#habitable

“This is what language is:

a habitable grief.”

I want to create a lot of planets that support human life. It's impossible to do in our solar system obviously. But based on this, does it only make sense that there could only be one planet per solar system that can support human life? Would its moons not support life? I can understand you could terraform a planet, but that won't change the sun. And wat about multiple suns? Do life supporting planets need to be twice the distance for a second sun so it'll give the right temperature?

Feral: As with any question of this nature, the answer I’m going to give comes with the caveat that it is perfectly acceptable and enjoyable to create a soft sci-fi setting, like for a space opera, that relies on Rule of Cool and common genre convention to break with physical reality as we understand it. In other words, the Star Wars galaxy has been a popular setting for film, television, traditionally published works, and fanfiction for over 40 years without too much thought put into the scientific plausibility of its solar system and planetary constructions, so you can take some liberties. It’s fine. You have our permission.

Too Long; Not Gonna Read: There’s actually a lot of math involved trying to make fictional astrophysics work, and this math is almost certainly not going to end up in a story, if that’s why you are worldbuilding. If you want to do a lot of math, that’s great! If not, generally speaking, if the star is an F-type (1.04-1.4 M☉), the planets aren’t massive compared to Earth, and they are next to each other in orbit, then sure, two or three habitable planets are scientifically plausible to occur within the same solar system.

Here’s how we get there…

The habitable zone (the range of distance from the star that a planet’s orbit can fall within to support human-like life) of a solar system is 95%-137% the square root of the star’s luminosity, which is in turn equal to the star’s mass cubed. Note that your star should be between 0.6 to 1.4 times the mass of Sol (our sun) for the solar system to be habitable.

So if your star is on the upper limit of the habitable mass, which is 1.4 solar masses, then its luminosity is 2.477 the luminosity of Sol, and its habitable zone is between 1.574 times the distance of Sol to Earth and 2.269 times the distance of Sol to Earth, which comes to approximately 236.1 million – 340.35 million kilometers (146.706 million – 211.484 million miles) sun to planet.

If you star is on the lower limit of the habitable mass, which is 0.6 solar masses, then its luminosity is 0.216 the luminosity of Sol, and its habitable zone is between 0.4415 times the distance of Sol to Earth and 0.6367 times the distance of Sol to Earth, which comes to approximately 66.225 million – 95.505 million kilometers (41.150 million – 59.344 million miles) sun to planet.

[note: I am actually plagiarizing myself; here’s the original ask where I did this math]

So, can there be multiple planets in the larger habitable zone? Eh. The shortest distance between any two planets in our solar system is Earth and Venus (Venus not being considered habitable), with a distance of about 38.2 million kilometers between them when their orbits are aligned properly; this fits inside the habitable zones defined above. Just remember to factor in the diameters of the planets themselves and the eccentricity of their orbits. An alternative would be to bring them much closer together and create a binary planet, or a pair of planets orbiting a common barycenter, which is a much more probable outcome than a habitable moon given the mass it would have to have to have the gravity, to have the atmosphere, to make it habitable. Edgar of Artifexian goes over how a habitable moon could technically happen in this video.

Keep in mind that when you’re determining the mass of your planets in a binary planet, they should be between 40% to 235% the Earth’s mass and have a radius of 78% to 125% the Earth’s radius and a gravity of 68% to 150% Earth’s gravity. The distance from the larger of the two planets to the barycenter is the average orbital distance (which ensures they remain within the so-called goldilocks or habitable zone) between the two planets divided by one plus the ratio larger mass divided by the smaller mass. So, if we take the largest (2.35 Earth Mass) and smallest (0.4 E.M.) planetary size possibilities, and the distance between the outer and inner goldilocks range of our larger star (0.695 AU), we find that the distance from the larger planet to the barycenter is 0.1 AU, which is about 15 million kilometers.

Furthermore, the mass of the larger planet divided by the mass of the star times the distance between the smaller planet and the star divided by the distance between the smaller planet and the larger planet squared should be less than 1.