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I would like to create a world with a temperate habitable zone as well as very hot and very cold zones. The hotter and the colder the better, but I don’t want it to be tidally locked. How can I do this?

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    $\begingroup$ Earth-like rotation, size and atmospheric pressure and composition? $\endgroup$ – Rodolfo Penteado Jun 13 '20 at 16:13
  • $\begingroup$ Earth like yes but with a lot of wriggle room. Needs to be habitable by humans or at the very least human like creatures $\endgroup$ – Slarty Jun 13 '20 at 16:25
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    $\begingroup$ You really need to be more specific about what you consider very hot & very cold. Otherwise, Earth satisfies your criteria, since it has very hot and very cold zones. Heck, you don't even need to get out of the western US: summer in Las Vegas certainly fits my definition of "very hot", and winter on the Colorado plateau - just a short drive away - can get pretty darned cold. $\endgroup$ – jamesqf Jun 13 '20 at 17:23
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    $\begingroup$ How steady do the temperatures have to be? For instance, deserts are often both extremely hot and extremely cold. This is because they lack water and other materials of high specific heat, which moderate temperature extremes by taking a long time to heat up and cool down (and absorb/let off a lot of heat to their environments by doing so). $\endgroup$ – Mary Jun 13 '20 at 17:33
  • $\begingroup$ @jameqf hotter and colder than Earth, to hotter and colder the better. The person suggesting the best method for the greatest range is the best answer $\endgroup$ – Slarty Jun 13 '20 at 17:44
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How about having a planet with an extremely high degree of obliquity/axial tilt- say, 75-85 degrees of axial tilt, comparable to that of Uranus in our solar system (which has an axial tilt of just over 82.2 degrees), perhaps with a large moon to stabilize this obliquity in the way that Earth's moon did? That way, you'd have a relatively thin 'temperate habitable zone', situated at its equator, whilst its Southern and Nothern hemispheres alternated between extremely hot summers and extremely cold winters on an annual basis. Effectively, you'd be compressing said planet's 'habitable temperate zone', with room for temperature climate extremes vaguely resembling those of Earth, into its equatorial region; whilst effectively turning its polar regions into nigh-uninhabitable zones.

Effectively, you could have a 'Urania' style planet, as postulated by Neil Comins as one of the alternate scenarios in What If the Moon Didn't Exist? Voyages to Earths that Might Have Been- one where Theia struck the Earth at a different angle, both forming an alternate version of the Moon and stabilizing the axial tilt of Earth/'Urania' at a far more extreme angle than it did in our timeline. The polar regions of this alternate version of Earth, in mid-winter, would be far colder than any part of our Earth ever has been, even during the 'Snowball Earth' period; meanwhile, the summer side would presumably equally be far, far hotter. Spring would foster enormous storms, as all of the ice (both water and other forms of ice, including dry ice/frozen CO2) in that hemisphere rapidly melted, a week or so after sunrise. All this water and heat being pumped into an entire hemisphere would foster gigantic storms, buffeting the small temperate zone near the equator with super-cyclones. Meanwhile, the hemisphere moving toward winter would rapidly cool once the sun went down, with all of the water there freezing solid within a few weeks of sunset, and much of the atmosphere itself beginning to sublimate and solidify into a huge polar icecap shortly thereafter.

As a result, not only temperature extremes, but CO2 levels, atmospheric pressure, and humidity levels, on said planet would also likely fluctuate wildly over the course of the year, with the atmosphere in the temperate, habitable equatorial region reaching its thinnest and driest extreme shortly after its summer and winter solstices (due to a large portion of the atmosphere itself being frozen into surface deposits in the seasonal, winter hemispheric icecap, and removed from circulation), and reaching its thickest and most humid extreme shortly after its autumn and spring equinoxes, during the height of the planetary storm season (when it'd all be melted and released back into the atmosphere again with the thaw ending on the bright/summer hemisphere of the planet, before commencing the process of circulating over to the dark/winter hemisphere, and being sublimated into dry ice once more).

What do you reckon, then- could the 'Urania' scenario, with your Earth-like terrestrial planet possessing a similarly large moon to our own to stabilize its axial tilt at a far more extreme angle, effectively spinning on its side like Uranus does, offer the kind of non-tidally locked, extreme-temperature, extreme-weather world you're looking for?

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  • $\begingroup$ Elaborating upon this- you'd imagine that, if this planet did indeed have its own indigenous life, you'd only see either aquatic or terrestrial complex lifeforms, not both, dependent upon what the planet's surface is mostly comprised of. Because plate tectonics would cause the mass extinction of any lifeforms which evolved on/in any continents/seas whenever they drifted out of the habitable bands, into the polar regions- in which, life on the surface would be all but impossible, and you'd be limited to deep-sea and/or subterranean fauna. $\endgroup$ – Aquar1animal Jun 13 '20 at 23:54
  • $\begingroup$ Or, alternatively, you could have an ecology wherein every living organism, on the planet's surface, goes through seasonal predictive and/or consequential dormancy, and/or has a annual, seasonal cycle of birth, life, reproduction and death as a necessitated, universal strategy for survival. You'd have plenty of possibilities, and room for imagination. $\endgroup$ – Aquar1animal Jun 14 '20 at 0:01
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    $\begingroup$ I like the idea. depending on the size of the orbit and the degree of the tilt conditions could be adjusted from flip flop Earth to a crazy atmospheric hurricane and everything in between. I imagine there would be a wide band of possibilities where the planet never reached thermal equilibrium. For example the air might start to condense for a few days or weeks and then start to evaporate again, much as happens here with snow. $\endgroup$ – Slarty Jun 14 '20 at 7:15
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If we want to avoid tidal locking, we need to find alternative mechanisms for heating (or cooling) particular regions of the planet. Heating is fairly simple: simply inject energy into a particular region. Observations of hot spots on several planets (unfortunately, gas giants) have shown that there are a couple of ways we could accomplish this:

