The galaxy's most abundant stars have barely featured in that picture at all
For three decades, astronomers mapped distant worlds through the lens of stars most like our own Sun, and a seemingly universal pattern took shape. Now, a McMaster University study using NASA's TESS satellite has revealed that this pattern was never universal at all — only local. Around red dwarfs, the galaxy's most numerous stars, the gas-shrouded sub-Neptunes that define planetary surveys around Sun-like stars are nearly absent, replaced by rocky, water-rich super-Earths. The cosmos, it turns out, builds its worlds differently depending on the star doing the building.
- Decades of planet-formation theory rested on observations of Sun-like stars — a minority in the Milky Way — leaving the galaxy's most common stars almost entirely out of the picture.
- TESS data reveals a striking absence: sub-Neptunes, abundant around solar-type stars, have virtually disappeared around red dwarfs, a gap too large to explain away with existing atmospheric-stripping models.
- McMaster researchers Erik Gillis and Ryan Cloutier propose that red dwarfs don't strip gas from planets after the fact — they may simply never build gas-shrouded worlds in the first place, favoring water-rich rocky planets from the start.
- The discovery forces a fundamental rewrite of planet-formation models and reframes the search for habitable worlds around the stars that actually dominate the cosmos.
For thirty years, astronomers cataloging distant planets built their understanding around a familiar picture: Sun-like stars hosting two dominant world-types, sub-Neptunes and super-Earths, in roughly equal measure. The field quietly assumed this pattern held across the Milky Way. A new McMaster University study has dismantled that assumption.
The oversight was structural. Sun-like stars, despite their familiarity to us, are a minority in the galaxy. Red dwarfs — small, cool, dim stars ranging from eight to forty percent the Sun's size — make up the vast majority of stellar populations. Their faintness long made them nearly impossible to study, leaving a critical blind spot at the heart of planetary science.
NASA's TESS satellite closed that gap. By completing a full-sky survey over 26 months, it gave PhD student Erik Gillis and supervisor Ryan Cloutier an unprecedented look at what actually orbits red dwarfs. The answer was striking: sub-Neptunes, so common around Sun-like stars, have essentially vanished. Rocky super-Earths thrive, but gas-shrouded worlds are almost entirely absent.
Existing theory blamed atmospheric stripping — young stars blasting away planetary gas — for the divide between these two planet types. Red dwarfs are energetically violent in youth and should excel at this. Yet the near-total absence of sub-Neptunes goes far beyond what stripping alone can explain. The McMaster team proposes something more fundamental: planet formation around red dwarfs may favor water-rich rocky worlds from the outset, never producing gas-wrapped planets in the first place.
Published in the Astronomical Journal, the findings arrive as a reminder that three decades into the exoplanet era, the universe still holds structural surprises. If planetary systems are built differently depending on their host star, then the search for habitable worlds — and for life itself — must be reoriented around the kinds of stars that actually fill the galaxy.
For thirty years, astronomers have been cataloging the planets orbiting distant stars, and a picture emerged that seemed reassuringly coherent. Around Sun-like stars, two types of worlds dominated: sub-Neptunes, gas-wrapped planets resembling a smaller version of Neptune, and super-Earths, rocky worlds up to ten times our planet's mass. These two populations appeared everywhere the telescopes looked, and the field quietly assumed they must be equally common throughout the Milky Way. A new study from McMaster University has just upended that assumption entirely.
The problem was not with the observations themselves but with what they represented. Sun-like stars, for all their familiarity to us, are actually a minority in our galaxy. The Milky Way is dominated by red dwarfs—small, dim, cool stars ranging from eight to forty percent the size of our Sun. These mid-to-late M dwarfs make up the vast majority of stellar populations, yet for years their faintness made them nearly impossible to study in detail. That gap in the data meant astronomers were building their understanding of planetary systems on an incomplete foundation.
NASA's Transiting Exoplanet Survey Satellite changed the equation. By scanning fresh patches of sky every 28 days and completing a full-sky survey over 26 months, TESS provided an unprecedented window into these previously elusive stars and their planetary companions. PhD student Erik Gillis and his supervisor Ryan Cloutier, Canada Research Chair in Exoplanetary Astronomy, used that data to examine what planets actually orbit red dwarfs. What they found was startling: the sub-Neptunes that dominate around Sun-like stars have essentially vanished around the galaxy's most common stars. Red dwarfs produce super-Earths in abundance, but the gas-shrouded worlds that define planetary surveys around solar-type stars are almost entirely absent.
The discovery forces a reckoning with existing theory. Astronomers have long explained the distinction between super-Earths and sub-Neptunes through photoevaporation—the intense radiation from young stars blasting away planetary atmospheres and stripping gas-wrapped worlds down to bare rocky cores. Red dwarfs are energetically violent in their youth and should theoretically excel at this process. Yet the near-total absence of sub-Neptunes around these stars goes far beyond what atmospheric stripping alone can account for. The McMaster team proposes a different mechanism: planet formation around red dwarfs may simply favor water-rich, rocky worlds from the outset rather than producing gas-shrouded planets that are later stripped away.
The findings, published in the Astronomical Journal, arrive at a pivotal moment for exoplanet science. Three decades after the first confirmed discovery of a world beyond our solar system, fundamental patterns are still emerging. Missions like TESS enable researchers to compare thousands of planetary systems simultaneously, revealing structures no one anticipated. As Gillis noted, understanding how planets form and what they are made of requires a complete picture—and until now, the galaxy's most abundant stars have barely featured in that picture at all. The implications ripple outward: if the mechanisms that build planetary systems differ fundamentally depending on the host star, then the search for habitable worlds and the origins of life itself must be reframed around the kinds of stars that actually dominate the cosmos.
Notable Quotes
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.— Erik Gillis, PhD student, McMaster University
The Hearth Conversation Another angle on the story
Why does it matter that sub-Neptunes are missing around red dwarfs? Aren't super-Earths still planets?
It matters because it tells us the rules of planet formation aren't universal. We thought we understood the game, but we were only watching it played on one type of stage.
So what changes about how we think planets form?
Everything. We assumed photoevaporation—stellar radiation stripping away atmospheres—was the main sculptor. But if that were true, red dwarfs should be doing it constantly. Instead, they're not making sub-Neptunes in the first place. The planets that form around these stars are fundamentally different from birth.
Does that affect the search for life?
Directly. Most stars in the galaxy are red dwarfs. If we've been studying planets around Sun-like stars, we've been looking at the wrong neighborhood entirely. The habitable worlds we're searching for are probably orbiting the stars we've barely examined.
How did TESS make this visible when other telescopes couldn't?
It's about scale and sensitivity. TESS looked at thousands of red dwarfs systematically. Earlier surveys focused on brighter, easier targets—which happened to be the rarer stars. You need volume to see the pattern.
What's the next question astronomers need to answer?
Why does planet formation work differently around small stars? Is it the star's gravity, its radiation, the disk of material around it? That's where the real work begins.