The most common planets in the galaxy don't appear around the most common stars, TESS observations suggest
Astronomers now estimate there is at least one planet for every star in our galaxy. These worlds, called exoplanets, are planets that orbit stars outside our solar system. But new research from McMaster University reveals a surprising twist: the most common planets in our galaxy don't exist around the most common stars.
Turning to small, dim M dwarfs
To fill the knowledge gap regarding planetary formation, McMaster researchers examined planets orbiting mid-to-late M dwarfs. These stars are smaller than Earth's Sun, having 8% to 40% of the Sun's mass, which constitute the majority of stars in the Milky Way. Historically, studying these dim stars has been challenging due to their low visibility.
NASA's Transiting Exoplanet Survey Satellite (TESS) has revolutionized our ability to investigate these celestial bodies. TESS observes a new patch of sky every 28 days within its two-year mission, which has provided unprecedented observational opportunities.
Sub-Neptunes vanish around M dwarfs
Using data from TESS, McMaster's team found that sub-Neptunes almost completely vanish around mid-to-late M dwarfs. Instead, a substantial number of super-Earths are found, indicating that existing theories of planet formation need reevaluation.
"We didn't just refine the picture—we changed it. Around these stars, sub-Neptunes effectively vanish, which means the mechanisms shaping planets here are different," says Erik Gillis, lead researcher on the project.
Rethinking photoevaporation and formation
Traditionally, astronomers have explained the difference between super-Earths and sub-Neptunes through photoevaporation, where intense high-energy radiation strips a planet's atmosphere. Although mid-to-late M dwarfs are remarkably active, the complete absence of sub-Neptunes around them does not fully align with this theory.
This observation implies that planet formation in the context of M dwarfs may more favor water-rich worlds rather than gas-rich sub-Neptunes. This shift prompts scientists to reconsider their understanding of how diverse planetary systems are formed.
A rapidly evolving exoplanet field
The findings, published in The Astronomical Journal, represent an important contribution to the fast-developing field of exoplanet science. Over the past 30 years since the first exoplanets were discovered, astronomers have only begun to scratch the surface of understanding the underlying mechanisms that define planetary systems.
"Historically, our solar system was a unique case. However, with advanced missions like TESS, we can analyze thousands of different systems and uncover patterns that challenge our initial assumptions," Cloutier concludes.
Publication Details
TESS Planet Occurrence Rates Reveal the Disappearance of the Radius Valley Around Mid-to-late M Dwarfs, The Astronomical Journal (2026). DOI: 10.3847/1538-3881/ae5810
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