He then applied the model to Venus. He varied the planet's rotation rate and tested hypothetical moons with masses ranging from half of Earth's moon to ten times its mass.

The results pointed in one direction. The moon eventually crashed into Venus. Larger moons fell even faster.

"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."

The work does not prove Venus ever held a moon. Kane considers it possible, but unconfirmed.

Could evidence survive inside Venus?

Spotting a crashed moon in the surface record would be hard. Roughly 80% of Venus's surface appears similar in age, which scientists interpret as the product of a global resurfacing event about a billion years ago. That event may have erased most older geology.

Signals could instead lie deep within the planet. Seismic studies of Earth have located unusual structures far below the surface that some researchers link to the moon-forming impact. Comparable measurements of Venus might one day reveal whether the planet absorbed a satellite.

Could a moon crash have reshaped Venus?

A moon impact could also bear on one of planetary science's biggest questions: whether Venus was ever habitable. Such a collision would dump enormous energy and angular momentum into Venus, potentially altering its rotation, geology, and climate. If Venus once held oceans, a major impact could have wiped them out.

The possibility carries weight because Venus sits as Earth's closest planetary neighbor. Some models suggest it may have once resembled a far milder world than the sulfuric-acid hothouse seen today.

What does this mean for planets around other stars?

The findings stretch beyond the Solar System. When astronomers hunt for Earth-like planets around other stars, they often ask whether those worlds have moons. Earth's moon drives tides, may sustain geological activity, and has shaped the planet's evolution.

Scientists still debate how necessary moons actually are for life. "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."

Kane's results suggest that forming a moon may be only half the story. A planet also has to keep it.

"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."

Slow rotators, in other words, may swallow their moons long before any life gets a chance to use them.