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cadoom
2009-Apr-16, 05:39 PM
Assume you have a star like 55 Cancri A, and what appears to be a gas giant orbiting in the habitable zone. Lets also assume that there is a moon roughly the size of Earth orbiting around it.

Wouldn't it be impossible for life to develop on such a world, as the radiation belts from gas giants are extremely intense, or is it possible to model a gas giant planet that doesn't have such a large radiative area, and thus allow life to potentially develop on the earth mass moon orbiting it?

Is this even answerable? Assume you have a Jupiter sized planet 1AU from its parent star, how would the magnetic field of such a planet be affected by being closer to the star, and would solar winds be dramatically stronger? How would the increased absorption of solar radiation affect a planet roughly equivalent to Jupiter?

antoniseb
2009-Apr-16, 06:12 PM
If the Moon was Earthlike, it would have an atmosphere which would stop the radiation, and it would probably have a magnetic field that would also help.

I think a bigger issue might be that such a Moon would have some serious tidal flexing going on, and earthquakes might make tall buildings kind of hard to maintain.

cadoom
2009-Apr-16, 06:32 PM
Thanks, I totally forgot to take the magnetic field of the earth sized world into account. I still wonder, although I don't know the math to figure it out, if the Earth's magnetic field would be able to adequately protect the planet in an environment, that I would assume, have a much greater level of incoming radiation.

But it gives me something else to consider...

chornedsnorkack
2009-Apr-16, 06:38 PM
I think a bigger issue might be that such a Moon would have some serious tidal flexing going on, and earthquakes might make tall buildings kind of hard to maintain.
Not if the orbit has low eccentricity.

NEOWatcher
2009-Apr-16, 06:42 PM
Not if the orbit has low eccentricity.
Give that comment a second thought.
Think about how the tidal forces on Earth are generally in a 12 hour cycle.

antoniseb
2009-Apr-16, 06:44 PM
... I still wonder ... if the Earth's magnetic field would be able to adequately protect the planet in an environment...

The magnetic field protects the outer layers of the atmosphere. The atmosphere stops the radiation very effectively.

chornedsnorkack
2009-Apr-16, 06:46 PM
Give that comment a second thought.
Think about how the tidal forces on Earth are generally in a 12 hour cycle.

Yes.

Earth rotates freely with respect to Moon and Sun. Therefore the pair of tidal bulges pass over the whole Earth in 12 hour 25 minute cycle.

A satellite of a bigger planet would be tidally locked, not freely rotating. The tidal bulges would be stationary.

cadoom
2009-Apr-16, 06:47 PM
Another related thing I just thought about.

Say you move Jupiter to 1 AU, and for the sake of simplicity, only bring Europa and Ganymede along as satellites. Would the increased sunlight be enough to melt the ice covering Europa, and what, if any effects would there be on Ganymede?

I think there are a vast number of ways to build solar systems, and I'm trying to imagine some realistic, possible alternative ways to for habitable planets to form.

NEOWatcher
2009-Apr-16, 07:41 PM
A satellite of a bigger planet would be tidally locked, not freely rotating. The tidal bulges would be stationary.
Would it be, or only over time? Of course, the bigger the tide the faster the lock, so maybe it balances out somewhere.

grant hutchison
2009-Apr-16, 09:33 PM
Would it be, or only over time? Of course, the bigger the tide the faster the lock, so maybe it balances out somewhere.Regular moons of gas giants spin down fast. Plugging in some likely figures for an Earth-like moon orbiting Jupiter at Callisto's distance, I get spin-down from a 24-hour rotation period to tidal lock in about 35,000 years. That's blindingly fast, geologically speaking.

What will maintain tidal heating, however, is if the moon's orbit is continuously perturbed from perfectly circular by the influence of other moons: that's what's going on with Europa and Io, for instance, and I presume that's what antoniseb was referring to.

Grant Hutchison

Noclevername
2009-Apr-17, 04:55 AM
A satellite of a bigger planet would be tidally locked, not freely rotating. The tidal bulges would be stationary.

It could be a 2/3 ratio like Mercury. And the moon ccould be outside the gas giant's radiation belts, as Callisto is to Jupiter. In fact the furter out the better, as far as tide is concerned.

rommel543
2009-Apr-17, 06:27 PM
What would the temperature be like on that type of planet. I am assuming that the orbit around the jovian planet would be quite large to keep from being pulled in so you would be going from close to the star, to far away. Also, depending on the orbit, you would also be in shadow on the far side of the planet. Being tidally locked the dark side of the moon would almost never get sun because when on the light side of the planet, the dark side of the moon is facing away from the star.

Of course if the jovian was tilted like Uranus then you would have one pole always in sun and the other dark. The convection winds would be devastating to any life not protected.

timb
2009-Apr-17, 09:34 PM
Earth-sized moons are improbable, according to models of how the regular moons of the Solar System's giants formed. According to the model (I've posted a link to the paper before) moons are limited by the fact that larger moons drop out of the planetary disk and onto the planet faster. A larger body might be captured.

