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Imagine a planet, 3/4 water and 1/4 landmass. No axis tilt so the climate is moderate throughout the year. IE peak temperatures at 100°F at equator.

It orbits a sun about the size of ours and has a slightly different orbit.

What is the smallest circumference I can make my planet be and still have ocean currents supported by ice caps?

Its okay if I have to change the orbit, and assume that whatever the size the magnetic field will still protect the planet.

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  • $\begingroup$ Presumably there's complex, multi-cellular life on this planet? (Because Mars at one time apparently had oceans. We don't know if Mars could have developed like a colder Earth if it also had an iron/uranium core and thus a magnetosphere.) $\endgroup$ – RonJohn Aug 29 '18 at 8:07
  • $\begingroup$ @Ron John Yeah, I was hoping to scale down earth so there wouldn't be as much landmass, because I mistakenly thought that our magnetosphere protected our atmosphere from being stripped off by solar winds, rather than our gravity holding it in place. It seems the gravity is more important though, and I can't scale it down as much as I wanted originally. $\endgroup$ – Clay Deitas Aug 29 '18 at 9:21
  • $\begingroup$ According to Wikipedia, the magnetosphere "mitigat(es) or block(s) the effects of solar radiation or cosmic radiation, that also protects all living organisms from potentially detrimental and dangerous consequences." Thus, it's dubious how much complicated life you could have without it, or even any life, if the young Earth was continually bathed in sterilizing solar and cosmic radiation. $\endgroup$ – RonJohn Aug 29 '18 at 13:14
  • $\begingroup$ What if the core is a 10m wide neutron star. I claim this world as planet volleyball. $\endgroup$ – anon Aug 31 '18 at 11:58
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The circumference does not significantly matter for this; the exact distribution of the continents actually matters more.

What is affected by radius/circumference is the planet's mass and gravity. If they are too low, then the planet will likely not have the large amounts of water that the question asks for. Playing with the orbit and just pure dumb luck can give a rough lower bound of 30,000 km for a planet with 75% water surface and Earthlike atmosphere. However it is possible to be much lower.

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  • $\begingroup$ How do the continents effect polar ice caps? $\endgroup$ – Clay Deitas Aug 29 '18 at 0:21
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    $\begingroup$ Not significantly at all, other than the lands near the poles themselves. Also it's "affect" not "effect" btw $\endgroup$ – Majestas 32 Aug 29 '18 at 0:22
  • $\begingroup$ But they do determine how ocean currents flow, not the heating and cooling of water in different parts of the ocean? $\endgroup$ – Clay Deitas Aug 29 '18 at 0:25
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    $\begingroup$ Yes. They can somewhat affect heating and cooling, but only in similar latitude belts to the continents themselves. If you have large north-south expanses of ocean, currents would be less pronounced than having large east-west expanses of ocean. $\endgroup$ – Majestas 32 Aug 29 '18 at 4:05

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