm^3 (according to the Wikipedia). Its 'surface', if we can call it that, would be just like star's: very hot (thousands of Kelvins); and it's still hydrogen.

Your numbers fit nicely into these definitions, so far.

It's dense, but will it be solid? Most likely no, hydrogen needs very special conditions to become a solid. It won't be in the liquid state as well: Wiki states that you need a much lower temperatures and much higher pressure for it to condense. It will be in a 'metallic' state, most likely. But it's called that because it conducts electricity, not because you can make spoons or nails out of it. As far as I've researched, it's still a gas.

(It makes me wonder how having an gaseous atmosphere that is much denser than densest construction materials on Earth would feel like. Unfortunately, it seems that I won't experience that in my life)

In other words, there will be no solid 'surface' with well-defined boundary, like Earth's walkable surface. It will be more like Earth's atmosphere: there is no clear boundary; but much hotter, and once you descend deep enough, very dense.

To sum up: its surface will look like any stars'; it will get much denser quickly if you decide to dive deep, but that's it.


Anyways, good question! Yes, Jupiter is a 'small brother' of brown dwarfs; yes, your star has to be order of magnitudes more massive than Jupiter to be a 'brown dwarf'. Numbers are chosen well, but it there won't be enough iron to have an iron rain. It is still a big gas cloud, a boring star. No cities on the surface, since there is no one.

If you need a cool setting for your story: I could not find how diffuse a brown dwarf's 'edge' is; but assuming your star is cold enough, and density gradient is extreme enough (~kilometer to reach >~ 10g/cm^3), you can make a habitat just above the star 'surface' that is supported by long pillars made of tungstein. It won't melt at 1000K, and it is less dense than brown dwarf's hydrogenic body, so it will 'float' in it, providing bouancy for the whole habitat. Add some shielding for the habitat (large sun shades at the bottom) and coolind (enormous radiator metal plates at the top). Now, you got almost what you requested, and it is unusual and cool (almost as walking on a star surface). Still no iron rains, unfortunately. Maybe some devices that capture evaporating tungstein vapor (due to reverse sublimation) - if you need it for some reason. With some handwaving, you can make it work.

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    $\begingroup$ Brown dwarfs are not stars, they are considered to be a separate class, more massive than planets and less massive than stars. So a brown dwarf would be somewhere between a gas giant planet several times as massive as Jupiter and a very small and low mass star star. $\endgroup$ May 18, 2023 at 17:14
  • $\begingroup$ Gasses and fluids in a mechanical sense both adhere to the laws of fluid dynamics. An extremely dense gas as you will find here, is probably behaving pretty much the same as a fluid of the same density. $\endgroup$
    – Tonny
    May 18, 2023 at 18:59
  • $\begingroup$ Brown dwarfs are "only" 13 times Jupiter's mass or greater, not multiple orders of magnitude greater than Jupiter's mass — a body with 100x Jupiter's mass is almost certainly a red dwarf, a true star, and not a brown dwarf. $\endgroup$
    – KEY_ABRADE
    May 18, 2023 at 20:38
  • $\begingroup$ "our Sun has density almost like water's" — surely you must mean average density, which would include it's atmosphere, which is pretty sparse the further you get from the sun. $\endgroup$ May 19, 2023 at 23:32

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