Venus rotates in the opposite direction to the other planets in the solar system, and astronomers aren鈥檛 sure why 鈥� but it may have been caused by the drag of an ancient moon in a backwards orbit.
The early solar system was a chaotic place, with rocks hurtling around at extraordinary speeds. at the United States Naval Observatory in Washington DC and at Teza Technologies in Chicago performed a series of simulations of Venus capturing one of these rocks in orbit.
They found that it would have been easier for Venus to capture a massive moon in a backwards, or retrograde, orbit than a regular, forwards orbit. If Venus started out spinning in the same direction as the rest of the planets, the gravitational pull of this hypothetical moon 鈥� nicknamed Neith 鈥� could have slowed it down and reversed its spin.
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But as this process continued, Neith would smash into any matter in the disc of material left over from Venus鈥檚 formation, slowing the strange moon down until it ripped apart and fell onto the planet. The whole thing would probably have taken between 100,000 and 1 million years.
This could also help explain why Venus has no moons now, unlike any of the other planets in the solar system apart from Mercury. 鈥淎 falling, massive, captured moon sweeps up and removes not only the primordial circumplanetary disk but any other smaller satellites,鈥� says Makarov. 鈥淣eith would clear up the vicinity of Venus relatively quickly before crushing itself.鈥�
While a captured moon could explain some oddities about Venus, the idea would be extraordinarily difficult to prove. 鈥淓ven with extensive geochemical data from Venus 鈥� a proposition that鈥檚 decades away 鈥� it seems essentially impossible to test their idea,鈥� says at Washington University in St. Louis. 鈥淭hey do offer an explanation for Venus鈥檚 retrograde rotation, but there are other explanations, too.鈥� The most popular of these other explanations is that Venus collided with a planet-sized object early in the solar system鈥檚 history.
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