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I've been reading a few articles about Earth-like planets that are both larger AND smaller than Earth itself. But size poses its own problems, thus giving the Earth-like planets their own characteristics.

Two of Earth's crucial features recommended for the possibilities of life are plate tectonics, which move continents and recycle rock; and an ozone layer, which shields the atmosphere from harmful ultraviolet radiation.

Based on what we know at the moment, what are the minimum and maximum size requirements for an Earth-like planet to have both an ozone layer and plate tectonics?

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  • $\begingroup$ would you mind to add "a rotating ferromagnetic core" to your question, because if you are going to find size-requirements for a planet that support earth like life you won't go a long way without a magnetic field that shields you from sun wind and other nasty stuff. I may answer myself to this if you share your thoughts about the need of this (and maybe a moon, which are said to be helpful in keeping the tectonics kind of alive) $\endgroup$ – Confused Merlin Oct 28 '15 at 9:53
  • $\begingroup$ I'm more interested in the ozone and the plates. $\endgroup$ – JohnWDailey Oct 28 '15 at 15:09
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    $\begingroup$ @ConfusedMerlin A follow on question about those two things would be great, but we prefer to have multiple specific questions rather than one overly broad one. For example if one person gave a great answer about the core and someone else a great one about the ozone layer and another one about the continents...who should be upvoted the most and who should be accepted? $\endgroup$ – Tim B Oct 28 '15 at 18:27
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    $\begingroup$ size isn't really the issue for ozone. Ozone requires the change of the atmosphere to have free oxygen, produced by living creatures. $\endgroup$ – Oldcat Oct 28 '15 at 23:16
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    $\begingroup$ What we know about plate tectonics at the moment is "not very much", there is still a lot of scientific debate about what actual forces are involved and how much each of these forces contributes to the process. However, size of the planet is not normally considered a contributing factor - more likely it involves variations in the density of lithosphere and/or tidal forces generated by the moon and the sun. $\endgroup$ – Nathan Griffiths Dec 3 '15 at 22:57
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It only needs to be big enough to hold on to oxygen.

The ozone on Earth is generated through the ozone-oxygen cycle. Any planet with oxygen in its atmosphere and bombardment from ultraviolet radiation will also have ozone. Mercury (0.055 mass of Earth) is large enough to have some oxygen in its atmosphere, though it's tenuous at best. Venus (0.815 mass of Earth) on the other hand has a nice hold on a lot of oxygen and it also has an ozone layer.

Tectonic activity is less of a matter of size and more of a matter of how recently the planet formed or if it's experiencing sufficient tidal forces. The Earth will eventually not have continental drift. Larger planets will maintain plate tectonics for longer than smaller ones all else being equal, but otherwise size doesn't determine whether a planet will have plate tectonics.

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In some circumstances, the size need not matter.
It is possible for a planet to have plates without having an atmosphere. Have your planet like this, then let oxygen slowly seep onto the planet's surface. Eventually, the planet will develop an ozone layer, and life would become possible, no size requirement needed.

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