If Venus was entirely inside the habitable zone, or life zone, would its proximity to the Earth provoke any remarkable changes to the Earth?
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$\begingroup$ I have problems understanding what the question is. If Venus was in the Goldilocks zone, then... what about it? Venus is actually really close to this habitable zone and in fact touches it on its aphelion, but it's not merely the distance to the star that defines its habitability. In Venus' case, it's its runaway greenhouse effects of its thick atmosphere and the fact that it rotates slower on its own axis than it does around the Sun. It would still remain uninhabitable even if it was co-orbital with Earth. $\endgroup$– TildalWaveJan 22, 2014 at 16:30
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$\begingroup$ Is not the distance beetween Venus and the Sun, but beetween Earth and Venus, since we are in the Goldilocks zone and Venus not, if Venus were inside too, the Earth would suffer any changes because the gravity of Venus? $\endgroup$– JeanPaulJan 22, 2014 at 16:35
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$\begingroup$ Well yes, but that again depends on where exactly it would be. If it was much closer to Earth, the two planets would be a subject to each other's tidal forces and eventually establish an orbital resonance, increasing Earth's orbital energy (higher orbit) and decreasing the one from Venus (lower orbit). I'm not sure where this energy to increase Venus' semi-major axis would come from, though? $\endgroup$– TildalWaveJan 22, 2014 at 16:41
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$\begingroup$ Loosely related, this is what Venus would look like from earth if it was as close as the moon. $\endgroup$– Eduardo SerraJan 22, 2014 at 19:53
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$\begingroup$ Hi @JeanPaul and welcome to the site, you might want to have a quick read of out help center to properly understand what this site is about and how it works. Specifically we try to avoid questions that are purely hypothetical like 'What would happen if a black hole was suddenly in the solar system'. Other than the little things we are a generally friendly community and I hope you stick around to ask and answer more questions! $\endgroup$– user96Jan 23, 2014 at 10:47
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2 Answers
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There are several scenarios: An Earth-Venus binary would be possible, or coplanar prograde or inclined up to retrograde on resonant or non-resonant orbits, elliptic or almost circular.
One option would be a co-orbital scenario, which is described in more detail here. The latter would allow Earth and Venus to stay both in the habitable zone. But the effects for Earth could be very remarkable in some cases: Seasons would change over time, since eccentricity of Earth's orbit would change. If Earth and Venus would follow a horseshoe orbit, we would get variations in the length of a year.
Co-orbital systems can become instable by the gravity of other planets.
In the long run tidal forces may lead to a collision of the two co-orbiting planets.
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Changes on Earth:
Except for possible tides and possible eclipses, if it were near enough, no.
Changes on Earth's orbit:
Venus would establish some resonance with Earth's orbit, if it were near enough or in one of the exact possible spots. This will not have a major impact except on year's lenght and Sun illumination on Earth.
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1$\begingroup$ I am not the downvoter but I would guess that it is because you are stating points without explaining why. For example, Why would there be an effect on Earths orbit, what would this change be? how much of a change would it be? Why would the length of the year or the suns illumination patterns be different. Its the description that makes the difference between a correct answer and a correct answer that is brilliant and informative! $\endgroup$– user96Jan 23, 2014 at 10:44