A groundbreaking study from McMaster University has fundamentally challenged our understanding of planetary demographics across the Milky Way Galaxy. For years, astronomers believed they had identified the most common planetary types in our cosmic neighborhood: sub-Neptunes and super-Earths. These worlds, discovered in abundance around Sun-like stars, were assumed to represent the galactic norm. However, this assumption was based on an incomplete picture—one that overlooked the vast majority of stars in our galaxy.
The revelation comes at a pivotal moment in exoplanetary science. As researchers expand their surveys beyond the familiar glow of solar-type stars, they're discovering that the universe's most common stellar inhabitants—red dwarf stars—tell a dramatically different story about planet formation. These findings, published in The Astronomical Journal, suggest that our previous census of planetary types may have been fundamentally skewed by observational bias.
The implications extend far beyond mere statistics. Understanding which planets are truly most common in the galaxy directly impacts our search for habitable worlds and our theories about how planetary systems form and evolve. As PhD student Erik Gillis and his supervisor Ryan Cloutier, Canada Research Chair in Exoplanetary Astronomy, demonstrate in their research, the answer to "what is the most common planet?" depends entirely on which stars you're looking at.
The Observational Bias That Shaped Our Understanding
For over a decade, exoplanet surveys painted a consistent picture: around stars similar to our Sun, two planetary types dominated the census. Sub-Neptunes—worlds ranging from roughly 1.5 to 4 times Earth's radius with thick hydrogen-helium atmospheres—appeared frequently in system after system. Alongside them, super-Earths, rocky planets up to ten times Earth's mass, populated the inner regions of planetary systems. Missions like NASA's Kepler Space Telescope found these worlds orbiting star after star, leading to a seemingly logical conclusion: these must be the galaxy's most common planetary types.
But this conclusion contained a critical flaw. The stars being studied—primarily F, G, and K-type stars like our Sun—represent only a fraction of the Milky Way's stellar population. In reality, M-dwarf stars, also known as red dwarfs, comprise approximately 70-75% of all stars in our galaxy. These small, cool, dim stars range from just 8% to 40% of the Sun's mass and shine with a faint reddish glow. Their low luminosity made them challenging targets for early exoplanet surveys, creating a massive blind spot in our understanding of galactic planet formation.
The situation was akin to conducting a demographic survey of Earth by only studying residents of major cities, then extrapolating those findings to represent rural populations, small towns, and remote communities. The NASA Exoplanet Archive contained thousands of confirmed worlds, but the sample was fundamentally biased toward the types of stars easiest to observe, not the types most common in nature.
TESS Illuminates the Galactic Majority
The game changed with NASA's Transiting Exoplanet Survey Satellite (TESS), launched in April 2018. Unlike Kepler, which stared at a single patch of sky for years, TESS employs a different strategy: it scans a new sector of sky every 27.4 days, building up a comprehensive all-sky survey over its primary two-year mission. This approach proved particularly valuable for studying M-dwarfs, as these numerous but faint stars are scattered across the entire celestial sphere.
TESS's wide-field cameras can monitor hundreds of thousands of stars simultaneously, detecting the tiny dips in brightness that occur when a planet passes in front of its host star—a phenomenon called a transit. For M-dwarfs, these transits produce relatively large signals despite the stars' faintness, because the planets are often comparable in size to their diminutive host stars. A Neptune-sized planet passing in front of a red dwarf creates a much more noticeable dimming than the same planet transiting a Sun-like star.
Gillis and Cloutier recognized an unprecedented opportunity in TESS data. By focusing specifically on mid-to-late M dwarfs—the coolest, smallest, and most numerous category of red dwarfs—they could finally answer the question: do these stars host the same mix of planets we see around solar-type stars?
The Vanishing Sub-Neptunes: A Startling Discovery
The results were stunning. Around mid-to-late M dwarfs, sub-Neptunes have effectively disappeared. The gas-enshrouded worlds that dominate planetary censuses around Sun-like stars are almost entirely absent from these systems. Instead, these red dwarfs produce super-Earths in abundance—rocky or water-rich worlds without the thick hydrogen-helium atmospheres characteristic of sub-Neptunes.
"If we want to understand the origins of planets and the origins of life, we need a complete picture of how planets form and what they are made of—and the most common stars in the galaxy have barely featured in that picture until now," explained Erik Gillis, highlighting the profound implications of this discovery.
This finding represents more than just a statistical curiosity. It suggests that planet formation processes vary dramatically depending on the type of star involved. The mechanisms that produce planetary systems around Sun-like stars apparently operate quite differently around the galaxy's most common stellar type. This has profound implications for our understanding of how planetary systems form and evolve throughout the cosmos.
The researchers found that the distinction isn't subtle—it's a near-complete demographic shift. While sub-Neptunes constitute roughly 30-40% of planets around Sun-like stars in the size range between Earth and Neptune, they're virtually absent around mid-to-late M dwarfs. This stark difference demands an explanation that goes beyond simple observational bias or statistical noise.
