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I had an idea for a planet which has very hot days, roughly 40 degrees C on average, and very cold nights, average -5 degrees. These figures are subject to change. Of course, there are places which are similarly variable on Earth, but I want this to happen globally.

The planet cannot be tidally locked, and must be habitable too. What kind of conditions would plausibly create such a scenario?

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    $\begingroup$ Don't you want to have oceans of this planet? Such a large variation in temperature is not really believable for lands near the ocean shore, and even less at sea. Moreover, on Earth such a large diurnal temperature variation is linked to deserts, where the rocky / sandy ground has very little ability to retain heat. $\endgroup$ – AlexP Jun 4 at 12:09
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    $\begingroup$ I want this to happen globally Single biome planet are not realistic. What happens near and at the poles ? There have to be differences in climate everywhere due to the distribution of land and sea and the effect this and the planetary rotation has on wind patterns. $\endgroup$ – StephenG Jun 4 at 15:01
  • $\begingroup$ I thought this weather forecast from BBC2s "The Planets" might be helpful (Facebook link). Not exactly your scenario but still helpful. $\endgroup$ – EveryBitHelps Jun 4 at 18:43
  • $\begingroup$ Does the planet have to be natural? Or can ancient terraformers be involved too? $\endgroup$ – Starfish Prime Jun 4 at 19:46
  • $\begingroup$ @StephenG: This is a point that is ignored far, far too often. The mere presence of an ocean current or Hadley cell or even a lump of rock in the right place ensures multiple states on all but the most lifeless of planets. $\endgroup$ – Joe Bloggs Jun 4 at 23:00
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Slow rotation.

Lots of time to heat up during the day, lots of time to cool down at night.

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  • $\begingroup$ This stretches the meaning of 'day' OP probably had in mind (my guess: '~24h', not 'one planet-rotation') --- Also, this would not get you the same temperatures at the equator and the poles, and only -5°C during the long 'night', with the planet facing the -4° K background? How would that work? $\endgroup$ – bukwyrm Jun 5 at 12:29
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You need to get the energy input equal over the whole of your planet - two options spring to mind:

1) The albedo (~reflectance) of the planet could be finetuned so the further to the poles you get the lower the albedo.

This could be a result of the prevailing sediment at the different latitudes, and would not even have to be visible to the eye, if you decree the differences in reflectance to be entirely in the invisible wavelengths (e.g. IR).

Varying the local albedo via bodies of water would be neat (you would just have to vary the size and distribution of ponds, which explains itself quite neatly along the lines of latitude-dependent geology), but this changes the local ability to retain heat, as water has a quite exceptional heat capacity, so you'd have to introduce a mechanism that radiates heat faster in the regions with more water, to get the requisite drop in temperature over night - possibly another geological feature like craggyness of the ground (like heat vanes)?

Note that vegetation might interfere in your scheme by covering the ground - OR the ground cover has for some reason evolved to produce exactly those parameters. That would be neat, as plants can change shape, color and vapor-losses in small timescales, thus making a globally-eualized climate even more feasible by introducing an active control of sorts.

Having a reflective soil at the regions with maximal irradiance (equator), and a much more (heat-)radiation-absorbing soil in the lesser lighted areas at the poles would offset the difference in energy input, and would thus make for globally equal average temperature. To get the 45° C day/night cycle you have to decree a lack of 'insulation', i.e. no clouds, or at least globally uniform amounts of clouds.

2) Your planet is a cylinder (thus probably of non-natural origin) with it's axis orthogonal to the radius of it's orbit. Ignore the endcaps. This also guarantees globally equal irradiance.

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Make your planet very dry.

The regions of Earth with the largest temperature swings are deserts. Without cloud cover providing atmospheric insulation, deserts heat up quickly in the day and cool down quickly at night. The same effect could take place on a planet-wide scale, with the only major driver needed to achieve such an effect being low planetary moisture. Your planet would effectively have a global desert, with hot days and cold nights globally.

Of course, planets being spheres, there will still be regional variance in terms of how hot the days are and how cold the nights. For a planet with Earth-like temperature ranges, the equator would resemble the hottest parts of the Sahara or the Middle East, while the poles would be similar to Antarctica, though with substantially less ice. Temperatures in intermediate latitudes would range between the two, but with consistently high daily temperature swings. Planetary oceans would be small or nonexistent, though some level of accessible water would still be necessary to support life.

Exoplanet research suggests that such planets could form and be habitable. Their habitable zone would be somewhat closer to the sun, as compared to the habitable zone for wetter, more Earth-like planets. Water serves as a strong greenhouse gas, so they'd be a bit cooler than an Earth-like planet with a similar orbital radius. (Source: http://hoffman.cm.utexas.edu/courses/Science-2013-Seager-577-81.pdf)

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  • $\begingroup$ Oceans also act as giant heat sinks, so when you're near an ocean it's much harder to have dramatic changes in temperature. $\endgroup$ – Rob Watts Jun 4 at 21:49
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I doubt that there is any way to get that temperature range globally with a spherical planet. But it may be possible to get much of the way there with the following:

1) First the planet should not have any tilt or obliquity in its axis of rotation, or eccentricity in its orbit. That will get rid of pesky seasons, so the climate doesn't vary throughout the year.

