Given the structure’s unusual characteristics, Lock and Stewart thought it deserved a new name. They tried several versions before coining synestia, which uses the Greek prefix syn-, meaning together, and the goddess Hestia, who represents the home, hearth, and architecture. The word means “connected structure,” Stewart said.

“These bodies aren’t what you think they are. They don’t look like what you thought they did,” she said.

In May, Lock and Stewart published a paper on the physics of synestias; their paper arguing for a synestia lunar origin is still in review. They presented the work at planetary science conferences in the winter and spring and say their fellow researchers were intrigued but hardly sold on the idea. That may be because synestias are still just an idea; unlike ringed planets, which are common in our solar system, and protoplanetary disks, which are common across the universe, no one has ever seen one.

“But this is certainly an interesting pathway that could explain the features of our moon and get us over this hump that we’re in, where we have this model that doesn’t seem to work,” Lock said.

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Among natural satellites in the solar system, Earth’s moon may be most striking for its solitude. Mercury and Venus lack natural satellites, in part because of their nearness to the sun, whose gravitational interactions would make their moons’ orbits unstable. Mars has tiny Phobos and Deimos, which some argue are captured asteroids and others argue formed from Martian impacts. And the gas giants are chockablock with moons, some rocky, some watery, some both.

In contrast to these moons, Earth’s satellite also stands out for its size and the physical burden it carries. The moon is about 1 percent the mass of Earth, while the combined mass of the outer planets’ satellites is less than one-tenth of 1 percent of their parents. Even more important, the moon contains 80 percent of the angular momentum of the Earth-moon system. That is to say, the moon is responsible for 80 percent of the motion of the system as a whole. For the outer planets, this value is less than 1 percent.

The moon may not have carried all this weight the whole time, however. The face of the moon bears witness to its lifelong bombardment; why should we assume that just one rock was responsible for carving it out of Earth? It’s possible that multiple impacts made the moon, said Raluca Rufu, a planetary scientist at the Weizmann Institute of Science in Rehovot, Israel.

In a paper published last winter, she argued that Earth’s moon is not the original moon. It is instead a compendium of creation by a thousand cuts—or at the very least, a dozen, according to her simulations. Projectiles coming in from multiple angles and at multiple speeds would hit Earth and form disks, which coalesce into “moonlets,” essentially crumbs that are smaller than Earth’s current moon. Interactions between moonlets of different ages cause them to merge, eventually forming the moon we know today.

Planetary scientists were receptive when her paper was published last year; Robin Canup, a lunar scientist at the Southwest Research Institute and a dean of moon-formation theories, said it was worth considering. More testing remains, however. Rufu is not sure whether the moonlets would have been locked in their orbital positions, similar to how Earth’s moon constantly faces the same direction; if so, she is not sure how they could have merged. “That’s what we are trying to figure out next,” Rufu said.