Elon Musk and Grimes secretly had another baby, a girl named Y

• Elon Musk and Grimes have welcomed a second child, a girl named Y, Vanity Fair reports.
• The baby was conceived via surrogate — the couple never announced they were expecting.

Elon Musk and Grimes are parents — again.

The Tesla and SpaceX CEO has secretly welcomed another baby with his on-again, off-again partner, the musician Grimes, she revealed in a Vanity Fair cover story by Devin Gordon published Thursday.

It's the second child for Musk and Grimes, but Musk's seventh and his first daughter. The couple's son, X Æ A-Xii — which is pronounced "X A.I. Archangel," Grimes told Vanity Fair — was born in May 2020.

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Their daughter was conceived via surrogate after Grimes had serious discomfort and complications during her pregnancy with X. The new baby's name is Exa Dark Sideræl Musk, but they call her Y.

According to Vanity Fair, "Exa" references the supercomputing term exaFLOPS, "Dark" is "the unknown," and "Sideræl" is an "elven spelling" of sidereal, which is "the true time of the universe," Grimes said. It's pronounced "sigh-deer-ee-el."

Musk and Grimes have never announced that they were expecting again, and it seems they had planned to keep baby Y a secret — Gordon, the Vanity Fair writer, discovered her existence after arriving at Grimes' house in Austin for an interview and hearing her crying upstairs.

"I'm not at liberty to speak on these things," Grimes said at first, visibly uncomfortable, according to Gordon. "Whatever is going on with family stuff, I just feel like kids need to stay out of it."

She added that "X is just out there" and that Musk seems to view the 2-year-old as a protégé, bringing him to public events.

After the baby began crying louder, Grimes laughed and told Vanity Fair that the baby is "a little colicky" right now. and added "I don't know. I don't know what I was thinking" trying to keep the baby a secret.