Aerosmith frontman Steven Tyler is neither Kate Hudson's father — that would be musician Bill Hudson — nor her stepfather figure — that would be Kurt Russell, longtime partner of Kate's mom, Goldie Hawn. So if they're not related, why does the 38-year-old actress keep referring to Steven as "Papa"?

Case in point: When she posted her Monday, April 16 Instagram pic of Steven with his arms around her pregnant belly, Kate wrote, "Baby girl got some good hugs from Papa Tyler this weekend." And in September 2016, Kate posted a selfie of herself with "Papa Steven" after they bumped into one another on a flight.

As it turns out, the rocker is sort of a father figure to Kate, and it's all thanks to The Leftovers star Liv Tyler, his actual daughter. As Yahoo reports, Kate and Liv were high school classmates at Crossroads School in Santa Monica, CA, during the '90s. Then the two actresses starred together in the 2000 rom-com Dr. T & the Women. Another coincidence? Kate and Liv both welcomed their first children in 2004. One more? Kate's breakout role in the 2000 dramedy Almost Famous, the groupie character Penny Lane, was partly based on fashion model Bebe Buell, Liv's mother and Steven's ex.

Kate and Steven's friendship is well-documented. They were photographed together at the 2013 Voice Health Institute's "Raise Your Voice" benefit, for example. And the following year, he attended a garden party celebrating her jewelry collaboration with Chrome Hearts.

So of course, this "Papa" would be one of the first people Kate would call with baby news! The Marshall actress announced her pregnancy to the world on Friday, April 6, revealing her third child — and her first with boyfriend Danny Fujikawa — will be a girl. "We have been trying to keep this pregnancy under the radar for as long as possible, but I'm a-poppin' now!" she told her Instagram followers. "And it's too darn challenging to hide, and frankly, hiding is more exhausting [than] just coming out with it! My kids, Danny, myself, and the entire family are crazy excited! A little girl on the way." (Sounds like "Sweet Emotion" to us!)