Grimes and Elon Musk secretly had another baby named Exa Dark Sideræl

Elon Musk and Grimes "semi" separated last year, but that hasn't changed their plans to have more children together.

In a new Vanity Fair profile, Grimes revealed she and the Tesla founder secretly had another baby together, a daughter named Exa Dark Sideræl Musk, who they call Y for short. She confirmed they welcomed their second child together via a surrogate in December after the information seemed to come out by accident, as the Vanity Fair journalist heard the baby crying in another room while interviewing Grimes.

The news comes after reports last September said Musk and Grimes broke up, and Musk at the time said they were "probably semi-separated." When asked about their relationship status, Grimes told Vanity Fair it's complicated.

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"There's no real word for it," she said. "I would probably refer to him as my boyfriend, but we're very fluid. We live in separate houses. We're best friends. We see each other all the time ... We just have our own thing going on, and I don't expect other people to understand it."

Grimes and Musk previously welcomed a son, who they call X Æ A-12 — a name Grimes told Vanity Fair she pronounces as "X A.I. Archangel." She confirmed she and Musk plan to have more children together, noting, "We've always wanted at least three or four."

Grimes also provided a breakdown of their daughter's name, Exa Dark Sideræl, which takes "exa" from "the supercomputing term exaFLOPS," combines it with "dark" as a reference to "the unknown," and adds a "more elven" spelling of sidereal, "the true time of the universe, star time, deep space time, not our relative earth time." Believe it or not, though, Grimes told Vanity Fair she worries the name is too boring and might even still change it.

"I was fighting for Odysseus Musk," she said. "A girl named Odysseus is my dream."