Quit Whining. Pluto’s Not A Planet, An Astrophysicist Explains

InnovationScienceQuit Whining. Pluto’s Not A Planet, An Astrophysicist ExplainsByAdam Frank,Contributor.Forbes contributors publish independent expert analyses and insights. Adam Frank is an astrophysicist exploring our place in the Cosmos.Follow AuthorApr 28, 2026, 08:17am EDT--:-- / --:--This voice experience is generated by AI. Learn more.This voice experience is generated by AI. Learn more.The distant not-a-planet PlutoNASAPluto’s been making news again. Jared Isaacman, the head of NASA, recently suggested that maybe astronomers should rethink their decision to demote the world from a “planet” to a “dwarf-planet”. That decision was made back in 2006 and it led to lots of wailing. This included the cries of many little kids who’d just learned about the 9 planets and felt like cute little Pluto got a bum deal. Now Isaacman seems like a good guy and I sure don’t want to make little kids cry. Still, there’s an amazing science reason why Pluto got kicked out of the planet club.It’s all about the rest of the solar system. It was only a few decades ago that we thought the solar system ended when you ran out of planets. We knew about the inner solar system where the “rocky” planets lived: Mercury, Venus, Earth and Mars. We knew about the the outer solar system where the gas giant planets lived: Jupiter, Saturn, Uranus (no snickering) and Neptune. And, of course, way out there where the Sun looks like just another star, there was Pluto. As we all learned in school, those 9 planets make up the solar system.Only they don’t. There’s more beyond Pluto and it matters. The Kuiper Belt, Pluto's orbit and the path of the New Horizon's mission we sent to explore those outer regions of the solar systemNASAIn the vast darkness extending beyond ice-giant Neptune lies a whole lot of stuff. Most important for today is the Kuiper Belt reaching from just beyond Neptune out to almost twice the size of its orbit. The Kuiper Belt is basically construction debris left over from th...