Can a planet have a warm, tropical climate both at the poles and at the equator?

Question

I am trying to create the perfect planet for life. It will mainly consist of shallow seas/oceans with archipelagos spread throughout the globe. The islands are the same size as New Guinea on average. Since rainforests have a lot of biomass and biodiversity, that will be the climate that almost all of the islands have (along with some grassy plain islands).

On Earth, the equator has plenty of hot deserts while the poles have cold tundras. Is it possible for both the equator and poles of a planet to have the same habitable temperature range? Assume that the mass and volume of the planet are the same as Earth. The planet also gets 10% more irradiance than Earth does.

Answer 1

Yes, with higher air pressure/density and temperature

See these pages for details: https://worldbuildingpasta.blogspot.com/2022/05/climate-explorations-temperature.html

https://worldbuildingpasta.blogspot.com/2020/03/an-apple-pie-from-scratch-part-via.html

Since you want a warm, tropical climate both at the equator and the poles, set your atmospheric pressure to be slightly higher than Earth’s and your average surface air temperature to also be higher. An atmospheric pressure of 2 or 3 atm should be enough (as long as you also increase the average temperature through more heating). This will reduce equator-pole temperature difference as the heat will be distributed more evenly across the planet.

An average surface air temperature of >20°C will give tropical conditions across more area of the planet. Do note though, that this will also increase precipitation and (depending on your setup) lead to more frequent and more severe storms.

Answer 2

You might like to explore Build Your Own Earth, at the University of Manchester. This lets you play with various settings, and adjust things like the amount of ocean.

Disclaimer: I played around with this site a few years ago, but haven't used it since. YMMV.

Answer 3

How to solve for a planet that is 100% true tropical zone.

Smaller Axial Tilt

Earth's tilted axis causes the polar regions to suffer from a year long day/night cycle which would be problematic to life even if it was relatively warm. Also, a tropical zone by definition has no winter/summer cycle. So, if the Axial tilt were essentially zero, it would not matter how far north or south you go: Day and Night would stay about 12 hours each, and there would be no noticeable seasons. Definitionally speaking, a world with no axial tilt is all Tropical Zone, even if some parts are cooler than others.

Thicker Atmosphere

But I know you are looking for more than just Tropical seasonal patterns; so, the next thing is a thicker atmosphere. In general, the thicker the air is, the more thermal energy gets moved around on the surface. This will help to stabilize the temperature between the equator and the pools because there will be stronger thicker wind currents circulating hot and cold air between the regions.

Shorter Days

While more air helps carry more energy for how much it moves, the force that actually gets these currents going is the Coriolis Effect. This is the principle that causes air currents to move in large circles around the planet bringing air between the poles and the equator. The faster your planet spins, the faster and stronger these currents will be that help mix the cold northern air with the hotter equatorial air.

Don't let it get too hot.

That said, when you increase the atmosphere and coriolis effect, the temperature does not just get more stable, it gets hotter and windier too. One way or another, this planet will be more prone to hurricanes than Earth is, but by keeping it relatively cool, you can control how strong the hurricanes can get. Hurricanes are powered by heat; so, as long as you keep the oceans around a modest room temperature, you don't have to worry about any high category destroying everything in thier path. To do this, you will want the planet to be farther from the sun relative to its luminosity compared to Earth.

How to make a planet as climatically homogenous as possible.

While a zero axial tilt gives each part of the planet a very stable climate, and the other stuff will help keep the whole planet about the same temperature, the poles may not get enough direct sunlight for good photosynthesis. There is another solution where your planet will still still have seasonal zones, but the whole plant will always get enough light to support life.

Binary Star System

Imagine a binary star system with one small dim star like a brown dwarf (StarA) and a much brighter main sequence star (StarB). Next place an Earth like world in a small short orbit around StarB such that orbital distances give both stars have about 1/2 of our sun's apparent luminosity. This is possible because a star's mass to luminosity ratio is not 1:1; so you can be gravitationally bound to just the smaller star while receiving equal amounts of light.

