Based on the real Alpha Centauri system, I want a harsh but technically human habitable planet around Alpha Centauri A. This is for a hard science-fiction PC game.

My question is: are these planetary characteristics realistic?

Star comparison:

Star name Sun α Centauri A
spectral type G2V G2V
Luminosity 1 1.519
distance from Earth 0 4.37
Absolute magnitude (MV) 4.83 4.38
Mass 1 1.1
Temperature 5772 5790
Age 4.6Gyr 5.3 Gyr
Habitable zone range: AU .9 to 1.4 1.37 to 1.76

At first I was worried about α Centauri being a trinary system, but I looked into how the stars interact and affect their planets: Considering the habitability of binary star systems, “the habitable zone for Alpha Centauri A extends, conservatively estimated, from 1.37 to 1.76 au[2] and that of Alpha Centauri B from 0.77 to 1.14 au[2]—well within the stable region in both cases.”

I don’t need it to be JUST like Earth, but simply survivable for humans; for eg. not bombarded by huge meteors every year. Or having its atmosphere stripped away often.

Comparing Earth, Mars, and fictional planet Rakuen (means paradise or pleasure garden in Japanese):

Earth Mars Rakuen Rakuen compared to Earth?
semimajor axis: au 1 1.5 1.55
position in habitable zone 20% 120% 46% 0% = nearest edge of HZ, 100% = farthest edge. Rakuen gets a bit less light and heat from its star.
sidereal period: yrs 1 1.88 1.9
rotation: hrs 24h 24h 37m 22h faster spin can create faster winds and Coriolis fx
axial tilt: deg 23 25.19 11 less pronounced seasons
surface gravity 1 0.38 1.1 slightly higher g
mass 1 0.107 1.584 M⊕ to generate 1.1g
radius re earth 1 0.533 1.2 to generate 1.1g
radius km 6378 3396 7653 More surface area.
surface temps:
min C -90 -143 -30 In degrees Celsius
mean C 14 -63 25
max C 57 35 60
greenhouse fx adds 33 6 30-50 due to Rakuen’s thick CO2 majority atmos

After those basic stats, I’m aiming for these characteristics COMPARED TO EARTH:

  1. More tectonically active. Thousands of active volcanoes and hot springs. More fault lines that are shifting faster; geological events like earthquakes are more frequent.
  2. There are more but smaller continents than Earth. Similar to Indonesia and other Pacific Ring of Fire areas. Lots of archipelagos and volcanic chain islands.
  3. Overall, there is less land surface than Earth, and more ocean at the surface.
  4. Volcanic ash and gases, like SO2 and CO2, dominate a thick hazy atmosphere.
  5. Due to higher gravity and above atmospheric composition, atmospheric pressure is slightly higher.
  6. Abundant metals near surface, especially iron oxides. The common volcanic eruptions should help this.
  7. No native life beyond single cell bacteria.
  8. I’d like wind velocities that are higher than Earth, with more frequent and larger storms. But I’m not sure how all the planetary factors will sum. Volcanic eruptions should disturb the heat distribution. With a thicker atmosphere, slower winds will have more force.

My question is: are these planetary characteristics realistic?

Planetary and climate sciences are rather complex, and I would really appreciate some help confirming or debugging these basic characteristics.

Thank you!

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    $\begingroup$ I'll start by saying I am not a meteorologist so what follows is a layman's opinion - for what that's worth. Your comment about the higher planetary rotation rate leading to higher wind velocities, more frequent and larger storms etc may not be accurate. The issue is one of thermodynamics. If the Earth for example had a slower rotation rate - the sun ward side would gain more heat from solar radiation than it does now. Likewise the night side would get cooler than it does now because there is more time for that atmospheric heat to radiate away. See Part 2. $\endgroup$ – Mon Mar 15 at 1:43
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    $\begingroup$ Part 2. The end result would be a higher overall level of thermal dis-equilibrium in the Earths atmosphere i.e. a 'hotter' day side and a colder 'night side'. Overall the amount of sunlight striking the Earth stays exactly the same but the distribution is changed. As the system attempts to equalize the result would be stronger winds and convection currents. More/hotter air rising day side more/colder air dropping lower night side then air from either side rushing in to fill the gap! So I think a longer rotation rather than a shorter one would give your the effects your looking for. $\endgroup$ – Mon Mar 15 at 1:52
  • $\begingroup$ @Mon ty for the feedback! I understand what you're saying--similar to how tidally locked planets get high winds as heated air on the sunny side races to the dark side. However I have also read about planetary spin effects on wind speeds and patterns as well, especially regarding Coriolis spinning storms. I'm not sure which are stronger... $\endgroup$ – Koon W Mar 16 at 1:15
  • $\begingroup$ Yes, you might want to dig a little - perhaps asked a meteorologist or climatologist which of the two options (faster or slower spin rates) is likely to have the most impact - along the lines of what you are looking for in your story. The key IMO would be finding out which of the two options gives you the wind speeds required for the smallest/least change in rotation rate. (So that you end up with a rate that is as close to Earths as seems reasonable.) $\endgroup$ – Mon Mar 16 at 3:41

Looks mostly OK, but a couple of things jump out at me.

atmospheric pressure is slightly higher.


greenhouse fx adds 30-50 due to Rakuen’s thick CO2 majority atmos

are a problematic combination.

Remember that nitrogen is cosmically common... it is the 7th most common element in space according to spectroscopic analysis (cos it is easy to make; just need big old stars, not supernovae) and so you should expect to find quite a lot of it in your fictional solar system. Furthermore, N2 is a relatively heavy gas and will be readily trapped by your world's gravitational pull, and it would be reasonable to expect it to form a relatively large proportion of your world's primordial atmosphere.

Any CO2 present will be in addition to the nitrogen. Remember that Venus has an atmosphere that's 3.5% nitrogen, but due to the density of that atmosphere that adds up to more nitrogen than you can find in Earth's atmosphere!

This implies that if you want a majority CO2 atmosphere, either you have to explain a weird lack of nitrogen (explained on Mars by atmosphere loss due to being small and without a magnetosphere, but you don't have that excuse) or you need an atmosphere that's got at least twice the pressure of Earth's and is 50%+ CO2.

I'd also worry that an atmosphere that thick, with that much CO2, on a water-rich world is also just gonna fill up with water vapour and you'll get a runaway greenhouse turning it into some kind of steam planet. Consider whether you really need that much CO2.

Volcanic ash and gases, like SO2 and CO2, dominate a thick hazy atmosphere.

SO2 has a fairly short residence time in the atmosphere, and it mostly comes from volcanic activity. For it to make up a signficant proportion of the atmosphere suggests that there's a terminal level of volcanism. Consider that Venus' massive spasm of volcanism about half a billion years ago might have been what tipped the planet's atmosphere over the edge, and big flood basalts on Earth have been associated with dramatic environmental change and mass extinctions, and Earth's atmosphere seems rather more benign than your world's.

If your intent is that the planet is already on its way to being a pressure cooker world then you'll be fine. Otherwise you might consider toning down something.

I’d like wind velocities that are higher than Earth, with more frequent and larger storms.

This has been a weirdly popular request on this site of late. Its like, a volcanic near-lifeless hellworld where it rains acid and the oceans are gonna boil away in a few thousands years isn't unpleasant enough... the weather has to be bad too.

Anyway, volcanism is a good source of stormy weather and fancy coloured lightning. Getting higher wind velocities seems a bit less practical, but really... do you need it?

  • $\begingroup$ ty! i didnt consider that mars had its N2 stripped away, and i was modeling my atmos on a mars composition. your explanation for venus's atmos also makes sense! i just looked up Titan also, and see 97% N, despite its smaller size. What would be good ballpark N, CH4, and CO2 levels for this planet? based on Archaean earth, i see about 70% N, 10% CO2, 10% CH4, 10% H2O vapor? $\endgroup$ – Koon W Mar 16 at 2:00
  • $\begingroup$ regarding SO2, that is very useful info as well, ty! it definitely wouldnt stick around long due to lots of rainstorms. High winds are optional, and it seems hard to predict the many factors involved. Pleasant planets are rare, no? (Rare earth hypothesis). From what I've read, most exoplanets will not be very pleasant for human habitation. $\endgroup$ – Koon W Mar 16 at 2:07
  • $\begingroup$ @KoonW sure, Earthlike planets may be rare, but your world is still more earthlike than not at this point in its life. Earthlike with bad weather is still Earthlike; it is not (yet) a Venus or Mars, after all. Those are unpleasant worlds. $\endgroup$ – Starfish Prime Mar 16 at 9:26

Turn up the SO2!

I share the concern that a "majority CO2" atmosphere will cause an intense greenhouse effect. Look how much damage we can do with 0.04% in the atmosphere. You can deal with it by turning down the CO2, by moving the planet outward, or ... by massively increasing the amount of SO2.

SO2 can be thought of as a geoengineering agent - according to Chemistry World at the cost of just 1000-4000 lives annually, airplanes can distribute a sufficient quantity to hold back the global warming clock by six months. (They weren't debating doing it, but stopping doing it) But Earth's atmosphere is still only around 1 part per million SO2, so there is a lot of room for improvement.

I'm not going to pretend I can calculate the effects of large amounts of CO2 vs SO2 - both will absorb certain frequencies, and both should have diminishing returns at high levels. But I can say how to turn up the SO2.

You have single celled organisms. Follow the sulfur cycle. On Earth, sulfate is reduced to sulfite, and then to hydrogen sulfide. Maybe their rudimentary biochemistry hasn't learned the last step. Instead, the sulfite remains, in equilibrium with sulfurous acid (H2SO3), which is in equilibrium with H2O + SO2. The organisms use sulfate in the water sort of like a substitute for oxygen in their metabolism - for each sulfate they convert, they get one oxygen atom.

If the planet's atmosphere is fairly oxidizing, sulfur compounds eventually get converted back to sulfate and washed into the ocean, so there is always a saturated sulfite solution to keep the atmosphere well supplied with SO2, creating a high atmospheric haze that reflects some substantial fraction of incoming light.

  • $\begingroup$ I'm researching more about archaean earth's atmos (more like 70% N, 10% CO2, 10% CH4, 10% H2O vapor). did you see starfish prime's link about SO2 not lingering long in the atmos? I'm also worried that the SO2 would imperil a human colony's survival w the acid rain problem? sulfur cycling bacteria is an interesting concept--surely smells awful! thank you for the info! $\endgroup$ – Koon W Mar 16 at 2:14
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    $\begingroup$ SO2 doesn't linger long in Earth's atmosphere. But if the local biology is not reducing the sulfur, and the oceans are saturated with sulfurous acid, it could linger longer. I think. (Yes, this would certainly imperil a human colony's survival - but if SO2 and CO2 dominate the atmosphere, I think that's already at issue! $\endgroup$ – Mike Serfas Mar 16 at 2:31

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