'Combo of Things' Led to Bill & Melinda Gates' Divorce, But 'Nobody' Wants 'More Scrutiny': Source

Bill and Melinda Gates may have announced their split to the world this week, but the decision to end their marriage was not one that came about suddenly.

A source tells PEOPLE that a "combo of things" led them to call it quits after 27 years of marriage, and that the recent timing reflects the fact that their youngest child, daughter Phoebe, is now an adult.

"It's absolutely because their youngest child is graduating from high school, and the idea was that they stayed together through that," the source says. "They limped through until their kids were out of school like a lot of people."

Phoebe celebrated her 18th birthday in September. Bill, 65, and Melinda, 56, are also parents to son Rory, 21, and daughter Jennifer, 25, who called her parents' split "a challenging stretch of time for our whole family."

Melinda filed for divorce from her billionaire Microsoft co-founder husband on Monday in documents that indicate she and Gates have a separation agreement in place — something the source says is likely part of a plan to keep things as civil and private as possible between the former couple, who have said they still plan to work together at the Bill and Melinda Gates Foundation.

"Nobody is going to want to invite more scrutiny because it'll hurt their credibility," the source says. "I don't think they're so angry that anybody wants to take each other down, like you sometimes see. [Melinda is] not incentivized for that."

Frederic Stevens/Getty Melinda and Bill Gates

With the pair putting on a united front in regards to their foundation, which they launched in 2000, they'll keep one specific aspiration at play, the source says: a Nobel Prize.

"They were really interested in trying to win a Nobel Prize," according to the source. "So one thing that was part of this is, if it gets worse, then it ends that. It seems as if that was on the agenda, and that's for both of them."

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The foundation, which is among the largest private foundations in the world, has more than $51 billion in assets, CNBC reported.

Reps and attorneys for Bill and Melinda did not immediately return PEOPLE's requests for comment.

Randy Kessler, a celebrity divorce lawyer, recently told PEOPLE that the Gateses and their lawyers have "done an exemplary job of resolving their issues privately and professionally" through the couple's separation contract.

"Such a separation contract is an agreement between the parties on the resolution of issues which would otherwise be left to the court to determine," Kessler said, noting the issues typically include division of property, support and child-related issues.

In announcing their divorce to the world, the couple said in a joint statement to Twitter that they "no longer believe we can grow together as a couple in this next phase of our lives."

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In Melinda's court documents, which were obtained by PEOPLE, the philanthropist said her marriage was "irretrievably broken," and that she was not requesting spousal support or child support — only that the court reinforce their separation contract.

Their split comes nearly three decades after they tied the knot on New Year's Day in Lanai, Hawaii after first meeting at Microsoft after she began working there in 1987.