Is Venus in our sun's habitable zone?

Question

I know that Venus is closer to the sun than Earth and if an article I read is to be believed, Venus is hotter than Mercury even though the latter is closer to the Sun. The explanation for this odd phenomenon is, per some scientists, runaway greenhouse effect (the CO2 content of Venus' atmosphere is very high).

It's safe to say that Venus would've been cooler without so much CO2 in its atmosphere. Question is would it be as cool as Earth to allow liquid water to exist if only there wasn't so much CO2 in its atmosphere? Is Venus in our sun's habitable/Goldilocks zone?

Answer 1

What are the inner and outer edges of the Sun's circumstellar habitable zone or Goldilocks zone?

Here is a link to a list of about a dozen estimates made in the last 60 years, some using complex computer modeling.

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates

Note that many of them disagree a lot with others.

The semi major axis of he orbit of Venus is 0.723 Astronomical Units (AU) from the Sun, and most of the estimates make the inner edge of the habitable zone farther than that from the Sun.

Only the first estimate is for worlds habitable for humans and life forms with similar requirements. The rest are for liquid water using life forms in general. On Earth there are many environments where humans would swiftly die which have lifeforms suited to those environments. So a planet habitable for some forms of liquid water using life is not necessarily habitable for humans.

So some of the estimates which extend the inner or outer edges of the habitable zone more than others do, do so by assuming specific atmospheric compositions which moderate the temperatures. And there is no requirement for such compositions to be breathable for humans or for humans to survive longer than seconds on the surfaces of worlds with those hypothetical atmospheres.

Anyway, most estimates of the present habitable zone of the Sun put Venus too close to the sun to be habitable.

Answer 2

Venus, currently is not in the habitable/Goldilocks zone. However once it had deep oceans of water, millions of years ago. based on computer modellings of the climate of the ancient Venus by scientists at NASA’s Goddard Institute for Space Studies (GISS).

This was because at that time Venus actually was in the habitable zone. This shift was because Goldilock's zone radius from the central body, depends upon it's thermodynamic temperature, as more heat is more black body radiation. The temperature is lower in ZAM age, or medium, because the helium fused would have sank down to the core, providing gravitational contraction, which would in turn heat it up.

The solar irradiance was low that time, because the temperature was low therefore the black-body radiation emitted that time was comparatively less. Because of this the Venus was actually in the habitable zone, until, the Sun brightened and made the oceans photocatalyze due to UV rays producing electronegative oxygen readily reacting with the carbon (maybe the comet deposits or solar wind?) making it's CO2 rich atmosphere (96%!).

But, if we somehow manage to deplete the CO2 level, by capturing the 90 bars in carbonates or other techniques, we probably may in fact be able to terraform Venus. Although it is practically possible only for Type II civilisations, theoretically, if we remove the CO2 and replace it with gases with higher albedo, we can cool down Venus. According to the inverse square law, the solar constant for Venus would be approximately 2800 watts per square meter. If somehow we manage to absorb only 50% of that solar energy and reflect the rest 50%, the temperature of Venus would be roughly be comparable to that of Earth with polar ice caps.

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