Because you are so close to the smaller star, you will also have a very short StarA year. Make this about 3 days so that you can give it as short of a year as possible without the day/night cycles of the two stars syncing up in counter productive ways. Next give the planet about a 45 degree axial tilt. This way the North pole gets StarA's light for about 1.5 days, and then the South pole gets it for 1.5 days. Not long enough to create a proper season, but it is long enough so that half the planet's heat and light comes from what hemisphere you are on and the rest of your heat and light will come from StarB's Day/Night cycle. StarA makes sure that each latitudes gets enough heat and light on a regular basis while StarB makes sure that each longitude gets enough heat and light on a regular basis.

Your planet will still experience latitudinal seasons like Earth does, so, it's technically not a 100% true tropical zone world, but StarA will significantly offset the the severity of the winter/summer cycle so instead of your planet having an average 50°F degree seasonal variance in the temperate zone like Earth does, it will be more like a 25°F degree seasonal variance.

Combine this with a thicker atmosphere and faster spin and you should be able to get the seasonal variance down to less than 10°F making it more or less unnoticeable.

The one thing of note in this setup is that far more of your light would have to come from infrared than on Earth; so, your native life forms may be adapted to see and photosynthesis in different parts of the spectrum than earth based life.

Answer 4

Thick atmosphere, high axial tilt, long years, and volcanic activity

First, as has been suggested, you need a thicker atmosphere to better distribute the heat.

Second, a high axial tilt could give you polar islands that are tropical during summers and are abandoned during the cold and dark winters (with an ecosystem adapted to this change). No axial tilt can give you polar regions that are always warm without making the equator blistering hot. This will work best with fairly long years that make summers last longer.

Third,your planet could have high volcanic activity that heats the islands from underneath. This could be because the planet is still young, or because of strong tidal forces from a large moon or being the moon of a large planet. Tidal heating will however imply very extreme tides. Volcanic activity will make the ground hot, and you will have hot springs that heat the nearby air. The caldera of near-dormant volcanoes will provide valleys that retain the heat from hot springs, and deep canyons could carry hot rivers to the sea (think Grand Canyon with a hot Colorado River).

Answer 5

Place it closer to a weaker star, to have shorter years

Stars have an advantage for this purpose in that they don't release as much energy when being only slightly smaller, this allows placing planets closer to smaller stars so that they would have shorter years, that would translate into shorter winters, less pronounced seasons and all that. Say, if you'd have a planet compared to Proxima Centauri b but just a tad further to not get tidally locked too fast, you can have a planet with years lasting an Earth month but just several local days, that would turn seasons into a kind of fluke.

Also, higher (not lower) axial tilt, from 30 to 45 degrees, would allow higher overall heating on poles and lower on equator, due to more time spent with higher elevation of sun over the horizon on poles and lower on equator, this, together with overall energy transfer of an Earth-like atmosphere would cause the poles to not freeze too deeply even during the polar night, that would last half a month or a tad less since the visible angle of the sun will also be larger.

Past this, play with atmospheric density, compound, water level and overall planet's stats like mass and gravity to cause enough energy transfer all over the planet, and voila, you have a planet with about equal temperature ranges on poles and equator; I expect such a planet to be able to be tuned into hot summer days but no freezing winter days or polar nights. It'll likely be painted red and black everywhere, regarding our color system, with faint blue sky due to blackbody spectrum having a tiny blue tail, but local life might have different visible spectrum range to compensate for this.

Answer 6

Yes, you could have such a world by having it be closer to it's star. in fact, 50 million years ago, the earth's poles were so warm that it had palm trees up there. in order to have a tropical climate at the equator, you could add more water, so the water near the equator would evaporate and make a cloud band, increasing the albedo around the equator, and keeping it from becoming a desert.