When alt-rockers Neptune’s Core picked their moniker in 2019, it seemed like a clever and ironic descriptor for their new band: Neptune is a gas planet and thus presumably coreless. Hence, they didn’t “exist.” During a later trip to the Adler Planetarium, however, the group — which comprises two sets of sisters in their midteens, Kaitlin and Jackie Cywinski and Sofie and Hannah Richter — learned that not only does Neptune have a core, but it’s one of the densest objects in the Milky Way.

The revelation just made them more confident in the name — and themselves. “Now we own it,” says Sofie, 17, who plays guitar and sings. “We’re dense. We are indestructible. We’re fierce.”

Those traits are readily apparent on the band’s latest EP, Called Upon, out August 25 via Side Hustle Records. It follows two self-released full-length albums: 2020’s Can’t Have It All and 2021’s Evolving. The group will celebrate the release with an August 26 show at Metro alongside fellow locals Motel Breakfast and OK Cool.

The band members, who attend CPS high schools and live in North Center, are all multi-instrumentalists who write their own material, drawing from rock, jazz, punk, new wave, pop, and classical influences. Jackie, 15, started learning piano when she was 3. At 8, she moved to guitar, forming chord shapes based on YouTube videos. That inspired Kaitlin, 17, to start playing drums when she was 11. Seeing the Cywinskis rock out led Sofie, who was Kaitlin’s classmate, and Hannah, 14, to take up guitar and bass. In addition to being classically trained singers, the Richters have studied piano, violin, and music theory.

They began playing together four years ago just for fun but were soon scouring venue listings for open-mic opportunities. They’ve made it a priority since then to play live as often as they can. The band’s profile has risen thanks to support from the local scene, including musicians such as Spencer Tweedy and Finom’s Sima Cunningham and venues like Schubas and Subterranean. But at the heart of their growing chemistry are their sisterly bonds, which the members say allow them a near-telepathic ability to perform in sync. “When one of us comes in with a song or melody, we know exactly where to go with it,” says Jackie. “It’s really helped unite us when writing songs.”

This year has seen Neptune’s Core play a variety of shows around Chicago and the Midwest, including in April at House of Vans alongside local stalwarts White Mystery and festival sets this summer at Do Division and Andersonville Midsommarfest. The attention has led to positive press, including mentions in the online music marketplace Reverb and the syndicated radio show Sound Opinions, as well as the band signing to Side Hustle, a New York–based indie label.

The real traction, though, came from opening for fellow Chicago act Dehd during its end-of-year stand at Empty Bottle. Neptune’s Core spent a week at the DIY venue the Red Room in preparation for the performance, practicing eight hours a day and writing new songs. “Ever since then, we have just been on this whole new level of togetherness and cohesion,” says Sofie. “We’re two sets of sisters, but now it really feels like we are four sisters.”