Photoevaporation: Necessary But Not Sufficient
The leading theoretical framework for understanding the divide between super-Earths and sub-Neptunes has long been photoevaporation—a process where intense radiation from a young star strips away a planet's atmospheric envelope. In this model, planets initially form with substantial hydrogen-helium atmospheres, but those located close to their stars receive such intense high-energy radiation that their atmospheres are gradually blown away into space, leaving behind a bare rocky core.
M-dwarf stars should theoretically be excellent candidates for driving photoevaporation. Despite their low overall luminosity, young red dwarfs are energetically violent, producing intense flares and high levels of ultraviolet and X-ray radiation during their youth. This activity can persist for hundreds of millions to billions of years—far longer than the brief violent youth of Sun-like stars. The stellar evolution of M-dwarfs suggests they should be particularly effective at stripping planetary atmospheres.
However, photoevaporation alone cannot explain the near-complete absence of sub-Neptunes around mid-to-late M dwarfs. While this process can strip atmospheres from some planets, creating a population of bare rocky cores, it shouldn't eliminate sub-Neptunes entirely. Planets farther from their stars, or those with sufficient mass to retain their atmospheres against the stellar wind, should still exist as sub-Neptunes even in the harsh radiation environment of an M-dwarf system.
A New Paradigm: Formation Rather Than Destruction
The McMaster team proposes a more fundamental explanation: planet formation around M-dwarfs inherently favors different types of worlds. Rather than forming gas-shrouded sub-Neptunes that are subsequently stripped of their atmospheres, these systems may primarily produce water-rich super-Earths from the outset.
This hypothesis connects to our understanding of protoplanetary disk composition. The disks of gas and dust around young stars—the birthplaces of planets—have different properties depending on the star's mass and temperature. Around cooler M-dwarf stars, the snow line (the distance from the star where water ice can exist) lies much closer to the star than in solar-type systems. This proximity means that rocky planets forming in the habitable zone—where liquid water could exist on a planet's surface—have greater access to water ice and other volatile materials during their formation.
Additionally, the lower mass of M-dwarf stars means their protoplanetary disks contain less material overall, and the dynamics of gas accretion onto forming planets may differ significantly. Planets around these stars might simply never accumulate the massive hydrogen-helium envelopes characteristic of sub-Neptunes. Instead, they form as rocky or water-rich worlds with thin atmospheres composed of heavier elements—if they retain atmospheres at all.
Implications for Planetary Composition and Habitability
If super-Earths around M-dwarfs are indeed water-rich worlds rather than gas-enshrouded sub-Neptunes, this has profound implications for the search for habitable environments. Water-rich planets, sometimes called "ocean worlds" or "water worlds," could potentially harbor conditions suitable for life, though their habitability depends on numerous factors including atmospheric composition, surface temperature, and geological activity.
The research also suggests that the galaxy's most common planets may be fundamentally different from anything in our own Solar System. Our system contains no true super-Earths or sub-Neptunes—we have small rocky planets (Mercury, Venus, Earth, Mars) and gas or ice giants (Jupiter, Saturn, Uranus, Neptune), but nothing in between. The most common planetary type in the galaxy may be one we can only study from afar, making comparative planetology both more challenging and more essential.
A Thirty-Year Journey and the Road Ahead
The confirmation of the first exoplanets occurred just three decades ago—51 Pegasi b was announced in 1995, earning its discoverers the Nobel Prize in Physics. In the cosmic blink of an eye since then, we've progressed from knowing of no worlds beyond our Solar System to cataloging thousands and making fundamental discoveries about planetary demographics across the galaxy.
The McMaster findings represent a crucial milestone in this rapid evolution of understanding. They demonstrate that we're finally developing a comprehensive, unbiased view of planetary systems across all stellar types. As Gillis noted, understanding the origins of planets and life requires this complete picture—one that includes the galaxy's most numerous stars, not just the ones most similar to our Sun.
Future missions will build upon these foundations. The James Webb Space Telescope is already beginning to characterize the atmospheres of planets around M-dwarfs, providing crucial data about their composition. Upcoming missions like the Nancy Grace Roman Space Telescope and ground-based facilities like the Extremely Large Telescope will further expand our ability to study these systems in detail.
Rewriting the Galactic Census
The question "What is the most common type of planet in the galaxy?" now has a more nuanced answer than astronomers suspected just a few years ago. While sub-Neptunes and super-Earths both appear common around Sun-like stars, the galaxy's most numerous stellar inhabitants—M-dwarfs—produce predominantly super-Earths. Given that M-dwarfs comprise roughly three-quarters of all stars, this suggests that super-Earths, not sub-Neptunes, are likely the galaxy's most common planetary type.
This discovery fundamentally reshapes our understanding of planet formation and the diversity of worlds throughout the cosmos. It demonstrates that planetary systems are not one-size-fits-all—the type of star fundamentally influences what kinds of planets form and survive. As we continue to expand our surveys and study planetary systems around all types of stars, we're likely to uncover even more surprises about the incredible diversity of worlds in our galaxy.
The research serves as a reminder that in science, assumptions must constantly be tested against new data. What seemed like a well-established understanding of planetary demographics was based on an incomplete sample. Only by studying the full diversity of stars—including the faint, numerous red dwarfs that dominate the galaxy—can we truly understand the cosmic story of planet formation and the variety of worlds that populate the universe.
