Hamilton, ON, April 29, 2026 — The cosmos is yielding yet another astonishing revelation about the nature of planets and their formation. A groundbreaking study led by Erik Gillis, a PhD student at McMaster University’s Department of Physics and Astronomy, has uncovered a striking cosmic anomaly: the most common types of planets in our galaxy simply do not orbit the most prevalent stars. This discovery challenges long-held assumptions in astrophysics and planetary science, reshaping our understanding of exoplanet populations across the Milky Way.
For years, astronomers have catalogued the exoplanets orbiting stars similar to our Sun, identifying two dominant classes: sub-Neptunes and super-Earths. Sub-Neptunes are planets smaller than Neptune, often assumed to have thick gaseous envelopes, while super-Earths are rocky, with masses up to ten times that of our Earth. These planetary classes appeared abundant around Sun-like stars, shaping the planetary system archetypes we use for comparative exoplanetology. Yet, Sun-like stars constitute a minority of the galaxy’s stellar population, leaving a significant gap in our understanding of planets around the vastly more common M dwarf stars.
M dwarfs, particularly mid-to-late types, account for the bulk of stars in the Milky Way, characterized by their low mass—ranging from approximately eight to forty percent of the Sun’s size. Their inherent faintness and small size have historically rendered them challenging targets for exoplanet searches. This difficulty began to diminish with NASA’s Transiting Exoplanet Survey Satellite (TESS), which scans swaths of the sky every 28 days, continuously providing comprehensive data on these elusive stars and their planets over a 26-month observational campaign.
Utilizing TESS data, Gillis and his team embarked on an exhaustive survey of planet occurrence around these small, faint stars. Their results were remarkable: sub-Neptune planets virtually disappear around mid-to-late M dwarfs. While super-Earths remain prevalent, the expected population of sub-Neptunes is conspicuously absent. This surprising deficiency upends the canonical theory that planets of these sizes and compositions should be common around all stellar types, forcing astronomers to reconsider the planetary formation and evolutionary mechanisms at play within these systems.
One prevailing theory to explain the dichotomy between super-Earths and sub-Neptunes involves a process known as photoevaporation, where intense radiation from the host star gradually strips away the gaseous atmospheres of young planets, potentially transforming a sub-Neptune into a super-Earth. Mid-to-late M dwarfs are known for their high stellar activity, producing significant ultraviolet and X-ray radiation. However, the degree of atmospheric removal expected from these stars does not fully account for the near-total absence of sub-Neptunes in their orbit.
Gillis proposes that the planetary composition and formation pathways around these stars might fundamentally differ from those around Sun-like stars. Instead of forming planets primarily shrouded in hydrogen-helium atmospheres, these systems may preferentially give rise to water-rich planets. This paradigm suggests a fundamentally different chemical and physical environment within the protoplanetary disks of M dwarfs, influencing initial conditions of planet formation and dictating subsequent atmospheric evolution.
This discovery holds profound implications for fields beyond planetary formation theory, touching upon astrobiology and the quest to locate potentially habitable worlds. If mid-to-late M dwarfs host mainly rocky or water-rich worlds rather than gaseous sub-Neptunes, their planets’ surface environments, atmospheric compositions, and potential for hosting life could be markedly distinct. Understanding the frequency and nature of such planets offers critical insights into where life might arise in our galaxy.
The impact of the McMaster study is amplified by the growing capabilities of space-based observatories. TESS’s unprecedented all-sky survey capability allows astronomers to move beyond the solar analogs that framed early exoplanet science, opening new avenues to map planetary demographics comprehensively. This holistic approach reveals nuanced distributions, helping to constrain theoretical models with empirical data and enlightening us about planetary diversity on a galactic scale.
According to Ryan Cloutier, assistant professor and Canada Research Chair in Exoplanetary Astronomy overseeing Gillis’s research, these findings catalyze a profound shift in how scientists consider planet formation around different types of stars. The recognition that the most common exoplanets do not exist around the majority stellar type introduces a pivotal layer of complexity to planetary formation models, one that demands reexamination of established processes such as disk chemistry, planetary accretion, and atmospheric retention.
Looking forward, these revelations invite further detailed investigations to unravel the physical processes sculpting the exoplanet population around M dwarfs. Are there compositional gradients within protoplanetary disks that favor water-rich planet formation? How do stellar magnetic activity and radiation environments interplay with planetary atmospheres over time in these systems? Addressing these questions will require synergy between observational campaigns, theoretical modeling, and increasingly sophisticated simulations.
In concert, the McMaster team’s findings exemplify the dynamic nature of exoplanet science—a field still in its infancy less than three decades after the first exoplanet discovery. The gradual assembly of a galactic census of planets, empowered by missions like TESS, continues to expose the complexity and surprising variations inherent to planetary systems. This evolving knowledge base ultimately enriches our grasp of the universe’s capacity to generate diverse habitats and offers a clearer lens through which to interpret Earth’s place in the cosmic theater.
As the celestial catalog continues to fill, the enigma of the missing sub-Neptunes orbiting the galaxy’s most populous stars not only underscores the uniqueness of planetary systems but provokes profound questions about the cosmic ingredients necessary for planet formation. Studies like this remind us that the universe often defies simplistic expectations and that only through extensive, detailed exploration can we begin to discern the intricate rules governing planetary emergence and evolution.
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Subject of Research: Planet occurrence rates around mid-to-late M dwarf stars and the absence of sub-Neptune exoplanets.
Article Title: TESS Planet Occurrence Rates Reveal the Disappearance of the Radius Valley Around Mid-to-late M Dwarfs
News Publication Date: April 29, 2026
Web References: https://doi.org/10.3847/1538-3881/ae5810
References: Gillis, E., Cloutier, R., et al. (2026). TESS Planet Occurrence Rates Reveal the Disappearance of the Radius Valley Around Mid-to-late M Dwarfs. The Astronomical Journal.
Image Credits: Erik Gillis / McMaster University
Keywords
Exoplanets, M dwarf stars, sub-Neptunes, super-Earths, planet formation, photoevaporation, TESS, radius valley, planetary atmospheres, astrophysics, exoplanet demographics
