Pluto Drama: Can Pluto Be Declared a Planet Again?

Science Friday hosts a discussion about Pluto's planetary status with Lowell Observatory's Amanda Bosch and Alan Stern of the Southwest Research Institute. They explore the IAU definition that led to Pluto's 2006 demotion, why many planetary scientists disagreed with the vote, and whether President Trump or any executive action could reclassify Pluto as a planet. The guests tie Pluto's story to New Horizons flyby discoveries, the broader diversity of planets in our solar system and around other stars, and the philosophical and practical ways science defines what a planet is.

Pluto's status and the IAU definition

The podcast centers on the ongoing debate over Pluto's planetary status, sparked by Jared Isaacman's campaign to declare Pluto a planet again and the longstanding IAU definition that led to Pluto's 2006 demotion. Alan Stern and Amanda Bosch explain how the traditional planetary-definition criteria—orbit a sun, be rounded by gravity, and clear its orbit—led to Pluto being labeled a 'dwarf planet' because it shares its zone with other Kuiper Belt objects and is in a resonance with Neptune. The host, Ira Flatow, emphasizes that science is not decided by votes and that consensus is built from evidence and reasoning rather than ballots.

"We don't take votes in science" - Ira Flatow

Why the debate endures and what scientists think

Amanda Bosch discusses Pluto's place in American scientific memory and education, noting that Pluto's status as a planet has cultural resonance beyond strict definitions. The conversation highlights that Pluto’s 2015 New Horizons encounter revealed a dynamic, fascinating world with mountains, glaciers, and an atmosphere that changes with distance from the Sun, reigniting public interest in Pluto as a world worthy of planetary study. The panel agrees that public engagement with Pluto is valuable for science communication and for inspiring future research.

"Pluto is our planet" - Amanda Bosch

Defining planets and what exoplanets teach us

The guests unpack the IAU definition and contrast it with the practical, physics-based criterion planetary scientists commonly use—an object large enough to be rounded by self gravity but not massive enough to ignite fusion. Alan Stern explains that the IAU's orbit-clearing criterion was a political decision aimed at managing an increasing number of planets, a decision many scientists regard as scientifically questionable. He also points out that planets exist around other stars, broadening the debate beyond our Sun.

"The simple definition is an object in space that's large enough to be rounded by self gravity, but not so large and massive that it ignites a nuclear fusion" - Alan Stern

From Pluto to exoplanets: variety and discovery

The discussion moves to how exoplanet discoveries reveal a surprising diversity of planetary types, including hot Jupiters, under-dense worlds, and planets around pulsars. Stern notes that planets are common around stars, and the study of these worlds helps reveal what makes a planet a planet, and how Pluto fits into the broader taxonomy of planetary bodies. The conversation concludes with reflections on how science learns about planetary diversity without relying on simple labels.

"There are thousands of exoplanets that show a range of types we don't have in the solar system" - Alan Stern

Pluto as a symbol of science and a lesson in the scientific method

The final section connects Pluto's story to broader questions about how science communicates its evolving understanding to the public. The panel reiterates that scientific consensus is built through incremental evidence and expert judgment, not a single vote, and discusses what this means for ongoing debates about Pluto and similar planetary classifications. The episode also touches on Pluto's status as Arizona's state planet in 2024, underscoring Pluto's cultural impact.

"Votes don't work in science" - Ira Flatow