2) Reduce the water content of the planet. It doesn't need to be as dry as Mars, but think global desert. There should be no large oceans. If you need seas then keep them located in equatorial regions. That will minimize the effects of oceanic heat capacity on climate and also reduce the amount of cloud cover.

3) Reduce the density of the atmosphere. The more the better. If you still need most of the surface to be compatible with humans breathing unaided then one option is to reduce total pressure but increase the proportion of O2 in the air. Maybe 1/2 earth's atmospheric pressure (50 kPa or 7.5 psi) but with 35% O2 content. That will also reduce the heat capacity if the atmosphere and allow for faster warming/cooling.

4) Get rid of most mountain ranges and height differentials. (but see section 6 below). This has three advantages: Firstly, with no significant altitude variation there will be no related climate variation. Secondly, no rain shadows so no especially wet or dry regions. Thirdly, it will make it easier for global winds to blow unopposed by terrain - these could transfer heat from warmer equatorial to cooler temperate regions - and from temperate to polar regions.

5) Increase the length of the day? This one is debatable. longer days and nights will increase day/night temperature differential but if you go too far then the nights could drop well below -5. Your call...

6) Add features to lock people out of some regions. In practice, the above ideas can all work to increase day/night variation and minimize regional variation. But in practice the tropics will still be significantly warmer than the poles. So one option might be, for example, to make the polar regions comprise of very high altitude plateaus - they would now be very cold and due to low altitude unlivable (think of the top of Everest). Similarly, you could put all the surface water form a single sea that encircles the entire globe around the equator - so nobody can live at the equator. Then although the planet no longer has a globally constant climate, the inhabitable regions may be close to approaching that.

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Rust planet

My thought is that the surface needs to be something which heats up quickly and loses heat quickly. Metals have low specific heats (which we know from experience - metal gets hot fast especially compared to something like water with high specific heat that gets hot slowly). I think the metal oxides have even lower specific heats than the metal - specific heat for iron oxide is hard to find but other oxides are here.

https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html

In any case the iron oxide will be dark and so will absorb a lot of incoming radiation. It will get hotter and hotter and heat the atmosphere immediately above it. At night there is little heat capacity of the rust and it will quickly give up its heat and the temperature will plummet.

We want to minimize heat capacity of the atmosphere as well. We want incident radiation to come on down and hit the surface, not hang up in the air or be reflected from clouds. This world is dry; water has a high specific heat and is a potent greenhouse gas as well. Argon has a low specific heat and is a reasonable constituent of a livable atmosphere - this world has a high argon atmosphere that will allow passage of infrared radiation both ways with minimal interference.


I can imagine a dry rust planet that could host life - not super exotic. If there is oxygen in the air you would need to scheme up some dry world biochemistry to allow it. Maybe something prying the oxygen loose from iron using radiant energy? Combining it with atmospheric hydrogen to make water?

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    $\begingroup$ Specific heat isn't the critical thing for temperature swings. It's effective heat capacity that matters -- a material that has a high specific heat but is also a good insulator will heat only at the surface, getting hot but storing very little energy. Because of this, you need to avoid water as much as possible: not only does it have a high specific heat, but it also moves that heat quickly through both conduction and convection. $\endgroup$ – Mark Jun 4 at 21:30
  • $\begingroup$ the heat-swings are observable in some regions on earth, no reason for special planets there. The crux in OP's demand is the blanket temperatures planetwide. $\endgroup$ – bukwyrm Jun 5 at 12:33
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Wouldn't this be what Earth would be like if there was slightly less atmosphere?

The atmosphere absorbs radiative energy during the day, preventing much of it from reaching the surface of the earth, and retains it as heat energy during the night instead of allowing it all to reflect back into space.

If you want a simple way for the Earth-like planet to heat up a bit more during the day and cool down more at night, reduce the total volume of the atmosphere by a few percent and keep everything else exactly the same as Earth.

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This is an answer to more than just your question in particular [CW: TV Tropes], but this seems like as good a place as any to put it:

Planets are big

If you care about making your world make sense, you should care enough to not make a single-biome planet (because they don't make sense). People in the other answers are offering partial solutions to your query, but they're generally failing to take into account secondary effects and the straightforward geometrical fact that incident radiance falls off sinusoidally with the distance to the equator.

In other words:

No

What you're looking for cannot be made to happen globally if you want your world to make sense.

If you don't care to have it make sense, just say that it does. If you care about it making sense, put some other constraint on the size of the geographical area in which your story takes place. It doesn't need to be a tight constraint; the kind of situation you're asking for is pretty much any desert, and we have deserts on Earth, like the Sahara, that cover a solid twentieth of the land on Earth.

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  • $\begingroup$ My answer covers the problem of unequal incidence - and why not have a planet that appears to be a one-climate-zone-planet trope, but actually spoofs it? $\endgroup$ – bukwyrm Jun 5 at 12:38

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