Venus may have swallowed its own moon, according to computer simulations that point to the planet’s slow spin as the culprit.
Venus is Earth’s near-twin in size, mass, and structure, but it has no moon. Scientists have long wondered whether the planet never experienced a moon-forming collision or whether a massive object smashed apart a moon it once had.
“My study shows Venus didn’t require a catastrophe to arrive at what we can see today,” said Stephen Kane, an astrophysicist at the University of California, Riverside, and lead author of the new research. “It turns out the gravity of the planet itself combined with the rate at which it spins naturally caused the moon to collapse on top of it.”
The study, published in The Astrophysical Journal, does not establish that Venus ever had a moon, although Kane believes it may have. His models explore what would have happened to a hypothetical companion.
Slow rotation can draw moons inward
The relationship between a planet’s spin and its moon’s orbit is measurable here on Earth. Mirrors left on the moon by NASA’s Apollo 11 mission allow scientists to precisely track its distance from our planet. “We know that the moon is slowly moving away from the planet, at a rate of around four centimeters per year,” Kane said. That is about 1.6 inches annually.
Earth completes a rotation every 24 hours, and energy transferred from that relatively fast spin to the moon moves it farther away. Venus takes 243 Earth days to turn once. Its slow rotation gives a moon a different fate, with gravity drawing it inward instead.
Kane tested that process with computer models based on the physics of gravitational interactions between planetary bodies. He first reproduced the evolution of Earth and its moon to check that the model accurately represented a known system. Then he varied Venus’ rotation rate and the masses of its hypothetical moons, testing companions from half to ten times the mass of Earth’s moon.
“When I made this discovery, I was shocked,” Kane said. “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.”
In most simulations, the moon crashed into Venus. More massive moons collided with the planet sooner. The results suggest that even if Venus had formed a moon, it could not have kept it indefinitely.
Does life need a large moon?
“When people think about Earth twins around other stars, one question they ask is, ‘Does it have a moon?’” Kane said. “My study shows a disturbing scenario for many of those cases. If these planets don’t rotate fast enough, the moon will crash to the surface, and that would change the course of history for those planets.”
Scientists searching for potentially habitable worlds often focus on planets resembling Earth, with moons among the factors they consider. Earth’s moon drives tides and may have helped keep the planet geologically active. Whether those contributions make a large moon necessary for life remains unknown.
“My feeling is there are benefits to having a moon, but it isn’t required for habitability,” Kane said. “The moon has definitely changed the way Earth has evolved through time, but we don’t fully know how important that role is.”
A moon crashing into Venus would have transferred enormous amounts of energy and angular momentum, a measure of rotational motion, to the planet. That could have changed its rotation, geology, and climate, with consequences for another unresolved question about Earth’s nearest planetary neighbor. If Venus once had oceans or other conditions favorable to life, such an impact could have altered the course of its evolution.
Finding physical evidence of a collision would be difficult. Roughly 80% of Venus’ surface is similar in age, evidence of a major resurfacing event about a billion years ago that erased much of the planet’s earlier geological history.
Scientists may have better prospects beneath the surface. Earth’s moon is thought to have formed after a massive collision early in our planet’s history, and seismic studies have revealed unusual structures deep inside Earth that may be remnants of that event. Similar measurements on Venus could offer clues about whether it once absorbed a moon.
Reference: “Tidal Demise: The Evolution and Fate of a Hypothetical Venus Moon” by Stephen R. Kane, Franck Selsis, Jérémy Leconte and Sean N. Raymond, 14 September 2026, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ae9d6c
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