The Case for Habitable Exoplanet Moons

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It's a fascinating thought. What would our culture be like if we evolved on a satellite to a gas giant? How would this impact the discovery that the moon was not the centre of the universe?

Taken further, what if they evolved on a moon that orbits a habitable planet?

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Assuming the satellite had a breathable atmosphere and reasonably clear skies (that disqualifies Europa and any other iced-over ocean), I suspect we would figure out that our world is not the center of the Universe much sooner than we did in real history. For starters, we would see other moons visibly grow and shrink as they approached and receded -- and their traversals in front and behind the giant planet would be very obvious. OTOH, it would be equally "obvious" that the striped God with angry red eye IS the center of the Universe -- it hangs motionless forever at the same spot in the sky while everything else, including the Sun-God, whirls around it.

"Joviocentrism" might prove as durable as geocentrism.

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Great article, thanks.

The prospect of habitable Jovian moons is almost more interesting than your typical vanilla Earth analogues.

There is only one issue that I worry about with respect to habitable Jovian satellites: their primary's magnetic field. Unless the moon had a powerful field of its own, it would be subjected to pitiless irradiation and steady erosion of its atmosphere, especially if the primary had radiation belts like Jupiter.

<<Taken further, what if they evolved on a moon that orbits a habitable planet?>>

I think that would effectively make exploring any other planet besides their primary a moot point, assuming that the others in the system are uninhabitable.

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

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Here is a link I keep handy for worldbuilding;
http://skyandtelescope.com/resources...icle_255_1.asp

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Unless there is a way for an earth-like body (even if a moon) to generate a strong magnetic field other than rotation, then that practically bars the moons closest in to the gas giant - those moons are likely to be rotation-locked (the same hemisphere always facing the planet).

However, I think a moon fairly far out from the gas giant might still have potential for life of earth-level sophistication (notice I said sophistication, not type)

A rotation-locked moon still rotates, and a moon CLOSE to a gas giant planet rotates pretty fast. Io makes one revolution every 1.8 days -- fast enough to generate magnetic field if it were big enough.

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Fiction has to be plausible. Reality is under no such constraint.

Close-in moons also have the issue of the planet's magnetic field - the surface of Io and Europa are both pretty much uninhabitable for that reason.

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Everything I need to know I learned through Googling.

If satellite atmospheres erode, how do we explain Titan's retention of a major atmosphere?

We keep going to Jupiter as an example, and what if that's a bad one? Maybe we should consider inhabitable moons around Neptune analogues? Smaller gas giants with proportionally smaller and less powerful magnetic fields.

And what if the inhabitable moon were actually a captured planet? Something maybe half again bigger than Mars to something Earth sized.

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Man, the religions of a planet with a sky like that would be something to behold. I think I would get vertigo just looking at such a sky. Natural philosophy would be kickstarted handily in such an environment; the people's wonder at their cosmos would be far greater than our own.

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It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

Or it could well be that they give it no more thought than we have given the Moon over the course of our history. Our own wonderment about the heavens hasn't been exactly without close orbiting inspiration. We might look down on the old Moon now because now that we know more about the universe, its kind of a mundane place, but that's really only been in the last century or so of our history.

Even worse, a gas giant is no place they will ever have a chance to visit, and the dangers of radiation for leaving their protective magnetic field will be FAR more costly. Leaving home could also be something they consider entirely too dangerous for them to do than it is us. To say nothing of having far less darksky time because of the constantly overhead planet and the rotation with respect to the star causing substantially less than half a day's worth of stargazing time.

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.

<<Or it could well be that they give it no more thought than we have given the Moon over the course of our history. Our own wonderment about the heavens hasn't been exactly without close orbiting inspiration. We might look down on the old Moon now because now that we know more about the universe, its kind of a mundane place, but that's really only been in the last century or so of our history.>>

Ditto. We're the only terrestrial planet with a large moon, and nobody outside of the scientific community really pays attention to it. It's just too familiar. Sort of like the running joke that the only people in San Antonio who have never been to the Alamo are...San Antonians.

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

I am told that Titan orbits mostly beyond the effects of Saturn's magnetic field. So a moon might be habitable around a Saturn-like planet (within the habtable zone, of course) at Titan's orbital distance from the planet.

Except this causes a different set of problems; Titan has an orbital period of sixteen days; if the moon concerned is tidally locked it will have a day length sixteen times as long as our own.

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Orion's Arm . The Starlark . Voices: Future Tense . OA Flickr set

On the other hand, if the moon is far enough from the planet to escape tidal locking, it might have its own rotation rate. Are any moons of our solar system's gas giants such worlds?

^
Hyperion, IIRC, is not tidally-locked. However, its rotation is also chaotic.

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

It might be uninhabitable to normal earthlike life. But intense radiation doesn't have to kill everything, esp if the cells divide fast enough, or keep their genetic instructions in many smaller self-sufficient units.

Saturn doesn't have nearly the magnetic field that jupiter does, but it could be that the magnetic field is proportional to the size of the gas giant.

Question: Wouldn't the radiation due to the magnetic field be entirely charged? If so, wouldn't any atmosphere worth it's salt handily absorb it?

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Looks like there is a limit on the size of exoplanet moons:

Gas Giants Gobbled Up Most of Their Moons
http://www.universetoday.com/2006/06...f-their-moons/
June 16th, 2006

Even though our Solar System’s gas giants vary widely in size and mass, they
do have something in common. Each planet is roughly 10,000 times more
massive than the combined mass of all their moons. During planetary
formation, rocky moons grew out of the solid material surrounding each
planet. As these moons grew larger, leftover gas slowed them down, and
they fell into the planet to be consumed. The moons we see today were the
last ones to form around their parent planets, after the gas had dissipated.


I don't know why this article gives wrong Giant Planet/Moon ratio?
In reality, it is not "roughly 10,000 times" but rather around 4000 times.
(Jupiter - 4828 times; Saturn - 4019; Uranius - 9514; Neptune - 4789 times)

It means, if limit is 4000 times smaller moon around Giant Planet,
then, in order to have at least moon with mass of at least 1/4 mass Earth,
the super giant planet MUST have at least 3 times the mass of Jupiter!

I picked mass of 1/4 mass Earth's, having in mind that that mass is
really quite enough for atmosphere to stay around that moon
for long enough time to support developing life up to the stage of
inteligent humans, or better say, intellingent ETs, our brothers and sisters
in the Universe.

Not a problem, they've been finding gas giants ten times more massive than Jupiter.

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.

Yes, and just for the information, here is most uptodated list of planets:
http://vo.obspm.fr/exoplanetes/encyclo/catalog.php

Woo, they inlcuded into a list planets with up to 20 mass of Jupiter!

So when does fusion start and you get a brown dwarf anyways?

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