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We still don't have a definitive answer to this, and we may never know for certain. There are so many variables involved, and this all happened so long ago, that important evidence may have been erased over time, or simply beyond our current ability to measure.
Venus definitely stands out among the planets in terms of its motion; while it moves around the Sun in the same direction as the other planets (counterclockwise) it rotates around its own axis in the opposite direction (clockwise), whereas all the other planets have counterclockwise rotation. Venus also has an extremely slow rotational period (what we would call a day); it actually takes Venus more time to complete one day than it does one year. This is to say that in the time it takes the planet to complete one 360 revolution around the Sun, it has not completed a 360 rotation around its own internal axis.
This doesn't seem to make sense when compared to the other planets; something different clearly happened to Venus. Our ideas generally fall into two camps;
- Venus experienced different forces during its formation, such as tidal forces between itself and the Sun, that scrambled up what would otherwise have been an orbit and rotation just like all the other planets.
- When it was young, Venus was struck by a large piece of interplanetary debris that altered its rotation, such as by reversing it completely or flipping the planet upside-down.
There are means of investigating either possibility, but we are limited by a variety of constraints. The first idea might be investigated by building computer simulations of the early solar system, but these are merely simulations; we have no way of knowing the exact conditions and positions of objects early in the solar system's history. We might test the second hypothesis by looking for evidence of impacts on Venus's surface, but this may have been eroded by Venus's intense weather, and that's assuming it was formed after the crust solidified.
Thus, the answer to your question is a resounding "we don't know for sure". Although this may not be a satisfactory answer, we must face the fact that the evidence will be very difficult (and, most likely, expensive) to find.
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