Elon Musk secretly welcomed twins last year.
The 51-year-old Tesla CEO fathered twins with Shivon Zilis last November, according to court documents published by the Insider Wednesday. The twins were born in Austin, Texas.
The papers reveal that in April, Musk and Zilis –– who is the project director at his Neuralink company –– asked a Texan county court to change their babies' names so they would "have their father's last name and contain their mother's last name as part of their middle name."
The twins' sexes were not made public and their names were redacted from the documents.
An Austin judge granted their request in May, per Insider.
A representative for Musk did not immediately respond to PEOPLE's request for comment. Zilis also did not immediately respond.
The twins' arrival came just weeks before Musk and his former partner Grimes welcomed a baby girl via a surrogate in December, despite the pair splitting up in September. She revealed the birth of Exa Dark Sideræl, who they've nicknamed Y, in a cover story for Vanity Fair. They also share 2-year-old son X Æ A-12.
Kevin Mazur/MG22/Getty; Shivon Zilis/shivonzilis.com
At the time, she also said that she and Musk want more babies — telling the outlet, "we've always wanted at least three or four" — but the day the article was released, she tweeted that they had broken up again.
After welcoming the twins, Musk is now a father of 10. He shares 18-year-old twins Vivian Jenna Wilson and Griffin Musk, along with triplets Kai, Damian, and Sax Musk, who were born in 2006, with his first wife, Justine Wilson. The two had another son, Nevada Musk, who died at 10 weeks.
Last month, Vivian was granted her request to legally change her name and gender, according to court records obtained by PEOPLE last month.
RELATED VIDEO: Elon Musk's Child Says She Doesn't 'Wish to Be Related' to Billionaire in Petition to Change Name and Gender
In her original petition, filed on April 18 — shortly after her 18th birthday — Vivian included a short sentence explaining her request for the changes. "Gender Identity and the fact that I no longer live with or wish to be related to my biological father in any way, shape or form," she wrote, according to court documents obtained by PEOPLE.
Musk does not often discuss his own family, but he does have strong opinions on increasing the birth rate — stating that there are "not enough people" in the world at Wall Street Journal's annual CEO Council in December 2021.
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"I think one of the biggest risks to civilization is the low birth rate and the rapidly declining birthrate," he said. "Please look at the numbers –– if people don't have more children, civilization is going to crumble, mark my words."