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I have planet with twice the radius of Earth, with its core being the same absolute size as Earth's in order to provide magnetosphere, and the rest of the planet made of silicates like Earth's mantle. There are oceans on the surface with similar depths to Earth's, covering 2/3 of the surface.

I chose those numbers in order to have the largest terrestrial planet, while keeping the mass below 10 Earth masses in order to not keep the hydrogen. With twice the radius and planet mostly made of silicates, I expect the density to be about 4.4g/cm$^3$ which would give me around 1.6g surface gravity. I don't know how to calculate the compression effect on the silicates.

The planet has a satellite similar in size to Mars, and it's placed in the middle of the habitable zone of an Orange dwarf in near circular orbit.

I want the planet to have the largest temperature variations (very hot summers, very cold winters) throughout most of the planet, not just in poles or just in the equator thus axial tilt of 90% doesn't work for me. I don't want people to live just near the poles, or just near the equator. I want to achieve large seasonal variations on as large area of the planet as possible.

What kind of axial tilt would you recommend choose, 45°, 60°?

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    $\begingroup$ Give it a 0% tilt but an eccentric orbit. $\endgroup$ – John Feltz Oct 20 '16 at 18:56
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    $\begingroup$ Why wouldn't a tilt of 90 degrees work? So long as the planet's not tidally locked to its parent star, it will indeed undergo drastic temperature variations. $\endgroup$ – HDE 226868 Oct 20 '16 at 18:58
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    $\begingroup$ @Root Humans could survive there, but complex life is unlikely to evolve under such circumstances. (Ugh, just realized I used % instead of ° in the first comment.) $\endgroup$ – rek Oct 20 '16 at 19:21
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    $\begingroup$ I'd recommend taking a look at Spiegel et al. (2008), which is great, and Williams & Pollard (2003). $\endgroup$ – HDE 226868 Oct 20 '16 at 19:35
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    $\begingroup$ As a sidenote: I heard, that planets with radius of at least 1.6 times the earth are unlikely to be stony planets like earth. Here the link if anybody cares: drewexmachina.com/2014/07/24/… $\endgroup$ – lurch Oct 20 '16 at 20:47
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Okay so you should check out Artifexian's Video on axial tilt.

But to sum it up, the higher the tilt the warmer your planet will be and have more extreme seasons.

It will also result in less snow, permanent ice, lower humidity and cloud cover.

Life like humans won't be able to survive on a tilt greater than 80 degrees. So to answer your question as close to 80 as your comfortable and it will still be plausible.

Final thing to note is that if you go above 53 degrees your temperature zones reverse, so your polar regions will be at the equator and your tropics will be be at the poles.

The video also has a link to interactive software where you can change the tilt of earth to see temperature differences and the like. (I can't copy the link as I am on my phone)

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    $\begingroup$ Thank you for your video, but the book he references seems to be quite dated 1964 rand.org/content/dam/rand/pubs/commercial_books/2007/…. According to this paper Climate at high-obliquity oceans.mit.edu/JohnMarshall/wp-content/uploads/2016/01/… the planet should be habitable if it has ocean, there's two obliquities covered 90 & 54 degrees $\endgroup$ – Root Oct 20 '16 at 21:09
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    $\begingroup$ Artifexian made a critical error in the video, forgetting that for half the year (two noncontinuous quarters) the equator will point at the star. $\endgroup$ – rek Oct 21 '16 at 13:31

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