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For large habitable planets, what's the largest a planet can be, and still have a decent iron core to provide a magnetosphere that can protect the biosphere from the solar winds of two or more suns?

Thats a loaded question.

You can figure this out on your own just using some rule of thumbs.
Know volume to radius (4:1) and energy fall off (1:4)

Two Suns means that you need about 25% more protection at the same distance. (falloff is the square root of the distance) This means you need about twice the core mass by volume 25% by radius. Mass of the planet would be about twice the mass, Grey zone between a water world and earth like.

There is an inherent problem with this set up though. Although you have successfully protected the planet from solar winds, the planet is still receiving 25% more light/heat. An easier solution place the planet 25% further then 1AU, the keep the plant the same.

Realistically? I'll say about 5 Earth-masses is the highest it goes for habitability, so about 1.3 - 1.8 Earth-radii? Somewhere in there?

MKruer wrote:Thats a loaded question.

You can figure this out on your own just using some rule of thumbs.
Know volume to radius (4:1) and energy fall off (1:4)

Two Suns means that you need about 25% more protection at the same distance. (falloff is the square root of the distance) This means you need about twice the core mass by volume 25% by radius. Mass of the planet would be about twice the mass, Grey zone between a water world and earth like.

There is an inherent problem with this set up though. Although you have successfully protected the planet from solar winds, the planet is still receiving 25% more light/heat. An easier solution place the planet 25% further then 1AU, the keep the plant the same.

So, for a planet that is 1.20 earth radii, 1.25 AU would be good distance from, say, Delta Trianguli?

I made a mistake in my original assessment, the numbers would be correct if the star was twice as hot, not for two stars in a system.
For all intents and purposes the "solar winds" strength can be equated to the temperature of the star(s), So...

Delta Trianguli A is a G0V Star and has a surface temp of ~6000K
Delta Trianguli B is a K4V Star and has a surface temp of ~4300K
Sol (Our Sun) as a G2V Star and has a surface temp of ~5775K

Based upon the above information you can deduce via [tex]4*pi*r^2[/tex] that in order for the planet to receive the same amount of light per square meter as earth, that the planet would need to be

~1.02AU away from Delta Trianguli A
~0.86AU away form Delta Trianguli B
1AU away form Sol

For the life of me I am drawling a complete blank calculating the mean temperature of both stars combined. I think its just the hottest star + it orbital distance away for the berry center

Delta Trianguli A is ~0.045AU away from berry center
Delta Trianguli B is ~0.065AU away from berry center

So the habitat zone is 1.02AU + 0.045AU = 1.065AU. This is not 100% accurate because the habitable zone will shift to 1.02AU - 0.045AU = 0.975AU. at is furthest.

Anyway for that particular star system you can google it and find of that the habitable zone is ~1.06AU
http://www.solstation.com/orbits/deltrisys.htm

Ok, thanks.