In an alternate universe, Earth orbits not one sun, but two.

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As you can see, both stars are orange dwarves, each one only 40% as bright as our sun. All the same, this alternate Earth orbits from such a distance that daylight is as bright as Mars. However, being situated inside a dense star cluster, nights are still 60 times brighter than they are back home. It also stands at an axial tilt of 42 degrees, which means that polar seasons can reach as far down in latitude as 48 degrees. 32% of its atmosphere is oxygen, and 1200 parts per million of carbon dioxide linger in the air.

Here is a map of this alternate Earth:

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And here is the same map but now in comparison to our Earth:

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Using all the info provided above, what would the climate look like in this alternate Earth?

  • 2
    $\begingroup$ "32% of its atmosphere is oxygen". so your planet has a rather low sealevel air pressure? Because at Earth atmospheric pressures, 32% oxygen will result in unabated, endless wildfires. $\endgroup$
    – PcMan
    Feb 5, 2021 at 12:07
  • $\begingroup$ Part A: Could well be wrong but I don't think the orbit of your planet would be stable for long enough to let life evolve. Problem being that while both suns have comparable mass one will periodically be significantly closer to your world than the other. This matters given the distance your planet has to be from the systems center of gravity in order to maintain a habitable climate. $\endgroup$
    – Mon
    Feb 6, 2021 at 0:21
  • $\begingroup$ Part B: Given the above I think the planet would very quickly be pulled out of its current orbit & inwards towards one or other star. And the only way to prevent this is by putting your worlds orbit so far out from the systems center of gravity that it is no longer habitable or alternately making the second sun way smaller than the main star. In which case your world also becomes much colder. Which you would then have to counteract by making the main sun much hotter/brighter. So no twin main sequence orange red suns. $\endgroup$
    – Mon
    Feb 6, 2021 at 0:26
  • 1
    $\begingroup$ @PcMan not quite, 32% is uncomfortably high, but possible -- this is right about the level on Earth during parts of the Carboniferous. $\endgroup$ Feb 6, 2021 at 7:21
  • 2
    $\begingroup$ this basic kind of system can work (a few exoplanets like this are now known e.g. Kepler-16b and Kepler-453b) but the two stars ought to be closer together, relative to the planet's orbital radius (so they act like a point mass to the planet) $\endgroup$ Feb 6, 2021 at 7:28

4 Answers 4


There is a possible ambiguity with "40% as bright as our sun" and "daylight is as bright as Mars" - orange stars will emit more infrared in comparison to visible light than the Sun.

If that refers to total energy received, you may need some other greenhouse gases in the atmosphere, such as methane, to keep it habitable. (1200 ppm CO2 is a whole lot relative to modern Earth, but in terms of a partial pressure of CO2 it's less than current Mars.)

nights are 60 times brighter

No effect on climate. (The Sun is ~400,000 times brighter than the full Moon, which is far brighter than starlight.)

32% oxygen

This is probably around the upper end of what is stable (before mega-forest fires use up a lot of the oxygen); Earth peaked at around this value during the Carboniferous. Things will be very flammable...

tilt of 42 degrees

This should basically prevent ice caps forming if the greenhouse effect is sufficient to make the global temperatures at all Earthlike. High tilt means strong summers at high latitudes, so winter snows melt rather than accumulate into glaciers.


The south pole is land, but the landmass extends near the equator, so there shouldn't be a "cold trap" effect as with the circumpolar Antarctic current on Earth - further working against glaciation.

  • $\begingroup$ If the 42° tilt means strong summers at high lattitudes, doesn't that mean you'd also get very cold winters closer to the equator? So in general seasonal variance would be higher than on our Earth? $\endgroup$
    – Whitehot
    Feb 8, 2021 at 10:56
  • 1
    $\begingroup$ In general seasonal variance would definitely be higher, yes. A lot higher. If the tilt goes really high the poles can actually be warmer than the equator, but this isn't high enough for that (this abstract says 54 degrees or more: oceans.mit.edu/JohnMarshall/eapsdb/… ) $\endgroup$ Feb 8, 2021 at 20:54
  • 1
    $\begingroup$ But the high latitudes are important here because polar glaciation affects the planet's overall climate (ice reflects sunlight so more ice = colder temperatures = more ice, a positive feedback). The high tilt acting against glaciation here might help compensate for less solar energy and help keep the planet out of a "Snowball Earth" state. Though I'd still put some more greenhouse gases in... $\endgroup$ Feb 8, 2021 at 20:56
  • In general, this is going to be a significantly colder planet than today's Earth.

  • Brighter nights won't affect the climate, but nocturnal animals on this planet will evolve a bit differently from ours.

  • Your large continents are going to have significant areas of inland desert, like the Gobi, or large parts of Australia.

  • There should be somewhat larger ice caps than present-day Earth- take a look at some maps of the Last Glacial Maximum.

  • With extreme seasonal temperature shifts, the areas covered by sea ice will shift significantly over the course of a year.

  • $\begingroup$ Actually, on Earth, increased tilt works against glaciation as part of the Milankovitch cycles (more extreme seasons at high latitude = snow/ice melt in summer, rather than accumulating over years). $\endgroup$ Feb 6, 2021 at 7:40
  1. Your level of greenhouse gases are 3x ours but your level of solar energy is less, at best this is a wash, more likely the world will be colder and have perm polar caps with the warmer seasons melting to some degree.

  2. Your days are significantly dimmer, especially when one "sun" obscures the other, but your nights are much brighter. Effectively you have a "dim" dusk and a "brighter" dusk but there is no day as we know it and no true night time.

  3. Everything will be adapted for 24-hours of dusk. There will be no true "nocturnal" life. Since everything that hunts can hunt all the time, there will be evolutionary pressure to sleep as little as possible if at all.

  4. "Seasonal Shift" in temperature will be brutal and extreme and may run on a multi-year cycle. Depending on time of year the amount of energy that you get from the sun will be cut in half and where it goes will also change.

  5. You have a lot of land tied up in your south pole. There will be a perm mass of ice there that will range between "really cold" and "insanely cold". It will grow huge ice glaciers.

  6. Given the huge shifts in the amount of solar energy, you might have huge shifts in the temp of the planet leading to those glaciers melting and reforming and thus large swings in the level of the oceans.

  7. The equator (and your islands there) may shift from being a tropical paradise to being frozen. And again, everything will be adapted for this. So you might have polar bears that shed their fur in the summer or some such. Life, if it exists, would largely hang out at the equator.

  1. Seasons would be long, so thermal extremes would be more severe on land. Even so,land masses seem concentrated around one of the poles, which suggests a planet in an ice age below about 50S.

  2. There's not much land in the equatorial zone, so huge amounts of rain fall on the oceans instead of land. That suggests a big Hadley Cell in the subtropical regions of the southern hemisphere -- and huge deserts.

  3. Because of all the warm water in the subtropical zone in the northern hemisphere, hurricanes would be devastating wherever they make landfall.

  4. West coasts in subtropical areas might be as dry as the Atacama Desert or at least Baja California even in the norther hemisphere. But these would be the only deserts of the northern hemisphere, which would be heavily forested just about everywhere there is land.


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