  • Have strong aurorae heat the upper atmosphere through x-ray spectral line emission (e.g. C VI, O II and O III), as observed on Jupiter (Dunn et al. 2017)
  • Take advantage of shocks or gravity waves produced in the upper atmosphere of planets close to their parent stars, as may be the case on Upsilon Andromedae b, producing a hot spot not on the daytime side but nearly perpendicular to the star (Crossfield et al. 2010)$^{\dagger}$

Though these effects have been observed on gas giants, presumably similar phenomena could be achieved on terrestrial planets. I wouldn't be surprised if interactions between stellar and planetary magnetic fields could also transfer energy into a spot in the atmosphere; this is know to lead to synchrotron emission in one or two cases, presumably accompanied by some heating.

Forming a cool spot is trickier. Jupiter has a cool spot, thanks (ironically) to the aurora, but it exists only at high altitudes, above a warmer atmosphere (Stallard et al. 2017). Perhaps there could simply be an area where air easily circulates with the polar regions but not with lower latitudes near the equator; a lack of circulation would be useful for both hot and cold spots, to maintain temperature differentials and prevent heat from diffusing outwards. I'll admit that I'm a bit stumped on the specifics, though. In general, it's easier to find a way to dump a large amount of energy in than to siphon the same amount out.


$^{\dagger}$While Upsilon Andromedae b may be tidally locked to its parent star, I believe tidal locking is not necessary in general to produce gravity waves or shocks, though many models assume tidal locking (see Watkins & Cho 2010).

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  • $\begingroup$ Some interesting ideas, but would such a world be habitable? $\endgroup$ – Slarty Jun 13 '20 at 17:57
  • $\begingroup$ @Slarty Although the models involving shocks or gravity waves would likely require the planet to be close to the star, I don't think anything outright precludes you from using any of the ideas on a habitable terrestrial planet. I think the auroral route in particular seems likely to cause few effects on an otherwise habitable world. $\endgroup$ – HDE 226868 Jun 13 '20 at 17:59
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There is a world like this in Asimov's Trilogy of the Foundation, 2nd book, chapter 16. The world's name is Randole.

Here, the planet's axis of rotation is always pointing towards the star (partially tidally locked?), and it has a dark side, where "oxygen runs liquid over the surface", and a light side, where lava might be a common sight.

It has a habitable zone in the twilight equator and "it has almost transformed into a producer of luxury articles".

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The only alternative to this that I can think of, would be a planet similar to Earth but with the rotation axis perpendicular to the orbital plane. There wouldn't be any seasons depending on astronomical events, but maybe depending in the positions of the continents and seas.

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    $\begingroup$ Nice idea, but note the science-based tag on the question, this constrains answers to be based on what's scientifically possible and since the conservation of angular momentum would be violated by a planet's axis pointing always towards the star, this won't work. $\endgroup$ – A Rogue Ant. Jun 14 '20 at 14:03
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    $\begingroup$ Randole is tidal locked. $\endgroup$ – Rodolfo Penteado Jun 15 '20 at 1:00
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The planet is slightly larger than the Earth, 34% more massive and 15% larger at the equator. The planet's average gravity is just 1% above, negligible.

The planet orbits around 1 AU and has an eccentricity comparable to Venus.

Your planet had a formation similar to that of Earth, with a strong impact like the Big Splash. Unlike the cradle of humans, your planet kept the mass of debris in a disk close enough that it precipitated again along the equator, forming a mountain range that, as it eroded, gave an unusual flattened shape.

The composition of the planet as a whole is slightly lighter (87% of density) than that of the Earth. These lighter materials form a much thicker solid crust similar of the Venus, that is still covered by the debris from the Big Splash.

The planet has neither a moon nor tectonic activity. The balance of the planet also means that there is no inclination of the axis of rotation. The atmosphere is similar to that of Earth, perhaps a little richer in carbon in its early days. As a whole, it must also be more massive, in order to have a similar pressure on a 33% larger surface.

The flattened shape of the planet makes gravity stronger at the poles than at the equator, attracting bodies of water. The planet has two polar oceans and a long mountain range at the equator, similar to that of Iapetus.

In addition to the water bodies, the heavier cold air is also trapped at the poles, the surface of the oceans near the poles freezes and makes the air dry. The albedo at the poles prevents any absorption of heat making the temperatures quite low.

A possible problem that such a planet can have for the formation of life is that the poles can become sinks of carbon dioxide. The gas would condense and be deposited on the glaciers, stack and interrupting the carbon cycle. Then the ice layer at the poles contains both water and condensed carbon dioxide from the atmosphere. The edges of the glaciers are eroded by the winds, sublimating carbon dioxide and melting the water. The deposition of the elements inside the glaciers is slow thanks to the circumpolar winds separating the air masses over the ice shield and over the ocean.

In the equatorial regions, intense solar radiation, added to the high altitude, forms a very hostile desert, with very high temperatures during the day. Heated wind currents in the daytime travel easily over the desert and prevent low temperatures at night.

Near the coastal regions in the middle latitudes where the polar and equatorial air masses meet there is intense condensation, creating conditions for the water cycle and an environment friendly to life as we know it.

Without seasonal variations, the planet's temperatures intensify. A long strip of clouds runs along the middle latitudes near the coast in both hemispheres.

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