map of planetary surface

According to several articles (such as this post), a habitable inner planet (more than 1.1x Earth’s insolation) could realistically form with liquid water (and avoid turning Venusian) through the following conditions:

  • a slower day length (enough to form a permanently cloudy day-side) or be completely tidally-locked to the star
  • have less water/no oceans on the surface so that most of the planet is a desert
  • have a thicker atmosphere with more nitrogen (but not greenhouse gases), or using gases like sulphur dioxide to form a haze

I want a habitable inner planet with oceans orbiting a K-type star and without any of the 3 above conditions. It has a fast rotation, similar water coverage to Earth (see the above map) and doesn’t have a hazy atmosphere.

Are there any other plausible ways I can save this planet from a runaway greenhouse (or a moist greenhouse) at 1.44x Earth’s insolation?

Note: 1.44x Earth’s insolation is equivalent to being roughly 0.83 AU from our sun. In the case of my planet orbiting a K-type star, the planet distance is 0.53 AU away from its star

  • 2
    $\begingroup$ Nitrogen is transparent both in visible light and in infra, it does not affect albedo. Your title is about fast rotation, your content is about slow rotation, now which? Your title says habitable planet, your body says little or no water, now which? $\endgroup$
    – Gray Sheep
    Commented Jul 8 at 20:25
  • $\begingroup$ The body was explaining the usual methods an inner habitable planet can form (through ways such as slow rotation or less water). My title and end of my body are asking how I can still have a habitable inner planet with fast rotation and large oceans etc. $\endgroup$ Commented Jul 8 at 20:54
  • $\begingroup$ I'm not seeing anything in the question about where in the habitable zone you are wanting to put this planet. What's wrong with the outer part of the zone? Slightly lower solar input would take care the fact that you want to have a higher greenhouse factor. $\endgroup$
    – Futoque
    Commented Jul 10 at 21:57
  • $\begingroup$ @Futoque 1.44x Earth’s insolation = 0.83 AU (if my planet orbited our sun). In the case of my system, the planet orbits a K-type star at 0.53 AU. $\endgroup$ Commented Jul 10 at 22:22
  • $\begingroup$ What sort of K-type star are you looking at? For a K0 star like Sigma Draconis the inside edge of the habitable zone is around 0.6 AU. For a K7 star like 61 Cygni B the outside edge of the habitable zone is around 0.5 AU. $\endgroup$
    – Futoque
    Commented Jul 10 at 22:47

2 Answers 2


In order to make this world habitable, it needs to have low amounts of greenhouse-positive gases in its atmosphere (such as carbon dioxide or methane), potentially high amounts of greenhouse negative gases (such as sulphur dioxide), and higher albedo.

Since the OP doesn't want a hazy atmosphere with high sulphates, it's basically going to come down to low carbon dioxide and higher albedo. A little sulphate in the atmosphere may be acceptable.

As the temperature of this world rises, the OP's large seas are going to experience higher evaporation, which will lead to increased cloudiness, thus increasing albedo. As long as global warming doesn't proceed too far, this should be acceptable.

Since this world has greater insolation by definition, it may be that the vegetation has evolved to absorb only the necessary wavelengths of light, and reflect other wavelengths more than terrestrial plants do. This would serve to increase the world's albedo in vegetated areas.

The world could also have large areas of highlands near the equator that are high enough that they care covered with snow and ice, which also have a high albedo. The OP's map appears to show just such white highlands. In arid lowland areas, salt flats also have a high albedo.

Water is reflective when there is a low angle of incidence between the surface of the water and the light, such as occurs near sunrise and sunset, though it becomes lower in the middle of the day. However sea-foam also has a higher albedo, so another way of ensuring that this world's albedo remains high is to introduce organic compounds into the water that increase the amount and longevity of foaming in choppy seas, such as proteins and lipids.

So, with the right combination of airborn sulphur dioxide, low carbon dioxide, equatorial ice and snow coverage, cloudiness, reflective plants and concentration of organics in sea water, this world could easily have a sufficiently high albedo and low amount of greenhouse-positive gases to have an earthlike temperature.

As to how it evolved, high amounts of sulphur dioxide and a position further away from the star early in its history, followed by inwards migration as the amount of oxygen rose and greenhouse gases fell as a result of the processes of life could account for this without ending up with a venus-like world.

  • $\begingroup$ Very good answer! And yes, some sulphates are acceptable, just not enough to make an ash-like volcanic haze. And as long as the sky is still blue and not hazy, and the topography and oceans should be still clearly visible from space. The planet should still look earth-like just a lot sunnier. More clouds are acceptable (and inevitable with more evaporation) too but not a permanent cloud cover. I assume the cloudiness you’re talking about would be similar to Earth’s tropics (such as the Amazon or Polynesia). $\endgroup$ Commented Jul 11 at 20:01
  • $\begingroup$ @casualworldbuilder Yes, pretty much blue skies with tropical clouds, silvery plants, icy peaks and foamy seas. $\endgroup$
    – Monty Wild
    Commented Jul 11 at 23:41
  • $\begingroup$ would covering the surface predominantly in calcium-based rocks not be enough? If for some reason or another, most of the surface lithosphere is chalky, it will not only vastly increase albedo, but soak in CO2 that loves to react with limestone-group of rocks. Im not sure if there is a geological way for the world to be surface-covered in limestone, but there is an easy biological one; have the oceans slosh about and the continents rise and sink often, so that the lithosphere gets layered with dead mollusk shells and some such (basically Dover, but over the entire world). $\endgroup$ Commented Jul 12 at 8:50

Space shades are a wild but potentially game-changing idea here. Picture massive, ultra-thin mirrors floating in space, strategically placed to cast a shadow on our toasty planet. We're talking about parking these behemoths at the L1 Lagrange point, right between the planet and its star. It's like giving the planet a pair of cosmic sunglasses. The goal? Knock that intense 1.44x Earth insolation down to something more manageable, say, close to what Earth gets. We'd need to block about 30% of the incoming starlight. It's a hefty engineering challenge, but it beats trying to tweak the planet itself. The beauty of this approach is that we don't have to mess with the planet's atmosphere, spin, or oceans. It gets to keep its fast rotation and Earth-like water coverage. Plus, we could theoretically fine-tune these space parasols, adjusting them as needed to dial in the perfect climate.

An interesting twist on this idea: instead of just blocking light, we could use reflective mirrors to bounce some of that energy to the poles or the night side, make an icewind dale-style city of eternal spring far up north. It's like playing planetary ping-pong with sunlight.

The end game here is to create a Goldilocks scenario where liquid water is stable and temperatures are comfy for life as we know it. It's a high-tech solution that lets us keep the planet's cool features while giving it a more Earth-like energy budget.

  • $\begingroup$ This is also a good answer for people wanting a terraformed inner planet. I was more looking for natural methods but keep this answer up for those who want an artificial route. $\endgroup$ Commented Jul 11 at 20:11

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