Earth and the Moon are sometimes called a binary planet, because they’re the most evenly matched pairing of a major planet and a moon in the solar system — the Moon is more than a quarter of Earth’s diameter. The list of thousands of planets discovered in other star systems doesn’t include any true binaries — pairs of planets with similar sizes. But recent research suggests that such systems aren’t impossible — just highly improbable. They’d require a set of circumstances that would be tough to produce. In fact, they’d have to be similar to those that gave birth to the Moon. The leading theory says the young Earth was hit by another planet-sized body. The collision spun off a ring of debris that coalesced to form the Moon. Researchers at Caltech found that to make a true binary, you need a “kissing” encounter between two similar-sized planets. The two would need to pass extremely close to each other — perhaps close enough to touch — at low speeds. In such an encounter, the gravity of e
