The galaxy’s most common stars almost never host sub-Neptunes, planets smaller than Neptune but larger than Earth, according to new research.
The study reshapes our understanding of planetary systems by showing that small stars form close-in worlds differently.
A missing type of planet
Inside six years of space telescope data, 8,134 faint red stars produced just five trusted sub-Neptunes.
Erik Gillis is a PhD student in physics and astronomy at McMaster University. By analyzing repeated dips in starlight, his team linked the absence of sub-Neptunes to the Milky Way’s most common type of star.
Many super-Earths were still present, suggesting planet formation was still active.
That split points toward a different recipe for planets around the smallest stars, not a simple shortage of worlds.
Watching for dips in starlight
NASA’s Transiting Exoplanet Survey Satellite (TESS), a space telescope that tracks changes in star brightness, supplied the raw view.
Each dip in starlight can mark a transit – a planet crossing in front of its star – which briefly blocks light.
The team checked how many test signals its software could recover, so missed planets did not masquerade as absence.
After several rounds of filtering, 77 vetted planet candidates remained around 65 stars.
Stars that make up most of the galaxy
Astronomers call the targets mid-to-late M dwarfs – small, cool stars only eight to 40 percent as large as the Sun.
These stars make up most of the Milky Way, so their planets shape the galaxy’s real average.
Within 30 days of orbit, the survey found a little more than one planet larger than Earth around each star on average. That result shows these small red stars hold a large share of the galaxy’s nearby planets.
Sun-like star surveys had drawn a split between super-Earths and sub-Neptunes, with fewer planets in between.
Astronomers call that gap the radius valley, a size range where fewer close-in planets appear.
Earlier studies of slightly larger red stars still showed a dip between two common planet sizes. Here, that dip disappears, and most planets cluster around a single, Earth-like size.
A watery explanation
Formation models had predicted that the valley could fade when small stars make more water-rich worlds.
In that picture, pebble accretion – planet growth from drifting grains and ice – packs water into young planets early.
Water can increase a planet’s size without requiring a thick gas envelope, allowing some worlds to blur the usual boundary.
This idea could explain the missing sub-Neptunes, but confirming it will require measurements of their masses and atmospheres.
The mechanism remains mysterious
Intense starlight can strip gas from a young planet in a process known as photoevaporation.
That mechanism should be strong around active red stars because high-energy radiation heats upper air until gas escapes.
However, Gillis’s team found too few sub-Neptunes for stripping alone to explain the pattern cleanly.
“Around these stars, sub-Neptunes effectively vanish, which means the mechanisms shaping planets here are different,” said Gillis.
Blind spot in the data
Careful filters removed many false positives – signals that look like planets but are not – before the final count.
Software rejected 427 of 532 initial signals, and later checks narrowed the sample to 77 candidates.
Survey sensitivity dropped for smaller planets and longer orbits, especially beyond 14 days for many targets.
Because of that blind spot, the numbers say less about tiny Earth-size worlds far from their stars.
That caution also applies to future searches for life within the habitable zone – the range of orbits where liquid surface water could exist.
Classic calculations mark that zone by tracking when incoming starlight drives water loss or freezing.
For rocky planets close to Earth’s size, the stricter habitable zone holds fewer than about one in three stars with such a world.
“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’re made of,” said Gillis.
Giant worlds stay rare
Large worlds stayed scarce across the same search, which found no Neptune-size planets among the trusted candidates.
A single Jupiter-size signal turned out to be a brown dwarf – an object heavier than a planet but lighter than a star.
With no true hot Jupiters – Jupiter-size planets orbiting very close to their stars – the survey set a ceiling of 0.012 per star within 10 days.
That rarity helps explain why the smallest stars mostly offer compact worlds rather than giant neighbors.
By linking missing sub-Neptunes, plentiful super-Earths, and a vanishing radius valley, the survey redraws the most common planetary neighborhood.
The next step is to measure the planets’ masses and analyze their atmospheres, since size reveals the pattern but not their composition.
The study is published in The Astronomical Journal.
Image Credit: NASA
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