Life on moons orbiting a brown dwarf (or gas giant)

[Brutal Mustang]

Okay. I'm about to crash my head into my keyboard trying to figure something out. Are there any extreme geeks on this board that can help me out?

What I need two worlds that are rather close together: a rich-in-life, fertile planet covered in oceans where humans will be living (about 1g); and a drier, harsher planet where an alien race will be living (less than 1g) - in fact, the alien planet can be so harsh that they mostly live deep underground.

There is the obvious: create two planets in the Goldilocks zone orbiting a star. Boring - I want a more alien backdrop for my characters than that!

I prefer the idea of them living on moons that are orbiting a brown dwarf (substar), which in turn would be orbiting a normal star.

Here is what I think I know:

The moons would be tide-locked to the brown dwarf.
A day on one of the moons would be the same as their orbit around the brown dwarf. Here is what I don’t know for sure:

They would have no seasons, right? Since their day is also their year?
Since the brown dwarf would be away from the primary sun (like Jupiter), it would be providing a good percentage of warmth to the moons, right?
The sides of the moons tide-locked to the dwarf would be the warmer halves of the moons?
The moon with the oceans would have more uniform heat?
What would radiation coming from the brown dwarf do to these moons?
What about volcanoes? Jupiter’s moon Io is so volcanic because Jupiter’s gravity is pulling too hard on it, right?
What length of day could these planets have and still support life? Is set up plausible at all?

[Brutal Mustang]

P.S. Sorry about the small font. It just went screwy on me, and I can't seem to fix it for the world.

[blacbird]

I think you have a great deal of artistic freedom here. Fact is, we really know dang little about brown dwarfs. Only a few have actually been seen, and all of these, as far as I know, in orbital association with brighter stars. There's little doubt that individual independent brown dwarf objects exist, but they are abominably difficult to detect, even with our best instruments. Chances are we'll find many more with the newer generations of infrared telescopes and detectors currently in the works.

Which brings up a major issue for you: These objects emit the vast majority of their radiation in infrared wavelengths, with little visible light spectrum involved. Your planetary inhabitants, if you plan such, will need to be able to work with infrared as their visual spectrum, I would think. And, of course, the issue of sufficient warmth for the moon/planet is also involved. But I don't think you need to go overboard on the science, given how little is really understood about these things.

caw

[Brutal Mustang]

Thank you Blacbird. I've been running all over the net like crazy, trying to find some answers.

Obviously, the bright light to my moons would come from the primary sun, not the brown dwarf. I would think the dwarf would be a huge, red glowing orb, always about at the same place in the sky, not emitting too much light (you could look at it easily with the naked eye).

Artistic freedom. I like that!

[Pthom]

A moon tide-locked to its planet has, as you suggest, a day the length of its orbit around the planet. However, it's year is the same as the planet's year: one orbit around the star. The moon will at times experience solar eclipses, that depending on the speed of the moon's orbit and the size of the planet could be quite long. This period might correspond to a sort of brief winter because of lack of solar radiation. For an interesting "seasonal" situation (and very alien) read Vinge's A Deepness in the Sky.

The face of the moon facing the planet is only warmed especially more than the other face if the planet radiates heat. Depending on the planet's proximity to its star, there could be more heat contribution from the planet, or more from the star. I think you can make that part up to suit your story.

If the gas giant is, as Jupiter is purported to be, close to a brown dwarf and produces quite a bit of not only thermal radiation but other types as well, couldn't you place your lush moon at a distance that is "safe" and have it orbit the giant at such a rate so that the "day" isn't too awfully long. You could place your other moon closer to the gas giant, subjecting it to extreme planetary radiation, or farther away with a much slower orbit so that its days are close to impossibly long.

Again, I think you can just make up a lot of this to support your story.

[Brutal Mustang]

However, it's year is the same as the planet's year: one orbit around the star.

So the tilt of the moons stays the same in respect to the primary sun, and not the brown dwarf?

