Venus’s Missing Moon Explained by New “Planet-Eating” Theory

Astronomers from the University of California have introduced a novel explanation for why Venus—the planet most similar to Earth in size, mass, and internal structure—lacks a natural satellite. Their theory, published Monday in The Astrophysical Journal, attributes this absence to Venus’s exceptionally slow axial rotation.

How Venus’s Rotation Doomed Its Moon

According to the model, any moon that may have orbited Venus early in its history would have been drawn inexorably inward due to gravitational interactions, culminating in a catastrophic impact with the planet. This “planet-eating” mechanism diverges from earlier hypotheses, which suggested either that Venus never acquired a moon or that one was destroyed in a massive collision with another celestial body.

Astrophysicist Steven Krane stated: “If it had a moon—and it most likely did at some point—it could not have retained it. That moon would have crashed into the planet within a relatively short timescale.” Computer simulations modeling gravitational dynamics between Venus and hypothetical moons of varying sizes confirmed that all such satellites, regardless of initial size, ultimately collide with the planet.

Timing and Early Solar System Conditions

Krane estimated that such a moon-capture-and-impact event could have occurred within the first billion years of Venus’s existence—approximately 4.5 billion years ago, aligning with the formation timeline of the inner solar system. He noted that the early inner solar system was densely populated with large rocky bodies, making high-velocity impacts common. Due to its proximity to the Sun, Venus would have been more vulnerable to such collisions than Earth.

This raises the possibility that Venus avoided the kind of giant impact widely believed to have formed Earth’s Moon—a collision between proto-Earth and a Mars-sized protoplanet named Theia. Yet no evidence exists confirming Venus ever hosted a moon, nor has any geological or orbital signature of such an object been detected.

Contrast With Earth’s Lunar Dynamics

A study led by the University of California, Riverside, focused on Earth–Moon dynamics and identified planetary rotation speed as the decisive factor. Unlike Venus—which rotates once every 243 Earth days, slightly longer than its 225-day solar orbit—Earth completes one axial rotation every 24 hours, over 200 times faster.

Earth’s rapid spin transfers angular momentum to the Moon, pushing it away at a measured rate of 4 centimeters per year. That figure has been precisely tracked since 1969 using retroreflectors left on the lunar surface by the Apollo 11 mission.

Venus remains the third-brightest object in the night sky after the Sun and Moon. Its atmospheric composition, surface geology, and evolutionary path hold critical clues for understanding Earth’s past and future. NASA’s upcoming DAVINCI+ and VERITAS missions aim to probe those questions by analyzing Venus’s atmosphere and geological features.