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Brown dwarfs are celestial bodies in the gray area between planet and star.

They're huge, gaseous, hot compared to planets, and come in all different kinds.

(1) Is it possible for life to develop in the outer layers of such an object?
(2) If so, how large and/or complex could it get before being constrained by resources, heat etc.?


  • Possible means able to happen at all. This is not to be confused with likely - I don't expect life to be common across brown dwarfs - but I want to know if it's realistic to even consider these circumstances happening once.
  • Life means something that reproduces and evolves, that adapts to its environment, and that responds to stimuli. DNA is in no way necessary, nor are cells, to meet this criteria. Complexity is not a requirement to be "alive".
  • To develop can include the creation of life by abiogenesis or the arrival of life through panspermia but either way it must not perish; it should be able to evolve and reproduce in its environment.
  • A brown dwarf for this question is an object with > 13 $M_J$, so that deuterium fusion occurs comfortably. Its composition must occur naturally, and its volume and weather dynamics should be realistic, but you may determine other characteristics if it helps life develop. It may not orbit another object but it can have natural satellites if you so desire.

Related to but not a duplicate of Could a society exist within a brown dwarf which asks about humans settling within the atmosphere of such an object, as opposed to microorganisms evolving and growing complex, in that environment.

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  • $\begingroup$ I'm on the border for voting to close for too broad. I don't see how an 'is it possible' question can be answered with anything other than yes. I read this as, basically, 'invent a way that life can survive on a brown dwarf'. And if you worded it that way, it would get closed. $\endgroup$ – kingledion Dec 18 '16 at 20:14
  • $\begingroup$ @kingledion The whole reality-check tag seems to work just fine. If the genuine answer is no then answer no as opposed to closing because people make up their yes answers. $\endgroup$ – Zxyrra Dec 18 '16 at 20:48
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There's a few major issues which I highly doubt can be reasonably overcome:

The surface temperature

The coolest brown dwarfs appear to have a minimum temperature of around 500k, or 226 celcius - well above the boiling point of water. That immediately means life as we recognise it would not be able to form; simply boiling something is an easy way to sterilize it.

This acts as a limit for Earth life because cells use water as the carrier for a cocktail of proteins moving back and forth through the cell walls. Boil that water, destroy the cell. There have been suggestions that it might be possible for non-water based life to form in the Universe, so this point in itself might not be a limiting factor for life in general.

KO: Water based life.

Radiation

The intense magnetic field generated by such enormous bodies causes dangerous particles to get 'stuck' in it. This would make it extremely unlikely for the life to form on some other planet and arrive e.g. by comet, as the comet passing through this intense radiation belt would be sterilized; similarly the radiation levels on the surface are dangerously high too. In short, radiation destroys large molecules, such as proteins.

KO: Protein based life. Large molecules like polymers (plastics).

The strength of gravity

Your average brown dwarf is not much bigger than Jupiter, but is much denser so its gravitational strength is considerably higher. To give it the best chance, let's do a quick bit of maths to figure out the acceleration due to gravity on the bottom end of the brown dwarf scale:

  • M: At a minimum, about 13 Jupiter masses
  • R: At a minimum, about 110% Jupiter's
  • Acceleration due to gravity: g = G * M / R^2

Drop in the numbers..

g = 1.898 × 10^27 * 6.67408 × 10^-11 * 13 / (1.1 * 69,911,000)^2

g = 278.4 m/s^2

That's 10 times higher than the sun.

So, let's say you had the average mass of a typical American - about 80kg - giving you an applied gravitational force (F=m*a) of 22,240 newtons.

And this is where the problem is. That's beyond the crushing force of bone. By comparison, it takes about 7,560 newtons to completely fracture a femur. Bone has an incredible strength to weight ratio, so if you swapped bone for something like steel, it'd be crushed by the far higher mass that it's now got to support.

What about carbon nanotubes?

Carbon nanotubes are about as good as it gets when it comes to the strength to weight ratio, but they achieve that by being hollow. That, in turn, makes them very weak to compression.

All in all, complex life - even non-water-based forms - would likely be crushed by the gravity, or at the very least, completely unable to move in any useful way.

KO: Metallic and carbon-nanotube based 'mechanical' life.

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    $\begingroup$ What about extremophile organisms? They're relatively small and tolerant of heat. $\endgroup$ – Zxyrra Dec 18 '16 at 17:01
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    $\begingroup$ It's worth noting that there are brown dwarfs much cooler than the temperatures you provide, but they do not fuse. I reasoned that there may be a happy in-between at which deuterium fusion occurs yet the temperature is minimal toward the outside. $\endgroup$ – Zxyrra Dec 18 '16 at 17:02
  • $\begingroup$ @Zxyrra Temperature in itself isn't the biggest issue; i.e. if the temperature was suitable for Earth-style water based life, the proteins would be destroyed by radiation and the gravity would crush virtually anything that tried to move around. Even the slightest bit of motion would require a lot of energy too. $\endgroup$ – Luke Briggs Dec 18 '16 at 17:06
  • $\begingroup$ (I say 'virtually anything' because the only thing that I can think of that could possibly be capable of moving around would be a boulder rolling down a very gentle slope. I can't see any way of making that intelligent though!) $\endgroup$ – Luke Briggs Dec 18 '16 at 17:08
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    $\begingroup$ @Zxyrra I'd like to add to Luke that every complex particle is very prone to be destroyed by radiation, it's not specific to proteins. $\endgroup$ – Borsunho Dec 18 '16 at 19:05
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Plasma-based life could exist in brown dwarfs. According to Vadim Tsyovich's computer simulations, life could exist because of plasma's electrical properties. The plasma self-organizes into a double helix and splits into daughter plasma-structures. Only the most stable of the organisms survive until they reproduce.

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The real problem with life on a brown dwarf is going to be the lack of a surface. You're floating in an atmosphere of mostly hydrogen--what's your solvent? And biochemistry needs a variety of elements.

You're also going to have a very hard time powering it--where's the energy source? We think of the brown dwarf as emitting energy, but something on it will simply see that as it's environment, there's no source and sink to actually extract useful work.

I disagree about the threat radiation poses. High in the atmosphere it certainly would be dangerous--but so is Earth's ionosphere. So what? It also wouldn't stop life from arriving from elsewhere--life can't travel on the surface of a rock anyway because it wouldn't survive the fiery plunge. Only life deep on a rock can make it to another world.

Likewise, gravity. Swimmers don't care. Rather, we come back to the same problem--there's no real environment for life.

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