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Court documents obtained by Insider show that the tech mogul Elon Musk quietly had twins last November with one of his top executives, Shivon Zilis. Musk, who has been an outspoken advocate of bringing more babies into the world, now has nine known children.

In April, Musk, 51, and Zilis, 36, filed a petition to change the twins’ names in order to “have their father’s last name and contain their mother’s last name as part of their middle name.” The order was approved by a judge in Austin, Texas, this May. (Insider is withholding the children’s names to protect their privacy.) The twins were born weeks before Musk and Claire Boucher, the musician who performs as Grimes, had their second child via surrogate in December.

Musk and Zilis did not respond to requests for comment.

Musk is the wealthiest person in the world, with an estimated net worth of about $220 billion. He’s at the helm of four companies: the electric-vehicle maker Tesla, the spacecraft manufacturer SpaceX, the tunnel-construction startup The Boring Company, and the brain-machine-interface implant company Neuralink, where Zilis works. This year he announced plans to purchase Twitter for $44 billion, though the deal has been held up over what the Tesla CEO has described as several unresolved matters. In May, Insider reported that SpaceX had paid a company flight attendant $250,000 to stay quiet after she alleged that Musk exposed himself and propositioned her for sex.

Zilis is a rising star in Musk’s empire. Born in Markham, Ontario, Zilis received her bachelor’s degree in economics and philosophy in 2008 from Yale, where she also played goalie on the women’s ice-hockey team. A lifelong athlete, she has been pictured on social media surfing, zip-lining, and ice climbing. After beginning her career at IBM, she joined the early-stage venture-capital fund Bloomberg Beta, where she led investment efforts in data and machine learning. She was on Forbes’ 30 Under 30 list in the venture-capital category in 2015.

Zilis puts Musk through her work with OpenAI, the artificial-intelligence research-and-deployment nonprofit Musk cofounded in 2015. She serves as the youngest member of OpenAI’s board of directors. In 2017, Zilis moved to Tesla, where she was a project director, deploying her AI expertise on the Autopilot and chip-design teams. Today she holds the title of director of operations and special projects for Neuralink, where Musk is a co-CEO. She previously worked as a project director in the CEO’s office. She’s recently been floated as one of the people Musk could tap to run Twitter after his acquisition.

During Zilis’ tenure at Neuralink, the company has conducted surgical trials of its Bluetooth-enabled brain chips on monkeys and pigs and enabled a nine-year-old macaque to play the video game Pong using its brain. Musk has suggested human trials could start by the end of 2022.

Zilis lived in San Francisco before buying a home in a gated community in Austin in August, about three months before the twins were born. The real-estate website Zillow estimates the home is worth more than $4 million. Musk has said his primary residence is a $50,000 modular house in Boca Chica, near SpaceX’s launch facility in South Texas, but in the court documents Musk and Zilis listed the same address — the multimillion-dollar home — in Austin. Musk has been spending more time in Austin after opening Tesla’s new Gigafactory Texas on the outskirts of the city.

Zilis shares many of Musk’s intellectual passions, including space travel, transportation, and, most of all, artificial intelligence. They appear to have a similar sense of humor — they’ve both tweeted AI puns and tongue-in-cheek references to Musk’s favorite cryptocurrency, dogecoin.

She has been an ardent defender of Musk in the face of criticism. In 2020, when a California State Assembly member, Lorena Gonzalez Fletcher, tweeted “F*ck Elon Musk” after Musk tweeted about moving Tesla out of California over COVID-19 restrictions, Zilis responded: “This makes me sad. No one’s perfect but I’ve never met anyone who goes through more personal pain to fight for an inspiring future for humanity – and has done so tirelessly for decades. Everyone’s entitled to their opinion but mine is that there’s no one I respect and admire more.”

—Shivon Zilis (@shivon) May 11, 2020

In August, she retweeted Musk’s photo of SpaceX technicians and engineers connecting a Starship to its Super Heavy booster, writing, “Humanity’s history and future in a single photograph.”

Musk says he’s built his empire with the goal of protecting humanity against what he sees as its biggest threats, including the climate crisis, artificial intelligence gone rogue, and, more recently, population collapse.

He began sounding the alarm about declining birth rates in 2017, when he tweeted“The world’s population is accelerating towards collapse, but few seem to notice or care.”

He revisited the topic during a debate in August 2019 with Jack Ma, the chairman of Alibaba Group, at the World Artificial Intelligence Conference in Shanghai. “The biggest issue in 20 years will be population collapse,” Musk said. “Not explosion. Collapse.”

