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Okay, so I've found some threads about the day/night cycle and seasons of habitable moons orbiting gas giants, but what if we replace the gas giant by a brown dwarf (i.e. the sub-stellar object class that's between a star and a planet in mass and properties)? The basic idea of my world is that the habitable portion consists of five habitable moons orbiting this brown, which itself orbits the system's star. If my understanding is correct, these moons would have different temperatures based on their proximity to the brown dwarf as it does emit a small amount of radiation. What messes me up a bit is the effect that orbiting a brown dwarf would have on the day/night cycle of the moons as well as the seasons. First, I know it's probably best to assume that all the moons are tide-locked, which means that one side of each has a day (that varies in length based on orbit) which the brown dwarf interrupts with an eclipse and one side which never sees the brown dwarf as it faces out. So what do we get, a day/night cycle in which one side has brighter days, darker nights, and more temperature fluctuations, and the other side has dimmer days, brighter nights and less temperature fluctuations? What effect would that even have on climate?

Now, what if I wanted to add some slight seasons in the mix? Do I just need to tilt the orbit of the brown dwarf and its satellites? And what impact would that have on the day/night cycle?

So as a summary, I'm asking 3 main questions:

  1. (Main question): What are the differences between living on a (hypothetically) habitable moon tidally-locked to a gas giant vs tidally-locked to a brown dwarf
  2. (Sub-question): What impact would the planet --> brown dwarf switch have on the day/night cycle of a habitable moon and its climate
  3. Assuming I am correct in saying that I can create some seasons by tilting the axis of the whole system, what would be the impact of doing this.

Pick and choose which questions you feel like answering, I'm not picky. Thanks!

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    $\begingroup$ You will probably get better results if you split your three questions up into... well, three different questions. $\endgroup$ Commented Dec 9, 2020 at 5:40
  • $\begingroup$ And you need to provide LOTS more information about your system. Type of parent star. Mass of brown dwarf. orbital distance of same. orbital distances of the moons. Are they tidally locked, resonant orbits, or just doing their own rotations? Just stating "brown dwarf" answers none of these needed data $\endgroup$
    – PcMan
    Commented Dec 27, 2020 at 20:28

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What is a brown dwarf?

The first thing to make clear is that a brown dwarf is what is called a substellar object, commonly called a failed star. While it bears some similarities to gas giants, there is one important criteria for an object to be classified as a brown dwarf: it must be able to sustain deuterium fusion, which puts it at minimum size of 13x mass of Jupiter. If its smaller, it's a gas giant. Additionally, large brown dwarfs can also sustain the fusion of lithium, though unlike main sequence stars they are too small to fuse hydrogen - hence "failed star".

With that said, there is nuclear fusion in there, so yes, it does produce radiation, and it is more accurate to treat a brown dwarf like a star rather than a planet. As such, your system would be a binary system and your moons would actually be planets.

With that out of the way, onto your questions:

  1. A planet tidally locked to a brown dwarf would have the side facing it constantly bombarded by radiation - primarily X-ray and infrared. It would also heat up in the process. It is theoritically possible that a very small habitable zone exists around the brown dwarf, though I would assume in your case the entire brown dwarf system would be located in habitable zone of the primary star. In any case the planets would need to have magnetosphere strong enough to protect both against the radiation of the dwarf and the primary star.

  2. I'd say the day-night cycle would stay similar, The brown dwarf produces radation in X-ray and infrared spectrum, and very little is in the visible light spectrum, though this can also change depending on the stellar class of the particular brown dwarf in question. Some scientists consider the M-class, which does glow, to be a red dwarf, for example. Regarding the day-night cycle I'd say that the planet-side facing away from the dwarf would have a normal day-night cycle, with half an orbit of sunlight and half an orbit of darkness. The side facing the dwarf would be warmer and a move from daylight to sunlight twice in one orbit - first facing away from the primary star, then moving into daytime as it moves a quarter of the orbit, move to nighttime again as it passes through shadow of the dwarf, back to daylight, back to nighttime as it turns away from the primary star. Perhaps there could even be a portion of surface that is constant night, depending on the orbit distance from the dwarf and planet tilt in relation to primary star. Depending on the class of the dwarf, the night on the planetside facing it could have a dim red coloration and it would likely be hot due to constant radiation from the dwarf.