If the gas giant is, as Jupiter is purported to be, close to a brown dwarf and produces quite a bit of not only thermal radiation but other types as well, couldn't you place your lush moon at a distance that is "safe" and have it orbit the giant at such a rate so that the "day" isn't too awfully long. You could place your other moon closer to the gas giant, subjecting it to extreme planetary radiation, or farther away with a much slower orbit so that its days are close to impossibly long.That's exactly what I was thinking. I like the idea of the harsher planet being volcanic, like Io. Maybe not to the same degree, but definitely creepy in that way. And hotter. So perhaps having it closer to the brown dwarf would be a good idea. That way it would be getting more heat plus the gravitational pull causing the volcanic activity.

[Pthom]

So the tilt of the moons stays the same in respect to the primary sun, and not the brown dwarf? In most planetary systems, moons orbit their planets more or less along the planet's equator. In our solar system there are few instances where that varies more than a few degrees. I know of no system where a moon might orbit a planet in a polar orbit. Such an orbit is unstable over the kinds of time periods necessary for life to have evolved on said moons. Similarly, planets orbit their stars more or less along the star's equator. Again, this situation seldom varies more than a few degrees.

I think if a moon's axis is tilted with respect to its plane of revolution, then it would be unlikely that the moon is in tidal lock with the planet. But for your story, is it necessary for the lush moon to be in tidal lock? If it had, say, an axial tilt, rotated more than once per revolution, then the environment there might be quite terrestial, or more likely so than otherwise.

The other moon, then, in tidal lock, suffering from impossibly long day/years, extremes in temperature, radiation, etc., is a place people would want to be from, but not go to so much.

That's exactly what I was thinking. I like the idea of the harsher planet being volcanic, like Io. Maybe not to the same degree, but definitely creepy in that way. And hotter. So perhaps having it closer to the brown dwarf would be a good idea. That way it would be getting more heat plus the gravitational pull causing the volcanic activity.

On Earth, volcanism is almost always a function of plate tectonics. A moon in tidal lock doesn't necessarily experience that, nor would it be, as far as I'm aware, likely. As I mention above, being in a high, slow orbit above a gas giant/brown dwarf is such a severe environment that something like a volcano would be a delight to experience. :)

In all this we're still assuming your gas giant/brown dwarf is in orbit about another, hotter star, at least as hot as our Sun or more so?

[Brutal Mustang]

But for your story, is it necessary for the lush moon to be in tidal lock?

No it's not. Somewhere I got the idea that it would be probable. It certainly would make it easier to describe the sky, though.:D

The other moon, then, in tidal lock, suffering from impossibly long day/years, extremes in temperature, radiation, etc., is a place people would want to be from, but not go to so much.Spindly legged aliens live there; one of the things keeping the aliens from invading the lush planet is that the gravity is a bit too much for their lofty physiques.

On Earth, volcanism is almost always a function of plate tectonics. A moon in tidal lock doesn't necessarily experience that, nor would it be, as far as I'm aware, likely. As I mention above, being in a high, slow orbit above a gas giant/brown dwarf is such a severe environment that something like a volcano would be a delight to experience. :)

In all this we're still assuming your gas giant/brown dwarf is in orbit about another, hotter star, at least as hot as our Sun or more so?Yes, another star as hot as our sun. But wait, I don't get it, what about Jupiter's Io? It's tide-locked, yet is it not it volcanic because of Jupiter's gravity pulling on it?

[Pthom]

High gravity and spindly legs don't go well together. It would be the opposite. On a planet with greater than 1 gee, beings would be stockier. (Ref: Niven's planet Jinx.) On a planet with lower gravity, the beings are more fragile, not needing thick bones and powerful muscles to move around.

The location of the moon with respect to it's primary has little to do with how much gravity there is there. More, it has to do with what the moon is made of--and its size. If you want your hellacious moon to have higher gravity, it must be more dense. If it is then smaller than a moon that is less dense, then it will also have different orbital characteristics. Move it too close to the primary and have it move too slowly, its orbit will degrade rapidly. Move it out from the planet too far and have it move too fast, you may just lose the thing to outer space.

Realize that such degradations take place over very long time periods compared to the lives of your characters. Yet you want your planets to have existed pretty much the way they are in your story for a very long time (millions of years), for their environments to stablilze and life to arise.

