# How far from a habitable planet must a red giant star be to not seriously affect it?

Can a habitable planet having complex life exist in a binary star system with a Red Giant? I am not talking about close binaries, but distant binaries (100 AU or more). Planets can orbit some smaller stars (F, G, K, M class) in an S-type orbit, in order to have a steady source of light and heat. Does the red giant in that large distance affect the habitability of this planet? Is a similar setup possible with a Yellow Giant?

Edit: In the original question there was a typing error when I wrote that separation of binaries is 100 ly or more, but I intended to write 100 AU.

• A binary star system whose components are more than a hundred lightyears from one another is not a binary star system, it is two unrelated stars. Maybe you're thinking of AU? Commented Aug 26, 2023 at 5:38

In a binary system where the stars are separated by 100 Astronomical Units (AU), the effect of a red giant on a planet circling a smaller star (like F, G, K, M class) would probably be minimal. Neptune is about 30 AU from our Sun, and the Sun's direct effects (solar storms for example) at that distance are already much weaker than the effect they have on Earth for instance.

Red giants do release a lot of mass through stellar winds. However, at a distance of 100 AU, these wouldn't cause much trouble for the planet. Recall that our Sun, when it becomes a Red Giant, might expand enough to consume even the Earth, which is 1 AU away. Thus at about 100 Astronomical Units the planet would be safe from any effects. Especially if it has a strong magnetic field in place take care of a possible stellar wind. The yellow giant star case would be thus similar.

So, a livable planet orbiting the smaller star should be mostly fine, even with a far-off red or yellow giant. Mainly, the planet's livability depends on its own star and local conditions.

• Sorry, I meant 100 AU, not 100 ly (I was tired when I was writing a question). I don't think that binary with 100 ly separation would be even possible :D. Commented Aug 26, 2023 at 13:04
• @Bedlasky that's what I thought! Commented Aug 26, 2023 at 13:07
• Edited accordingly. As I mentioned, I don't think the red giant would be a huge issue, especially if the livable planet has a magnetic field Commented Aug 26, 2023 at 13:14
• And what about the end of giant's life? When it expels outer layers as a planetary nebula and become white dwarf. How it would affect life on the planet? Commented Aug 26, 2023 at 14:34
• @Bedlasky, our Sun will become a Red Giant but it'll only swell up to about the Earht's radius or 1 AU according to estimations. So, I speculate that your planet would still be fine as it is very far away from the "action". Commented Aug 26, 2023 at 15:08

Red giants eject planetary nebulas. It's speed is tens of km/s and mass is 0.1 to 1 solar masses.

Surface of the sphere with 100 AU radius is 2.5 10^27 m^2 so that is about 100-1000 kg of gas per m^2 (only 1-2 orders of magnitude less then the mass of atmosphere) incoming at ~50 km/s.

So I suspect that this is enough to sterilize the planet.

But this ejection happens at late stage of red giant life so it can easily be hand-waved away.

• But planetary nebula wouldn't be just on surface of a sphere, but in full volume. 100 AU sphere have volume V = 4π/3 * (100*1,496*10^11)^3 = 1,402*10^40 m^3. If it have 1 solar mass, than it will have density ρ = 1,419*10^-10 kg/m^3. However you are still right with this, I found this thread about effects of planetary nebula on the planet which says that at this distance planet would be probably stripped of its atmosphere: worldbuilding.stackexchange.com/questions/1304/… Commented Aug 29, 2023 at 21:31

Having a Red Giant 100 AU away is probably fine for your habitable planet as explained in cconsta1's answer but getting there may not be.

Note that a Red Giant is a relatively short phase (in astronomical terms) in the evolution of a star. So this red giant was a star somewhat similar to our sun for billions of years before it became a red giant. This phase is even more harmless for your habitable planet. However, the shift from sun-like to Red Giant might have caused a helium flash and this is probably very bad for the habitable planet:

This runaway reaction quickly climbs to about 100 billion times the star's normal energy production (for a few seconds)

• I was thinking if A or B class star can be added on to a distant orbit. These stars have short life time and they become red giants before multicellular life have even chance to evolve. That's why I ask this question. Commented Aug 29, 2023 at 21:36