My binary planets are roughly the size of earth and they are tidally locked with one another. Both planets support life. If the day/night cycle is roughly 100 hours (50 hours of day, 50 hours of night), how widely would the temperature vary from the warmest part of day to the coldest part of night?

The distance between the planets is approximately 12,000 miles (19,312 km). I came up with this distance from this article, http://phys.org/news/2014-12-binary-terrestrial-planets.html, which suggests the centers of the two planets would be separated by approximately three planet radii.

The distance from the planets to their star is at least 1/2 an astronomical unit but they could be much further away. The planets are orbiting each other in the same plane as they orbit their star so they experience a solar eclipse once a day. Axial tilt is zero. The atmosphere on both planets is similar to Earth's atmosphere. I would like to keep the average temperature as similar to Earth as possible.

I believe temperatures would vary less near the coasts and more inland, and humidity would also decrease variance in temperature.

With such long days and nights, is a diurnal temperature variance of 50-75F possible in certain areas? I need these planets to support life.

  • $\begingroup$ You can read the following article to calculate the temperatures: dangermouse.net/gurps/science/temps.html There are an excellent chapter to Tidally locked planets $\endgroup$ Jun 21, 2016 at 16:17
  • $\begingroup$ Nice update. A lot more to go on. Would still need stars output/classification to actually determine energy being received at a given distance. $\endgroup$ Jun 21, 2016 at 22:56
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    $\begingroup$ Since there is no axial tilt: With constant sunlight from a very low but constant angle (0°), both north and south pole should have fairly stable conditions if there are no other geographic patterns that interfere. They would also be incredibly romantic places - with an eternal sunrise/-set. $\endgroup$
    – 0range
    Jun 22, 2016 at 0:36
  • $\begingroup$ There is simply not enough information to answer that. Being a calculation rather than creative writing advice, try asking on Astronomy ! $\endgroup$
    – JDługosz
    Jun 22, 2016 at 2:16
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    $\begingroup$ Something I thought of that hasn't been mentioned is that with the close proximity and size of the planets, their atmospheres will be tidally bulging toward each other to the degree that there will experience an atmospheric exchange. $\endgroup$
    – user21896
    Jun 22, 2016 at 2:51

2 Answers 2


Twice the daily variance of earth and no seasons. Extrapolate the climates you know with those rules.

During daylight weather is always an extremely hot spring day and nights are always an extremely cold fall. My hometown's climate was high altitude arid. We had spring and fall swings of 60F-80F daytime to 25F-50F during the night.

If my hometown were on one of your planets I'd expect it to have afternoon temperatures at 110F and frost every night. No crops would grow outside of greenhouses and in most climates plants would have to be very hearty to handle the high temperature of the day and freezing temperatures at night. (most climates can see frost early in the spring and late in the fall extend that to every day and you've got serious problems for food production)

Our temperature swings were extreme due to thin dry atmosphere. You'll see much lower swings at low altitude and with more humidity in the air. I currently live in a very moist sea level area. We see 60F-70F basically all year (with some outliers up or down by 10F)

I'd expect the long intense eclipses on the near side to actually be a nice respite from the long and hot day. I'd expect wind due to temperature variance at the edge of the eclipse area and eery twilight darkness sometime every day. On the far side of each planet people wouldn't have even seen the companion! (imagine growing up on the far side of the moon, you'd have never seen the earth)


Assuming a similar distribution of surface landmass and water, and a similar atmospheric composition, these planets could absolutely support life.

Temperature swings would be least extreme near the poles because, as on Earth, the day is not as different from the night as it is at the Equator; less radiation hits it at daytime.

Temperature swings would also be relatively meaningless in sufficiently large bodies of water, and in caves.


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