I am trying to make a universe using math and this is a planet from the first solar system I am making. Firstly the star. This star, which I don't have a name for, has a mass of 1.3 solar masses, a luminosity of of 2.19 times that of our sun, a Diameter of 3049964.86 km (1895160.3 miles), a surface temp. of 6596 K ,or 1.14 times that of our sun. The habitable zone is located within 1.4 AU to 2.02 AU. The frostline is at 7.17 AU and the inner and outer limits of the system is located at 0.13 AU and 52 AU. Now the planet. This planet ,once again I have yet to make a name for, has a mass of 2.8 earth masses, a radius of 9556.6 km (5938.2 miles), a semi-major axis of 1.72 AU, a surface gravity of 13.3 m/s^2, a eccentricity of 0.1, and a axial tilt of 25.4 degrees.

P.S. Ask for more details if you need some because I can't think of any other details that I should add.

  • $\begingroup$ I hope you don't mind, I've added a few tags to attract those with the appropriate expertise. Please feel free to revert the edit if you see fit. Welcome to worldbuilding. $\endgroup$ Commented May 25, 2021 at 21:38
  • $\begingroup$ I think the question can be altered. Intelligent life as we know it starts with a species that gets by on it's intelligence, getting tracking and using tools down in the long run. At a point where there's abundance of resources to thrive, intelligence can be boosted greatly to a point where its not a detriment to put so much energy into it. Then they survive periods of hardships better. The question is "can my planet support life", "can my planet support tools" and "can there be a period of abundance for a fledgling Intelligent species to grow through the barrier to 'sustainable' intelligence" $\endgroup$
    – Trioxidane
    Commented May 25, 2021 at 21:46
  • $\begingroup$ This Blog should really help you with bottom up worldbuilding. $\endgroup$ Commented May 26, 2021 at 4:13
  • $\begingroup$ Minor query/quibble, with 1.5 Earth radii & 2.8 Earth masses I calculate the surface gravity to be 12.221 m/s^2, compared to your 13.3 m/s^2. The escape velocity for ET to visit the cosmos will be 15.28 km/s compared to 11.184 km/s for Earth. Average density of your planet will be 4.575 g/cm^3 compared to 5.514 g/cm^3 for Earth. $\endgroup$
    – user81881
    Commented May 26, 2021 at 10:19

2 Answers 2


Mass-luminosity relationship is proportional to the 4th power, so I'd expect a mass 1.2 times that of the Sun for 2.19 times the luminosity, but that's close enough for demiurgical work. [Habitable zone] should be proportional to the light received, which decreases by inverse square, so for 2.19 times the light I'd expect something centered on 1.5 AU. Still, the "habitable zone" is a concept I don't actually believe in - there are habitable zones for specific planets, not habitable zones of space in the abstract - and in some formulations Earth is practically on the inner edge of a zone that extends out almost to Mars. I'd say it's a fair assertion. Star temperature and luminosity are a bit loosely related; this doesn't seem implausible to me. The gravity, and radius look like they work out. The planet is a little less dense than Earth despite its large size, which hints a bit at light elements - potentially deep oceans or something stranger like a massive abundance of carbon.

With a 2.25-year year, and eccentricity quite high by our standards. The apoapsis/periapsis ratio is 1.1/0.9, which is squared to determine illumination, so the planet receives 1.49 times more light in "summer" (orbitally defined) rather than "winter". Now insolation on Earth can vary quite substantially also - see maps for a practical view of it. This tells me that if your axial tilt happens to line up with the semimajor axis of your ellipse, weather in one hemisphere of the planet might be fairly moderate (still perhaps more extreme than Earth), while on the other we are definitely talking about deep freeze and burning heat. I'll call the freeze-and-burn hemisphere "south" in honor of Earth's much smaller correlation, but it's a 50-50 chance.

The mass of your planet makes it harder for it to lose its primary atmosphere, but that depends on facts not in evidence, such as the flare history of the star and the orbital migration of the planet, not to mention whether it gets hit by Theia. It could be a Neptune but it might be a super-Earth, and I don't think any facts we know can kibosh either alternative. I'm going to suppose that it comes out with an Earthlike atmosphere by favorable chance. There is certainly a possibility of some more interesting circulation patterns, with carbon dioxide freezing out of the atmosphere in the southern hemisphere for an Earth year or so each orbit. (This can happen in Antarctica, but only conceptually, because there is very little CO2 in the atmosphere, much like it doesn't really snow on a cold dry day. Your atmosphere gets much colder, and it might have more CO2 based on that low density and the difficulty of stripping the heavy atoms) This might affect plant growth and lead to evolution of CO2 reservoirs that go a little beyond crassulacean acid metabolism! I suppose the freezing and thawing of CO2 might even make the southern seasons a little more extreme. Still, even the night side should be habitable in spots due to volcanic hot springs - it might have a bit less radioactive material internally, but it still has a much larger internal store of heat. (The vulcanism would depend on chemical particulars of the planet's crust and plate tectonics...)

My overall expectation is that intelligent lifeforms ought to be able to live on such a planet, if we assume it developed in a way they can. There are many free parameters left to fudge to try to make things work out the way you want, and of course, nobody has ever seen a planet like this as anything but a speck of light.

  1. 1.3 solar mass -> 5.2 billion year star lifespan. Tricky.

With stellar evolution you end up with shifting habitable zones. (Earth has something like 500 mln years left before from perspective of not-technological intelligent specie becomes uninhabitable).

Without handwaving, you'd need:

(a) much faster evolution than on Earth, OR

(b) some orbital changes that have been moving the planet on higher orbit, OR

(c) life evolving somewhere else and just being seeded on this planet

  1. 2.8 earth masses - on its own it seems fine, except issue from which massive planet are so good at keeping volatiles that they tend to become water worlds. Proper heavy bombarding or forming close to home star recommended.

[other parameters seem fine, so I'm not addressing something clearly workable]


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