I have a small planet with a Radius of 3,900 kilometers and a density of 8.9 g/cc. It has an extremely Earth-like climate and has been extremely volcanically and geologically active in the "recent" past, creating numerous fissures, lava tubes, and the like. This planet will only have a few freshwater lakes about the size of the Mediterranean sea, no large salty oceans. Is it feasible through a network of underground aquifers and rivers to have a planet largely (at least 85%) forested?

  • $\begingroup$ Your planet would be quite a bit bigger than Earth. Earth got a radius of ~6.400 km and a circumference of ~42.000 km. Your planet got a circumference of ~49.000 km. That would be a planetary mass of ~17.69 * 10^24 kg, nearly three times the mass of Earth. Maybe your planet isn't so small after all... $\endgroup$
    – DarthDonut
    Commented Jul 6, 2018 at 11:38
  • $\begingroup$ I just realized I put the diameter as the radius! @ DarthDonut $\endgroup$
    – Thalassan
    Commented Jul 6, 2018 at 15:00

3 Answers 3


Rainforests generate their own rain

Forests, if extensive and active enough, can generate their own rain. Plants transpire, which is the movement of liquid water from the roots to the leaves of plants, and which results in evaporation from the leaves.

If you have enough plants in the same place, their transpiration can cause humidity to rise enough to cause cloud formation. If clouds form during the day, and the temperature drops (as, in the evening), then rain will fall. This will be the same water molecules that were transpired by the trees earlier in the day.

Depending on atmospheric circulation, it is certainly possible for a planet to be so heavily forested that all the rain that falls was previously transpired by those same trees. In essence, it is possible for a planet to be completely forested, and for its forested condition to cause it to rain every single day.

If the forests generate their own rain, then, given time, an entire planet could be covered with such forests.

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    $\begingroup$ Without extra lakes to provide more evaporation this process would slow and eventually stop. its impossible to have the plants use the water, transpire it, evaporate it and then reuse it with 100% efficiency. and even if somehow it was 99.9% efficient, it would eventually slow and stop. in a lot less time then it would take for the plants to grow significantly and cover the planet. $\endgroup$ Commented Jul 6, 2018 at 8:03
  • $\begingroup$ @BladeWraith I'm pretty sure the process is at least 100% efficient. Water just doesn't disappear. As long as seepage into ground water is equalized by tree roots pulling moisture out of that same ground water, you can get a closed cycle with no loss. $\endgroup$
    – kingledion
    Commented Jul 6, 2018 at 12:32
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    $\begingroup$ Yes most of the water the plants use is evaporated, but they still need water to grow. so if it were 100% efficient then the plants would never grow, never create seeds that would then create new plants, and no animals what so ever can exist that would either eat the plants/seeds. by our current understanding of physics and biology. without a lot of water, life cannot exist as we know and understand it. All i'm saying is there should be a lot of lakes around, to provide more evaporation to provide more water to younger plants with smaller roots that haven't reached underground aquifers yet $\endgroup$ Commented Jul 6, 2018 at 14:15

The Earth-Like climate is incompatible with the lack of oceans. Earth-like climates rely upon ocean currents to move massive amounts of heat from the tropical zones, warming chilly places like Europe and western North America to temperate. Without ocean heat movement, you will have more clear, discrete boundaries between the tropical, temperate, and arctic zones. The arctic zone will be bigger than Earth's, and the temperate zone smaller. There will be no seasonal monsoons or cyclones/hurricanes/typhoons.

Evaporation, mostly from oceans, provide 86% of the water in the Earth's air. Even quadrupling the amount of transpiration cannot make up for the lost evaporation. That means less rain: Winter snowpacks will be thinner, rivers will be smaller, erosion and deposition will be slower, glaciation will be much less pronounced on landforms. That means fewer and less-widespread porous strata like limestone and sandstone, and relatively more non-porous strata like granite. In turn, that means smaller and fewer aquifers, and fewer springs.

Less rain, smaller rivers, fewer springs, and smaller aquifers means that the planetary surface will be relatively dry. Less polar glaciation means more hills and valleys (instead of flat plains) in the temperate zone. You will see more bigger trees where the water table is high in the valleys, but transitioning to mere scrub just a few meters up the slope away from that water table. With forests chopped up by hill/valley terrain, rainforests seem likely to be scattered and small.

Instead of competing for light, plant life will compete for water - trees in the wide marginal areas will be broad and short and spaced far apart, scrubby above ground while pushing deep roots in search of water, rather like alpine or sub-arctic trees.

With reduced seasonal rains (or snowmelt), steppe grasses and forest undergrowth will be reduced.

  • $\begingroup$ As mentioned in the answer, you need oceans for a planet to be earth-like. But if you have an earth-like planet, it is easy to have all land covered with forests. The natural state of earth is forest; there war forest in noth america before the first humans came there, for europe its the same. So without human interference you would propably have continents with forests, some deserts (but much much smaller than on earth, if any), some lakes, and of course the oceans. $\endgroup$ Commented Jul 6, 2018 at 8:21

Given the information stated, this is not an Earth-like planet. Under many definitions it is a super-Earth. Diameter is larger than any terrestrial planet in the solar system and surface gravity is 2G. The density is impossibly high - it is more dense than iron, and iron is the final abundant product of stellar fusion.

Secondly, it is unavoidable for the final outflow bodies to be salty unless there is (somehow) no salt on this planet - which means no metal salts. Chemistry that weird immediately stops it being an Earth-like planet.

Finally, without some bizarre characteristics of the primary and orbit, there will not be a uniform climate and associated ecology across an entire planet. If you want all of the land mass to have the same climate then it all needs to be at roughly the same latitude under most circumstances. Star Wars has popularised the idea of a "desert planet", an "ice planet" and a "forest moon", but actual worlds will inevitably have temperature variations between different areas based on the sunlight received.

  • $\begingroup$ For example, you could have fairly uniform temperature zones by combining a high inclination with a very fast orbital period. However to get that fast orbital period, you will be very close in -- so either the uniform temperature is "molten lead", or the primary has very low luminosity: a very dull red glow filling half the sky. Still not very "foresty" ... $\endgroup$
    – Securiger
    Commented Jul 6, 2018 at 11:30
  • $\begingroup$ Assuming oceans, you can get a reasonably similar climate pole to pole. During the Cretaceous, you had a lower temperature gradient from pole to equator, average temperatures up to 10 C higher than now (largely due to CO2 levels 2.5 times higher), and forests from the top of Ellesmere Island to the South Pole. There were still deserts (the Gobi being an obvious example), but nowhere near as extensive as today, while temperate forests were much greater in area, even discounting the effect of humans. $\endgroup$ Commented Jul 6, 2018 at 14:51
  • $\begingroup$ I just realized I put the diameter as the radius! @KerrAvon2055 $\endgroup$
    – Thalassan
    Commented Jul 6, 2018 at 15:04

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