Galactic Planet Census Rewrites the Rulebook

A new study from McMaster University has upended our understanding of planetary demographics, revealing that a common class of planet is mysteriously absent around the most populous stars in the galaxy.

"We didn't just refine the picture – we changed it. Around these stars, sub-Neptunes effectively vanish."
— Erik Gillis, PhD student, McMaster University

A Universe of Planets

Astronomers have long known that our galaxy is teeming with planets. Current estimates suggest there is at least one planet for every star in the Milky Way. Around Sun-like stars, the most common types are "sub-Neptunes" (smaller than Neptune but larger than Earth) and "super-Earths" (rocky worlds slightly larger than our own).

However, Sun-like stars are a cosmic minority. The vast majority of stars in our galaxy are "mid-to-late M dwarfs"—small, cool, and long-lived red dwarfs that represent the most common stellar type.

The Great Disappearing Act

Using data from NASA's TESS (Transiting Exoplanet Survey Satellite), the McMaster research team made a startling discovery: around mid-to-late M dwarfs, sub-Neptunes are virtually absent, while super-Earths remain just as common as expected.

This finding directly challenges the leading theory of planetary evolution known as photoevaporation, which suggests that planets lose their atmospheres due to intense stellar radiation.

"It was already astonishing to learn that the most common planets in our galaxy do not exist within our own solar system."
— Ryan Cloutier, Assistant Professor, McMaster University

A New Formation Mystery

If photoevaporation were the primary driver, scientists would expect to see a gradual transition from sub-Neptunes to super-Earths as stellar radiation increases. Instead, the data shows a sharp cut-off: sub-Neptunes simply do not form around these common red dwarfs.

This suggests that fundamentally different formation mechanisms are at play—mechanisms that our current models cannot explain.

The findings, which reshape our understanding of planetary system architecture across the galaxy, have been published in The Astronomical Journal.