Radiation might be more of a problem for the planet's ability to retain hydrogen than it is for potential surface life. Tidal locking implies slow rotation which implies a weaker magnetic field.

chornedsnorkack
2009-Apr-18, 07:22 AM
What would the temperature be like on that type of planet. I am assuming that the orbit around the jovian planet would be quite large to keep from being pulled in
Not a safe assumption. Io is rather close to Jupiter, yet safely far outside Roche limit. As are the inner big satellites of Saturn.

so you would be going from close to the star, to far away.
No. The distance between Earth and Sun varies by 5 millions of km, and this is a minor effect - seasons are caused by axial tilt. The diametre of Callisto´s orbit is less.

Also, depending on the orbit, you would also be in shadow on the far side of the planet. Being tidally locked the dark side of the moon would almost never get sun because when on the light side of the planet, the dark side of the moon is facing away from the star.


But the far side of Moon is actually sunlit for half the month - at new moon, when the near side is dark, the far side is fully lit.

EDG
2009-Apr-18, 09:25 PM
It could be a 2/3 ratio like Mercury. And the moon ccould be outside the gas giant's radiation belts, as Callisto is to Jupiter. In fact the furter out the better, as far as tide is concerned.

IIRC Callisto is still in side Jupiter's magnetosphere. Titan on the other hand goes in and out of Saturn's magnetosphere (which is probably why it still has an atmosphere - there are less highly energetic particles outside the magnetosphere to sputter it all away).

Earthsize planets will tidelock very very quickly to the primaries - which causes a problem in that if they're outside the magnetosphere then their rotation period would effectively be very long (like, several days to a couple of weeks long). But if they're close enough to have a more earthlike rotation periods (around 24 hours) then they're darn close to the primary and well within the magnetic field, which means a constant assault on their atmosphere/magnetic field by the charged particles, and also much greater tides if they're in a resonance with another moon that pumps up their eccentricity.

Noclevername
2009-Apr-18, 11:01 PM
IIRC Callisto is still in side Jupiter's magnetosphere.

Yes, but the radiation at that distance is far weaker than for the other large Jovian moons.


Not a safe assumption. Io is rather close to Jupiter, yet safely far outside Roche limit. As are the inner big satellites of Saturn.

But none of them are large enough for their Roche limit to tear them apart. An Earth-sized moon at the same distance would be in a lot more danger.

grant hutchison
2009-Apr-18, 11:37 PM
But none of them are large enough for their Roche limit to tear them apart. An Earth-sized moon at the same distance would be in a lot more danger.To a first approximation, the Roche limit depends only on the distance of the secondary from the primary and the relative densities of primary and secondary. If anything, an Earth-like moon would be denser than a smaller moon, and therefore more stable against tidal disruption.

Grant Hutchison

timb
2009-Apr-19, 12:31 AM
To a first approximation, the Roche limit depends only on the distance of the secondary from the primary and the relative densities of primary and secondary. If anything, an Earth-like moon would be denser than a smaller moon, and therefore more stable against tidal disruption.


Possibly the PP means that very small moons could be maintained by their rigidity, which is impossible for large moons.

NEOWatcher
2009-Apr-20, 12:14 PM
Regular moons of gas giants spin down fast.
Then I guess my use of "faster" was appropriate, even if it was vague.;)

Plugging in some likely figures for an Earth-like moon orbiting Jupiter at Callisto's distance, I get spin-down from a 24-hour rotation period to tidal lock in about 35,000 years. That's blindingly fast, geologically speaking.
Yes; I was thinking "blindingly fast" in my head, but even my wildest ideas weren't that fast.

I guess now my question would be what would be the maximum* time for tidal lock, with a minimum* tidal force to do some considerable seismic harm?

*of course, I'm looking at ballpark ideas just to get a feel for how bad the tidal forces are.

Noclevername
2009-Apr-20, 08:50 PM
To a first approximation, the Roche limit depends only on the distance of the secondary from the primary and the relative densities of primary and secondary. If anything, an Earth-like moon would be denser than a smaller moon, and therefore more stable against tidal disruption.

Possibly the PP means that very small moons could be maintained by their rigidity, which is impossible for large moons.

:doh::doh: Uh, yeah, sure, that's what I meant! I hadn't forgotten about density at all!:doh: That's the ticket! :doh::doh:

rommel543
2009-Apr-21, 04:12 PM
Not a safe assumption. Io is rather close to Jupiter, yet safely far outside Roche limit. As are the inner big satellites of Saturn.

:doh: I completely forgot that the Roche limit for a gas giant is irrelevant when dealing with rocky bodies. Too many years programming an not enough reading.



No. The distance between Earth and Sun varies by 5 millions of km, and this is a minor effect - seasons are caused by axial tilt. The diametre of Callisto´s orbit is less.

Yes that 5 million km is varied throughout the year not in 10-15 days like a moons orbit would be. As well thats the earth being the primary planet, not as a moon. If you combine the orbital radius of the primary planet and it's eccentricity and the orbit of the moon (which taking the orbits of Io, Titan, Callisto, etc) which could very well be over 1 million km, that's going to play havoc with the temperature. Additionally if the eccentricity of the primary is greater than than of earths, the orbit of the moon has the possibility to move outside of the habitable zone (~.95 AU for our sun).



But the far side of Moon is actually sunlit for half the month - at new moon, when the near side is dark, the far side is fully lit.

Yes but the side facing the planet would be in shadow when on the star side of the primary planet, and in the shadow of the planet when on the far side. The planet facing 1/2 of the moon would get very little light. The Jovian sized planet would block much more light than the earth would be.