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Some details required are,

The right mass, should it be higher or lower than Earth?

The right rotational period obviously the current Venus is rotating far to slowly.

This planet needs to be able to support Earth like life.

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Mass is mostly irrelevant, though you'd probably want plate tectonics and that is thought to be related to mass. Something roughly Earth mass should be OK, anywhere from maybe 0.6-0.8 Earth masses, you might still get plate tectonics and have a magnetic field to up to 2 possibly 3 or 4 Earth masses. Note, 8 earth masses, equal density = 2 Earth gravities, so 2, 3 or even 4 earth masses wouldn't be over the top gravitation wise, but anything over 4 earth masses, you might run into problems with the planet retaining too much hydrogen which might interfere with oxygen formation and if it gets too heavy you lose plate tectonics, so, 3 or 4 earth masses might be the upper limit.

Rotation is important but flexible. You probably wouldn't want days that last 3 months like they do on Venus, but 8-10-12 hour days or even 48 hours might be OK, though the longer the day, you run the risk of scorching under the mid-day sun. If I was to choose, I'd go with about 16 hour days. 8 hours of sunlight on average, to avoid too much mid-day heating.

The real trick is atmosphere and oceans. I think you'd want a dry planet, cause too wet, water can be a powerful greenhouse gas and that close to the sun, the hotter it gets the more water would evaporate, and that's a problem. If you have a not entirely desert, but say 50% desert with tiny desert plants that suck the water out of the air, you could get a low heat trapping, rapidly cooling planet at night, with some spots of green on the wind side of the oceans where there would be more rain.

Such a situation might have high winds with all the rapid warming and cooling, but, that's what I'm thinking might work.

You could also have a planet with many active volcanoes. Volcanoes help make the soil fertile, so it's not all bad. Volcanic soil over time can be very good, and with enough SO2 released in the air, that would increase reflectivity and create a haze of sorts over the planet, which would have a cooling (and darkening) effect. Personally, I'd rather low water than haze - but pick your poison. I think you need one or the other. That's the only real answer to this question. Significantly lower greenhouse gas or significantly higher natural upper atmosphere reflection, either SO2 or perhaps tiny dust clouds constantly being kicked up into the higher atmosphere like Mars has. Venus is generally considered inside the Goldilocks zone, though that's not a well defined border, so you'd need a pretty darn big heat reflection or lack of heat retention designed into the planet.

Venus has sulfuric acid rain, and with enough SO2 to keep the planet cool, acid rain could be a problem, but with enough water (Venus has essentially no water), but with water to dilute it, the acidity would probably be manageable for life.

A rather, perhaps unlikely idea could be natural formation of white rocks, sand and lighter colors. Plants, in abundant sunlight, might develop white leaves that reflect a lot of the sun away - why would plants do that naturally? Water retention and a cool undergrowth, with white leaves on top. The cool wet air like you get naturally in jungle would be the goal, and absorbing sunlight could be more secondary under a very bright sun. I'm not saying that's likely, but it might be possible.

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  • $\begingroup$ Excellent answer! I'm surprised how it only got one upvote other than mine. //(^_^)\\ I wonder about SO$_2$ being an atmospheric coollant however. Also large quantities of SO$_2$ would indirectly contribute to buildup of large quantities of CO$_2$ very soon, considering that SO$_2$ forms sulphonic acid - H$_2$SO$_3$ - which would react with carbonates to form sulphites and release CO$_2$ as a result. That would be a disaster for a planet as close to its parent star as Venus is. $\endgroup$ – Youstay Igo Mar 20 '16 at 11:27
  • $\begingroup$ Very good, it's a very detailed answer, that covers everything someone would want to know! Only a little question: isn't SO2 a greenhouse gas? If a gas reflects heat, it may also be true that the heat that enter the atmosphere heat more the planet...? $\endgroup$ – Eithne Mar 21 '16 at 7:59
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    $\begingroup$ $SO_2$ raises the albedo of the planet (making it more reflective). You could consider it to be an anti-greenhouse gas. $\endgroup$ – Jim2B Mar 21 '16 at 14:34
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As you get closer to the parent star, the slower the rotation of the planet gets until at one point it gets tidally locked to the star. Venus is in many ways different than most other planets (one being that it revolving around sun in retrograde direction, opposite from most other planets).