Since the beginning of 2022, the mogul has tweeted more than a dozen times about population issues. On May 24, he posted a graphic from The Wall Street Journal showing a “fertility slump” in the United States.

“Contrary to what many think, the richer someone is, the fewer kids they have. I am a rare exception,” Musk said. He returned to that thread several times over the next month, adding on June 14, “I mean, I’m doing my part haha.”

On June 17, Musk tweeted the opening scene of “Idiocracy,” the 2006 Mike Judge comedy depicting a world in which highly intelligent people are reluctant to reproduce while those with low IQs continue to have large families, dumbing down the Earth’s population. “When I ask my friends why they’re not yet having kids (very few are), it sounds exactly like the movie,” he wrote.

Experts are divided on what our demographic future holds. After years of concern about an overcrowded planet further straining Earth’s limited resources, some studies are beginning to suggest that the global population may peak and then begin to decline sooner than expected. Replacement-level fertility, or the average number of children a woman would need to have in her lifetime in order for a generation to replace itself, is 2.1. In 2020, as The Journal pointed out, the total fertility rate in the United States was down to 1.64, though the Centers for Disease Control and Prevention reported that in 2021 the number of births in the US rose for the first time in seven years.

Part of Musk’s concern about declining birth rates appears to stem from the idea that there won’t be enough humans to populate Mars.

The Tesla founder is a proponent of making humans what he calls a “multi-planetary species.” He’s said SpaceX aims to put humans on Mars via its Starship project by 2029. As he wrote in a 2017 paper published in the academic journal New Space, he sees a human presence on the red planet as a safeguard against “some eventual extinction event. “

In response to a tweet in 2021 praising him for his large family, he wrote: “I’m trying to set a good example! Population collapse is a much bigger problem than people realize and that’s just for Earth. Mars has a great need for people, seeing as population is currently zero.”

Musk’s first wife, the author Justine Musk, told Musk’s biographer Ashlee Vance that he “wanted to get married and have kids early on.” In 2002, the couple had their first child, Nevada Alexander Musk, who died of sudden infant death syndrome at just 10 weeks. After the baby’s death, the couple turned to in vitro fertilization. Justine gave birth to twins, Vivian and Griffin, in 2004, then triplets, Kai, Saxon, and Damian, in 2006.

This June, Vivian was granted approval to legally change her name and gender. In her petition, she wrote that she no longer wished “to be related to my biological father in any way.”

Following Musk and Justine’s 2008 divorce, Musk married, divorced, and then remarried and redivorced the actor Talulah Riley, with whom he had no children. In 2018, he was romantically linked to Grimes. She gave birth to their first child, X Æ A-Xii Musk, in 2020. (The couple had to change their son’s name from X Æ A-12 to comply with California law.) In March, the existence of their second child, Exa Dark Sideræl Musk, was revealed when Exa started crying while Grimes was being interviewed by a Vanity Fair reporter.

In the interview, Grimes described the complicated nature of her relationship with Musk. “There’s no real word for it,” she told Vanity Fair. “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.” She also spoke of plans to have more children with Musk, saying, “We’ve always wanted at least three or four.”

The day the article was published, she tweeted, “Me and E have broken up *again* since the writing of this article haha, but he’s my best friend and the love of my life, and my life and art are forever dedicated to The Mission now.” (She later defined “The Mission” as “sustainable energy, making humanity a multiplanetary species and the preservation of consciousness.”)

Musk and Amber Heard — who dated for about a year in 2016 — may also have frozen embryos together. During Heard and Johnny Depp’s high-profile defamation trial this spring, Jennifer Howell, Heard’s sister’s boss, tested that she had heard secondhand from Heard’s mother, Paige Parsons, that Heard “was in a legal battle with him over the rights to embryos that they had created together,” adding, “He wanted to destroy them, and Amber tried to keep them to have a baby.”

Though Musk speaks reverently about fatherhood, he has also been up front about his limited role in raising his children when they are younger.

“Right now there’s not much I can do,” he told The New York Times about his relationship with X Æ A-Xii as an infant. “Grimes has a much bigger role than me right now. When the kid gets older, there will be more of a role for me. I think just doing what I’ve done with my other kids. If I have a trip for Tesla to China, for example, I’ll bring the kids with me and we’ll go see the Great Wall or we took the bullet train from Beijing to Xian and saw the Terracotta Warriors.”

Last week, Musk took his four eldest sons along with him as he met the pope. After pulling his kids out of Los Angeles’ Mirman School, he cofounded a specialized school for them to attend along with the children of some SpaceX employees called Ad Astra (which means “to the stars” in Latin).

On Father’s Day last month, Musk tweeted“I love all my kids so much.”