So I've ran into a bit of an issue. Not knowing much about geology, I thought that I could get away with making a planet with 150% Earth radius with it still having a geology identical to that of Earth and 9.8 m/s gravity. I was wrong, to say the least. After having done a bit of research, I came across this question which made me realize how much more alien a simple increase in radius can make my terrestrial planet. Diamond sand? Cool. Possibility of no tectonic plates and no ores in the crust? Not so cool. As a compromise I tried doing a bit more math and got the following figures :

Planet : 150% Earth radius, 280% Earth mass, 11.54 m/s gravity, 14.868 km/s escape velocity, 4.315 g/cm3 density, 2.25 AU orbit, atmosphere same as Earth

Star : 506% Sun luminosity, 134% Sun diameter

Taking this into account, what would I need to change about my planet in order for it to:

  • Have tectonic plates
  • Have a magnetic field capable of protecting the atmosphere
  • Not have a gravity higher than 13 m/s
  • Not have a radius lower than 140% that of Earth

And in the end would...

  1. The landscape be flatter, the same as on Earth, or with taller mountains?
  2. The oceans be shallow and flat or like on Earth?
  3. Would tectonic plates act different in any meaningful way?

This is a part that confused me a bit as the two answers to the question I linked were a bit contradictory in that regard.

EDIT: Clarified my question a bit.

  • 2
    $\begingroup$ ??? A planet exactly like Earth but with a radius 1.4 times as large as Earth's would have a gravitational acceleration at the surface of 1.4 g, that is, 13.7 m/s²; close enough?. A planet exactly like Earth but with a gravitational acceleration at the surface of 13 m/s² would have a radius 1.33 times as large as Earth's; close enough?. (And anyway the heavy metals in the crust, such as iron, were kindly provided by asteroids coming from outer space after the planet had cooled enough to form a solid crust.) $\endgroup$
    – AlexP
    Oct 3 '21 at 17:42
  • $\begingroup$ Related, but I believe it's not a duplicate due to the main focus being about its moon. $\endgroup$ Oct 3 '21 at 17:49
  • $\begingroup$ I did not know that asteroids were the ones that seeded the crust with metals. And I might have also not worded the question too well. However, that still does not tell me if mountains would be nonexistent or taller. $\endgroup$
    – Pipi Caca
    Oct 3 '21 at 17:56
  • $\begingroup$ Also consider that the question doesn't need to be answered for a good story. No one would question it if the planet is 150% larger, but is otherwise exactly like Earth. Not to dissuade you from asking your question, but it's an important consideration. $\endgroup$
    – Trioxidane
    Oct 3 '21 at 18:32
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    $\begingroup$ I apologize, but VTC:NMF ("This question currently includes multiple questions in one. It should focus on one problem only.") Further, from the help center, "Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much." Note, also, that sometimes the quest to be as "scientifically accurate" as possible is a fool's chase. The answer to your question is, "it won't be." And considering we have one (and only one) data point to work with - Earth - it's the best you're going to get. $\endgroup$ Oct 3 '21 at 22:15

Ok, make the core of the plant a little hotter, say by 500K, to make sure :

  • the outer core thick-ish and mobile and generating a strong enough magnetic field and the inner core still small
  • the mantle is fluid enough to allow some dynamics
  • the crust stays around 20-25km on the continental side

1.5R means 2.25 larger surface to radiate inner heat, so lets make sure we have enough of it to start with

Last two bullets should ensure you still have tectonics without running-amok vulcanism.

For a good measure, place an order for asteroids with everything Earth has in terms of the concentration of surface available elements and primary substances after the crust was formed - me thinks you'll need a wee more than twice the amount that Earth used.
Water is important enough for the fluidity of the mantle and to encourage the subduction by providing some lubrication, so make sure you order enough of it to have some oceans as a reserve for... ummm... rainy days
Also, ask for a surplus of oxygen. Not much, perhaps 5-10% will suffice, you'll see a bit later why.

At 10-15% extra gravity:

  • if you planned well the volatiles, you'll have your atmosphere a bit thinner than Earth's at maybe an slightly increased pressure at sea level, but not enough to mess the radiative absorption/rejection/cooling-at-night-time or to increase the eolian erosion.
  • the maximum height of the mountains will be lower than the same on Earth, because at equal compressive strength of the rocks, a higher gravity means the rocks will crumble at lower max height. But even Earth doesn't have mountains to the max of allowed by the compressive strength of the rocks, so you can still have a Himalayan range.
  • besides, with a continental crust about the same as Earth, those mountains (and glaciers) will push the crust about the same depth into the mantle; after all, is a matter of buoyancy, thus independent of gravitational intensity, so pretty much the same isostasy that you see on Earth

Ok. now we get to the need of enough magnetic field, even when diluted in a larger volume and the need of a bit extra oxygen. You see, that:

Star : 506% Sun luminosity, 134% Sun diameter

buggered me. While you took the pain to get the same radiance per unit of surface on the planet (at 2.25AU), you needed to pump up the star's radiation power but could not afford to increase its size. As such, the power increased by a factor of 5, but the "reaction" volume increased only by 1.343 = 2.4 and the radiative surface by 1.342 = 1.8. Stefan-Bolzmann law says that the temperature of your star is about 7500k and Wien's law says peak emission happens at 390nm - which is near-UV. Your star is a F-type main sequence and the habitability in Terran terms requires UV shielding. One on top of the other, you will need to:

  • use some oxygen to make ozone and keep it well inside the Van Allen belts
  • make sure you have enough magnetic field to sustain those belts against the stellar wind
  • never ever let the sapiens species there use (H)CFC for their refrigeration needs

With all that, you should have a planet as good as Earth, just a bit larger.


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