Assuming an earth-like planet, but with several billion more years of cooling and settling (and with no moon to create tides). I'm imagining this planet would be geographically and tectonically tedious, with most mountains worn down and the sea levels relatively even across the surface apart from a few flat islands.

My questions: Assuming the sun is still relatively consisted, Is there some reason human(like) beings wouldn't be able to inhabit said planet? As a follow-up--what would the atmosphere and weather look like?

  • $\begingroup$ how is your planet different from Mars? It is a smaller than Earth, but not by a lot (and it explains why it cooled faster). Mars has moons, but they are way smaller than Earth's Moon, so they cannot produce tides. $\endgroup$
    – Bald Bear
    Commented Nov 14, 2018 at 21:37
  • $\begingroup$ This video might be of interest: youtube.com/watch?v=GP8MjPQrpiU $\endgroup$
    – Nathan
    Commented Nov 14, 2018 at 21:37

2 Answers 2


If the planet has reached an age where internal heat is no longer sufficient to maintain active tectonics, it will enter what is known as a "Stagnant-lid Regime".

One of the first things that happens at this point, is that the global carbon cycle grinds to a halt. Volcanism is severely reduced and ultimately extinguished, which means that carbon ceases to be recycled into the atmosphere. Within a handful of millions of years, the atmosphere of the planet will be devoid of significant carbon dioxide.

The climate of the planet will suffer a reduction in greenhouse effect due to loss of carbon dioxide, so it will be cooler that is otherwise would be, and furthermore, in the absence of carbon dioxide, photosynthesis as we know it becomes impossible. Plant life, and all life that depends on it, goes extinct.



  • $\begingroup$ On Earth, carbon dioxide contributes to a very small fraction of the greenhouse effect. The bulk of the greenhouse effect is due to water vapor. But you are right about the end of photosynthesis. $\endgroup$
    – AlexP
    Commented Nov 14, 2018 at 22:32
  • $\begingroup$ @AlexP This is true. I'll edit to reflect this. $\endgroup$ Commented Nov 14, 2018 at 22:43
  • $\begingroup$ Your first reference says plate tectonics helps sequester carbon, preventing runaway greenhouse effects. Isn’t that the opposite of your claim? $\endgroup$
    – cms
    Commented Nov 15, 2018 at 1:20
  • $\begingroup$ @cms Tectonic activity regulates the carbon cycle by sequestration and reintroduction of volatiles. $\endgroup$ Commented Nov 15, 2018 at 1:36

Once the planet has dissipated most of its internal heat, also the internal convection will be significantly reduce or even stopped (look at Moon or Mars).

Based on our current knowledge, a lack of internal convection means weaker or no magnetic field.

A magnetic field is a good shield against solar wind and helps protecting the atmosphere from its energetic action, again look at Mars.

Also, volcanic activity helps in supplying Phosphorus to the biosphere, which is need for vegetation to thrive.

In conclusion, I doubt that a geologically old (or quiescent) world would still be habitable.


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