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In my story, I plan to make a double-planet between a gas dwarf and an Earth-like planet, so is it plausible in a red dwarf or Ultra-cool red dwarf system? and if so, could the gas dwarf protect the Earth-like Planet from solar flares/winds in such a System?

And if it wasn't plausible for a red dwarf system, could it work in a normal one(Like K/G type Star)?

Will the radiation of the gas-dwarf affect the Earth-like planet?

Assuming both planets have enough day-length(Tidally locked) to be able to generate magnetic-filed and being in the habitable zone

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    $\begingroup$ Gas dwarfs are very unlikely to be able to exist in star's habitable zone. Sub-Neptune planet won't be able to hold to its hydrogen and helium if it is as warm as Earth or even Mars. $\endgroup$
    – Alexander
    Aug 11, 2021 at 17:22
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    $\begingroup$ @Alexander I think if the gas dwarf has 4x earth radius, and 12x earth mass it will be able to have an atmosphere of helium at least even with Earth-like temperature. $\endgroup$
    – Khalid
    Aug 11, 2021 at 17:35
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    $\begingroup$ Yes, this should be realistic on the heavier end of sub-Neptune planets. $\endgroup$
    – Alexander
    Aug 11, 2021 at 18:17
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    $\begingroup$ The problem you start to run into with red dwarf star systems is that they tend to tide-lock planets in their habitable zones, and tide-locking of the planet spells doom for moons, as planet-moon tidal interactions crank them inward to destruction. $\endgroup$
    – notovny
    Aug 11, 2021 at 21:36
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    $\begingroup$ @notovny - I think a binary planet should be immune to tidal locking to their star. They should be tidally locked to each other instead. $\endgroup$
    – Willk
    Aug 12, 2021 at 0:39

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If body A shields body B from the stellar wind, body A is always between B and the star.

For this to happen their orbits need to take the same time.

Satisfying both conditions means that A and B overlap on the same orbit. Any other orbit would not allow continuous shielding.

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  • $\begingroup$ I mean a double-planet where a planet has another planet as a moon. <en.wikipedia.org/wiki/Double_planet> but what u said is interesting. $\endgroup$
    – Khalid
    Aug 11, 2021 at 9:30
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    $\begingroup$ Actually a planet can hide behind another planet in orbit. They are called Lagrange points and there are 5 of them, one in front, one behind, one a specific amount ahead in orbit, one a specific amount behind in orbit, and one on the opposite side. If the smaller planet is in one of these points it should be fine however having any other planets in the solar system could and will very easily screw up the very fine balance and chaos ensues $\endgroup$
    – NuclearWill
    Oct 8, 2021 at 18:32
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The solar wind is blocked by magnetospheres, and these can be big.

https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter

The magnetosphere of Jupiter is the cavity created in the solar wind by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosphere is the largest and most powerful of any planetary magnetosphere in the Solar System, and by volume the largest known continuous structure in the Solar System after the heliosphere. Wider and flatter than the Earth's magnetosphere, Jupiter's is stronger by an order of magnitude, while its magnetic moment is roughly 18,000 times larger.

Bold emphasis mine. In addition to blocking solar wind via bulk mass (as per L.Dutch's answer) solar wind is deflected by the magnetospheres of planets. We are lucky that it works that way or the solar wind would blow down on us and it would be hard to listen to the radio. The magnetosphere of earth extends far out into space and it deflects / blocks the magnetosphere. Our own moon is far enough away that it is outside the Earth's magnetosphere for a lot of the time and so during those times it is hit by the solar wind. When it is within the Earth's magnetosphere it is shielded.

Jupiter has a much bigger magnetosphere which extends far out from the planet and envelops all of its moons and then some. If in your binary system one planet had a strong magnetosphere it could extend out to encompass and protect its sister from the solar wind.

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  • $\begingroup$ I know, but this half of the question, is the idea of having a double planet plausible on red dwarf or ultra-cool red dwarf? $\endgroup$
    – Khalid
    Aug 11, 2021 at 14:50
  • $\begingroup$ I don't see how the type of star makes a difference. $\endgroup$
    – Willk
    Aug 11, 2021 at 15:02
  • $\begingroup$ , In an ultra-cool red dwarf system, planets are very close to each other (To the extinct that a close gas giant could look like a moon in the sky), so it will affect the passability of having a moon, maybe in a normal red-dwarf it won't have that problem $\endgroup$
    – Khalid
    Aug 11, 2021 at 15:06
  • $\begingroup$ I can’t see why it isn’t possible and if it is possible I don’t see why you can’t do it. Even if it is very unlikely, the universe is so big, heck the galaxy is so big, that it’s bound to happen somewhere eventually from random chance from the trillions of stars out there $\endgroup$
    – NuclearWill
    Oct 8, 2021 at 18:15

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