  3. Yes you can tilt the axis of the whole brown dwarf system and the impact would indeed be creating seasons. The angle of the tilt would dictate how extreme they are. You can achieve that by tilting the planets relative to the primary star. Either tilt the orbit of the dwarf itself or tilt the plane in which the planets orbit the dwarf. Technically you could also tilt the axis of planets themselves, but this would mean they are no longer tidally locked. It would also have a lesser impact, as you're altering the effect of brown dwarf's heat on climate rather than the main star's.

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  • $\begingroup$ Awesome answer. Thank you so much! $\endgroup$
    – WordNerd
    Commented Dec 12, 2020 at 15:20
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Lots of Question

That cant be fully answered. So if i get you right, we have a Brown Dwarf, Orbiting a Star while having 5 Moons ?

The Answer as to how this would look depends a lot of the Distance and mass of each Body. A Brown Dwarf can be pretty big. And if the central star is small enough, you would have a Binary System between the Star and Dwarf.

But ok, lets just assume it works out fine.

What is it like

It depends. Brown Dwarfs barly glow at all. Most of there "Light" is emitted over Infrared. Thus, you wouldnt get any additional Light in your Orbit.

This is very much like what a Moon around a very dark Gas Giant would see. But then again, pretty much all Gas Giants are not Black, thus in General, your Moons have less light.

To make it short, there is really no major differnece between a Brown Dwarf and a Gas Giant besides the fact that the Dwarf dosnt reflect as much light. But the Mechanics of the Orbits, day night Cycle and stuff would be more or less the same.

The one major difference is that the reflected light from the Gas Giant would not exsist in this case. Or not so strongly. Thus, your nights are a lot darker.

Impacts on the Day Night Cycle

Non.

Besides that the Brown Dwarf dosnt reflect as much light on the Moons surface.

Seasons

Nobody knows. The impacts are depended on the Angle. You can sort of create Seasons by tilting the system, but the Impacts are well beyond anyone tbh. If we dont know the angle, the Impacts range from "Non at all" to "apocalypse now".

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A Brown Dwarf is just another heavy planetary body.
It does not generate its own heat via hydrogen and helium fusion, although they do tend to start out very hot and stay hot for extended periods due to gravitational contraction. But hot here means hot on planetary scales, not anything like a star.

A lot of the other answers and comments here presuppose that the brown dwarf is orbiting very close to an active star... this is completely unrelated.
The only reason that most brown dwarfs currently known do orbit close to their parent star, is because the means we have to detect brown dwarfs can only detect exactly those in this situation. A brown dwarf in a pluto-sized orbit would be utterly undetectable to our current methods, and one in a Mars-distance orbit would be very hard to detect. But one that is rubbing right up to its parent is readily visible using 2010-2020 technologies.

They also assume that anything orbiting around the brown dwarf will be close-in, and tidally locked. This, too, is a baseless assumption.

So a brown dwarf, the same as any large planet or small star or large star, can have things orbiting it. At many possible orbital distances. And itself can be orbiting a star or stars, at orbital distances from so close it almost touches the star, out to many times the distance of Pluto's orbit.

It is also quite possible for a brown dwarf to exist without a parent star, out in the void. With or without its own entourage of moons.

Really, all that one knows if you have a brown dwarf is that you have a really large planet (at least 13 times Jupiter mass), but one that is not large enough to sustain hydrogen and helium fusion in its core (about 90 times Jupiter mass).
Its orbital parameters, parent object, and moons could be almost anything.

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