As for the sky? Well. Your avatar pic looks like the sky in the situation you describe. There could be also the smaller moon, other (uninhabited moons), and the main star. All in the sky at once. And don't forget the wow factor of the eclipse that happens once each "day" on your lush moon.

[Smiling Ted]

It's tide-locked, yet is it not it volcanic because of Jupiter's gravity pulling on it?

Tidal stresses can sometimes create vulcanism by squeezing and elongating the moon.

[Pthom]

But such a body would need a molten core, no? If tidal forces are enough to melt rock I imagine they are also capable of destroying the moon, long before life might arise there.

[blacbird]

Tidal stresses can sometimes create vulcanism by squeezing and elongating the moon.

The most obvious example being Jupiter's moon Io, and less obviously, but intriguing, perhaps Saturn's moon Enceladus.

caw

[lpetrich]

... What I need two worlds that are rather close together: a rich-in-life, fertile planet covered in oceans where humans will be living (about 1g); and a drier, harsher planet where an alien race will be living (less than 1g) - in fact, the alien planet can be so harsh that they mostly live deep underground. ...

I prefer the idea of them living on moons that are orbiting a brown dwarf (substar), which in turn would be orbiting a normal star.
Good idea.

The moons would be tide-locked to the brown dwarf.
Very likely, given how our Solar System is.

A day on one of the moons would be the same as their orbit around the brown dwarf.
Yes, but that period would not be the year, which would be the orbital period of the brown dwarf around its primary.

They would have no seasons, right? Since their day is also their year?
There would be no seasons if the BD's spin axis is close to its orbit axis, but the example of the Solar System suggests that it may be tilted enough to make noticeable seasons.

The BD's rotation would make an equatoral bulge, which would make its satellites' orbits precess around its equatorial plane, which could produce additional climate complications.

Since the brown dwarf would be away from the primary sun (like Jupiter), it would be providing a good percentage of warmth to the moons, right?
Maybe, maybe not. From Brown Dwarf (Wikipedia) (http://en.wikipedia.org/wiki/Brown_dwarf), they have the size of the planet Jupiter and surface temperatures from 750 K to 2200 K.

T(distance a, max) = T(source with radius r) * (r/a)^(1/2)

T(distance a, avg) = T(distance a, max) * 1/sqrt(2)

The last factor is a reradiation factor.

For r = 70,000 km and a = 630,000 km, the planet's BD-induced surface temperature is 250 K to 700 K (max) or 180 K to 500 K (avg). If the planet was 4 times further away, the temperatures would be reduced by a factor of 2.

The sides of the moons tide-locked to the dwarf would be the warmer halves of the moons?
Yes, though it may not be by much if the planet is too far away.

The moon with the oceans would have more uniform heat?
Yes.

What would radiation coming from the brown dwarf do to these moons?
THe BD may have a big Jupiter-like magnetosphere, complete with trapped particles, which may give the planets continual auroras.

What length of day could these planets have and still support life?
Offhand, I can't think of any upper limit -- it would depend on what sort of organism. They'd likely be growing and active during the long day, and hunkered down and hibernating during the long night.

There would be an interesting complication if the planet received close to the same amount of light energy from the star and the BD. The planet would alternate from being about equally illuminated (full-BD) to being illuminated on one side (new-BD).

So the tilt of the moons stays the same in respect to the primary sun, and not the brown dwarf?
Their spin axes will likely be aligned or close to aligned to to their orbit axes around the BD.

(Io and its volcanoes...)
Tidal stresses can sometimes create vulcanism by squeezing and elongating the moon.
That happens to Io as a result of having an orbit with a forced eccentricity.

That eccentricity makes Jupiter's tides on it vary over its orbit, distorting it and thus heating it.

The forced eccentricity is the result of an orbit resonance with Europa, Ganymede, and Callisto; their orbit periods are in approximate proportions 1:2:4:8. Such resonances could exist in this hypothetical system.

[Brutal Mustang]

Ah, thanks Ipetrich! Your post is very informative. I get it now: the tilt of the brown dwarf gives its moons its seasons. Sigh. Thank you! :kiss: