Is this fictional habitable exoplanet of Alpha Centauri A realistic?

Question

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!

Answer 1

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

atmospheric pressure is slightly higher.

and

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?

Answer 2

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.