# In a Red Giant Binary System, How Far and How Wide Can the Habitable Zone Be?

Our estimates of our own habitable zone--a piece of space in which liquid water is possible--have varied over the years, but the current estimate is by Ramirez and Kaltenegger in 2017. Based on an expansion of the classical carbon dioxide-water vapor habitable zone model and assuming a volcanic hydrogen atmospheric concentration of 50%, they have estimated our habitable zone to be from 0.95 to 2.4 AUs from our sun.

In this scenario, we have discovered a solar system orbiting two stars--both of which are red giants (which isn't unheard of, as KIC 9246715 can vouch for that.) Each is eight times as wide and as massive as our current sun. One star orbits the other from a distance of one-and-a-half AUs.

Using the specified information above, how far would the habitable zone be from those two suns, and how wide would that zone expand to?

• I think that the Ramirez and Kaltenegger habitable zone is imagined as a zone where carbon based life using liquid water could live, and not as a zone where humans could survive on planets with liquid water and nitrogen-oxygen atmospheres. If a planet's atmosphere is 50 % hydrogen, it can't have any oxygen in the atmosphere, since fires would combine hydrogen and oxygen into water. A much wider habitable zone would combine the inner limit of Zsom et al (2013) with the outer limit of Pierrehumbert and Gaidos (2011), and a much narrower one would be that of Hart et al (1979). – M. A. Golding Jan 19 at 18:07
• If you want habitable planets for humans, or planets with advanced lifeforms, orbiting the red giant stars, there is a problem: such planets should have had steady temperatures for billions of years, while a star will be a red giant for only hundreds of millions of years. You might solve that problem by having the habitable planets form around other stars and be moved to the red giants by highly advanced alien civilizations who intend to move them to still other stars in just a hundred million years or so. – M. A. Golding Jan 19 at 18:11
• See here for a list of solar system estimates: en.wikipedia.org/wiki/… - There is a box at the top of the page to search the forum. A search for stellar habitable zone has 77 posts worldbuilding.stackexchange.com/search?q=stellar+habitable+zone - here is a question about red giant habitable zones worldbuilding.stackexchange.com/questions/15435/… - and here is a site about solar systems with many habitable planets planetplanet.net/the-ultimate-solar-system – M. A. Golding Jan 19 at 18:13
• @M.A.Golding None of those were what I was looking for, and no one answered my question on whether or not ONE red giant is bright enough for a habitable zone spanning 56-84 AUs. – JohnWDailey Jan 19 at 18:25
• I feel like your habitable zone will be subject to extreme change over time. Even if your stars spends a billion years in the red giant phase, their continuous habitable zone (CHZ) will be narrow because they will brighten relatively quickly. A cool glacial planet on the outer edge of the HZ may become a hot desert on the inner edge in a short amount of time. – Zxyrra Jan 19 at 18:40

In the mare magnum of the Internet, one can find anything, literally anything.

Well, it looks like someone created also a habitable zone calculator*!

The calculator takes as input the luminosity of the star, which you don't give. However you give the estimated masses, thus we can estimate the luminosity of the star using the mass-luminosity relation

$$L\over L_s=1.4({M\over M_s})^{3.5}$$

which for the given ratio of 8 solar masses gives a luminosity of 1448 Suns.

Let's use an axe to make a rough, 0th order approximation, and put the two stars at the center of the system, and use 2896 times the luminosity of the Sun as input for the calculator.

We get that the habitable zone ranges between 40 and 94 AU in the optimistic case, between 51 and 90 AU in the conservative case.

Probably having the two stars orbiting each other would affect those limits, but it's beyond me to evaluate how much. Some engineering common sense would suggest me to cut it to stay not closer than 55 AU to avoid getting too hot.

*Calculator based on Kopparapu et al. (2013), Habitable Zones around Main-sequence stars: New Estimates. Astrophysical Journal, 765, 131

• 51-90 AUs would be most interesting in the terraforming scheme. – JohnWDailey Jan 19 at 20:06
• In binary system suns may eclipse each other, if everything rotates in one plane. That would make light and energy flow to drop catastophicaly for qute a long periods of time. This would greatly reduce HZ radius and width – ksbes Jan 20 at 9:58
• Didn't JohnWDailey say that the two stars he wanted were red giants? Isn't the habitable zone calculator based on an article about habitable zones around main sequence stars? iopscience.iop.org/article/10.1088/0004-637X/765/2/131/meta Red giants are not main sequence stars, and if the calculator works only on main sequence stars it won't work for red giants. – M. A. Golding Jan 20 at 17:01