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Humans are changing the world in profound ways. Some scientists think those changes have launched a new epoch in Earth’s history....

Squabbling sibling planets may have hurled space rocks when they were young.

Simulations suggest that space rocks struck both the newborn Earth and Venus, but many of the rocks that only grazed Earth went on to hit — and stick — to Venus. That difference in early impacts could help explain why Earth and Venus are such different worlds today, researchers report September 23 in the Planetary Science Journal.

“The pronounced differences between Earth and Venus, in spite of their similar orbits and masses, has been one of the biggest puzzles in our solar system,” says planetary scientist Shigeru Ida of the Tokyo Institute of Technology, who was not involved in the new work. This study introduces “a new point that has not been raised before.”

Scientists have typically thought that there are two ways that collisions between baby planets can go. The objects could graze each other and each continue on its way, in a hit-and-run collision. Or two protoplanets could stick together, or accrete, making one larger planet. Planetary scientists often assume that every hit-and-run collision eventually leads to accretion. Objects that collide must have orbits that cross each other’s, so they’re bound to collide again and again, and eventually should stick. Each rocky planet in the solar system should have very different chemistry and structure depending on which scenario is true. But scientists know the chemistry and structure of only one planet with any confidence: Earth. And Earth’s early history has been overwritten by plate tectonics and other geologic activity. “Venus is the missing link,” Jacobson says. “Learning more about Venus’ chemistry and interior structure is going to tell us more about whether it had a giant impact or not...