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I am designing a planet that orbits the star Toliman, (Alpha Centauri B), which orbits well beyond the conventional Goldilocks zone. This planet has a similar radius to earth (6240 km) but its mass is much less (3.06x10^24 kg) due to its density being similar to that of Europa. Surface gravity on this planet is approximately 0.53g.

This planet has a nitrogen-dominated atmosphere with a pressure of about 3 bars. It consists of an ice crust, liquid water mantle and solid rocky core.

I would like this planet to have large bodies of liquid ammonia on its surface; if not vast oceans, then at least a lot of lakes and small seas akin to the hydrocarbon lakes of Titan.

My question is this: how far must this planet orbit from Toliman in order for surface temperatures to support liquid ammonia oceans?

Link to the Wikipedia article on Toliman below:

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

If any additional data is needed, please state so in the comments. I don’t want to give you guys a hard time with this one.

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  • $\begingroup$ IANAChemist but I'm pretty sure this the same as trying to store antifreeze in a bucket made of ice. $\endgroup$
    – g s
    Commented Mar 10 at 17:20

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Looking at a phase diagram (Richter & Niewa 2014) and doing some estimating, at 3 bars, ammonia is liquid at temperatures between ~200 K and ~275 K. Since Alpha Centauri B is about half the luminosity of the Sun, then assuming the planet has a surface albedo comparable to that of Europa (about 0.67), applying a simplified equation for planetary surface temperature says that ammonia could be liquid at orbital radii between roughly 0.37 AU and 0.70 AU. In reality, this should be shifted outwards a tiny bit because Alpha Centauri A is also in the system, but it lies ~11-35 AU away, so its contribution is smaller. To ensure that the ammonia isn't constantly subjected to phase transitions by being near the boundaries of that temperature range, a semi-major axis of perhaps 0.55 AU is reasonable.

(For what it's worth, some models of planetary formation suggest that this is in the inner part of a reasonable region for planets in the Alpha Centauri system to have formed around the secondary star (Thebault et al. 2008)!)

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