Venus may have once possessed a moon that was ultimately destroyed and absorbed by the planet, a new study suggests, potentially explaining why Venus currently has no natural satellites. The research, conducted by scientists at the University of California Riverside, explores the dynamic between Venus’s slow rotation and the orbital behavior of a hypothetical moon.
Unlike Earth, which completes a full rotation in 24 hours and hosts a stable moon orbiting every 27.3 days, Venus rotates extremely slowly, taking 243 Earth days to turn once on its axis. This slow spin means a Venusian day lasts longer than its year, which is about 225 Earth days. The study’s authors argue that such a slow rotation would cause tidal bulges on Venus to lag behind the orbit of any satellite, creating a braking effect that gradually pulls a moon closer to the planet.
As the moon’s orbit decays inward, it would approach the Roche limit—the critical boundary where tidal forces become too strong for the moon to remain intact. At this point, the moon would be torn apart, with debris either forming temporary rings or eventually falling onto Venus’s surface. Larger moons would accelerate this process and could also further slow Venus’s spin.
The team tested numerous scenarios and found consistent results pointing to the likely destruction of any prior moon. “When I made this discovery, I was shocked,” said Stephen Kane, lead author of the study, published in the Astrophysical Journal. “I thought surely the broad range of scenarios I was exploring would lead to a variety of results. But it all went pretty much in the same direction.”
Historically, early astronomers in the 17th and 18th centuries reported sightings of a possible moon orbiting Venus, sometimes referred to as Neith, but subsequent observations failed to confirm its existence. This new study does not prove Venus ever had a moon but offers a plausible mechanism to explain why the planet now stands alone, unlike Earth.
The implications extend beyond our solar system. Kane noted that many exoplanets identified as "Earth twins" may face similar fates if they rotate too slowly, leading to the loss of any moons. Moreover, the impact of a moon colliding with Venus could have affected its climate, potentially contributing to its current hostile environment marked by extreme heat and dense clouds of sulfuric acid.
This research adds a significant piece to the puzzle of Venus’s unique evolution and may influence how scientists understand the formation and stability of moons around rocky planets in general.