Venus rotates on its axis far slower than any other planet in the solar system, and in the opposite direction of most — a phenomenon now potentially explained by a single catastrophic collision. Researchers propose that roughly 4.5 billion years ago, a moon-sized object struck the young planet at a high angle and high velocity, fundamentally altering its spin.
The impactor's size and velocity are key to the hypothesis. Unlike typical accretion events that might have a minimal effect on a planet's rotation, this specific encounter was energetic enough to slow Venus's spin dramatically and reverse its direction. The study models how such a collision could produce the planet's current 243-Earth-day rotation period, a rate far slower than even its 225-day orbital year.
This ancient event would have occurred during the solar system's chaotic early formation period, when large impacts were common. The timeline aligns with the formation of Earth's Moon, which is also thought to be the result of a giant impact. For Venus, however, the aftermath left it with no large moon and a bizarre rotational state that has puzzled scientists for decades.
The finding carries implications for understanding planetary evolution and habitability. Venus, often called Earth's twin, experienced a radically different fate — one that may trace back to this single primordial blow. The work supports the idea that planetary rotation is not fixed at formation but can be drastically altered by chance collisions.
Critics note the scenario, while plausible, remains difficult to prove without direct samples from Venus's deep interior or precise seismic data, which no current mission can provide. The impact hypothesis, though compelling, is one of several explanations for Venus's slow retrograde spin.