With changes in our solar system, such as planets moving away from the sun and the reduction in solar radiation, which other planets in our solar system could have been, in the past, in a suitable zone for life to originate?
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I think this might be more on topic in the physics SE, no? – Atl LED Aug 23 '13 at 04:30
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I'm not sure what physicists would know of biology (origin of life). It seems on-topic to me. – Randy Aug 23 '13 at 04:41
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Cf. http://biology.stackexchange.com/questions/8972/why-must-a-planet-be-in-a-stars-habitable-zone-to-have-life?rq=1 – Oreotrephes Aug 23 '13 at 06:27
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@AtlLED it is equally suited here as it is specifically asking about where conditions are suitable for life. – Aug 23 '13 at 14:54
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@Oreotrephes do you think what I have below is biological enough? – Aug 24 '13 at 16:56
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1Looks, great to me. IMO the word "habitable" makes biological enough. OP should accept! – Oreotrephes Aug 24 '13 at 23:58
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This answer is also speculative, but according to the planetary habitable zone models proposed by "Habitable zone for Earth-like planets in the solar system" (Franck et al. 2000), a key finding was that:
an Earth-like planet at Martian distance would have been habitable up to about 500 Ma ago while the position of Venus was always outside the habitable zone.
So, Mars could have been habitable, but as it is much smaller than Earth, it has a smaller gravitational field presumably a major factor in how it lost much of its atmosphere.