You can have planets equal or larger than Earth at Venus distance without any issue. Bellerophon and Osiris are orbiting much closer to their parents stars than even Mercury. And they are both giants (as in, gas giants). Even imagining them fills a person with sheer horror. So the point is that you can keep an Earth sized planet at Venus distance from its parent star without any major (orbit stability related) consequences.

If you want to make it habitable, you would not have to twerk heavily with planetary physics, but also the size and luminosity of the parent star. If the parent star is smaller than our sun, your planet could lay in the habitable zone and also get to evolve an ozone layer one day in its history. Large stars would have too strong magnetic and charged particles storms to allow any ozone-layer buildup.

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You could use the example of Venus directly. The only (big) reason why Venus is not habitable today is due to its surface temperature. This temperature is not caused (mainly at least) by its proximity to the sun. It's caused by its greenhouse atmosphere which traps the planet heat.

That being said its proximity to the sun makes it far more vulnerable to solar winds among other sun related phenomena. This means that although Venus could be an habitable planet it is unlikely that life, as we know it, could have evolved there. It seems to me colonization of this hypothetical Venus-like planet is possible. Complex life evolution on the other hand (scales in billions of earth years) is very unlikely.

Venus gravity is 8.87 m/s² which is very close to Earth (9.8). In fact I believe it's the closest in the solar system. So to be better it should have just a bit more mass (although the current is tolerable) Venus is a far more similar planet to Earth, than Mars for instance.

Check this video for more details.

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  • $\begingroup$ Venus is hotter than Mercury, so there's more going on than just proximity. It's very thick atmosphere, greenhouse gas rich atmosphere is why it's so hot. Venus' atmosphere is so good at trapping heat that it's the same temperature on the north pole mid winter than on the equator mid summer. It's the same temperature day and night, even though the nights last over a month. $\endgroup$ – userLTK Mar 19 '16 at 21:19
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A great deal of how a planet analogous to Venus can be terraformed or made habitable depends a great deal on the planet itself.

Venus may not be a great example, the strange retrograde rotation might have been caused by a giant impact similar to the one thought to have created our own Moon, which would have upset many other factors that made Earth habitable (for example, disrupting the mechanisms that made a strong magnetic field like the one we have).

So if we take these factors out of the equation, we are left with one very important issue: the evaporation of the oceans.

Venus is close enough to the sun that if we replaced it by present day Earth, we would encounter a problem called a "wet greenhouse". Water vapour is by far the most efficient "greenhouse gas", and the amount of heat energy the planet would receive would start to generate a "wet greenhouse", ramping up the temperature in a positive feedback loop. (For those of you who are worried, Earth will suffer this effect in about one billion years in the future, as the sun's luminosity increases).

The wet greenhouse will be bad enough, but the ultimate issue is when the moisture in the atmosphere reaches the point that the stratosphere is saturated. At that point, another effect takes over: solar ultraviolet radiation cracks the water vapour into hydrogen and oxygen. There is actually an excellent set of answers to the question Would it be possible to terraform the moon? which shows the escape velocity of different gasses; short answer is the hydrogen will rapidly escape into space and the planet's supply of water will be expended in several million years.

So now you have a very hot, very dry planet orbiting the star, which causes even more complications.

Water being sub ducted into the mantle is thought to act as a lubricant for plate tectonics, so a dry planet could possibly "seize up" and its tectonic engines stop. The planetary heat in the core will still need to escape somehow, but the mechanisms will not work anymore. The planet will be rocked by supermassive earthquakes as the pressure builds against locked plates which cannot "creep" past each other like they used to, and eventually at least one plume of magma will be able to melt its way through the crust and create a series of super volcanoes like the Tharis group on Mars.

The elimination of water and ending of crustal movement and subduction also means the end of the carbon cycle. Carbon dioxide is not going to be dissolved in water to eventually create calcium carbonate, and the calcium carbonate isn't going to be sub ducted and melted in the mantle to be re injected into the atmosphere by volcanic activity. How this plays out on the hypothetical planet will depend on factors like how much carbon was locked in the mantle before the plates seized and the initial amounts of carbon in the protoplanet during formation; you could end up with either extreme amounts like Venus or a very tenuous atmosphere like Mars.

So the short answer is that any planet in the orbit of Venus is inside the inner edge of the habitable zone, and absent of massive technological intervention like a sunshield at the Venus-Sun L1 point, the planet is doomed to be a hot, dry husk.

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