M Dwarfs Dominate the Galaxy’s Stellar Landscape (Image Credits: Unsplash)
Astronomers have confirmed that our galaxy teems with planets, boasting at least one world for every star. This abundance fuels hopes of finding habitable environments beyond Earth. However, researchers at McMaster University analyzed data from NASA’s Transiting Exoplanet Survey Satellite and found a counterintuitive pattern: the Milky Way’s most common planets rarely form around its most numerous stars.[1][2] The discovery, detailed in a study published today in The Astronomical Journal, reshapes assumptions about where to search for potentially life-friendly worlds.TESS Planet Occurrence Rates Reveal the Disappearance of the Radius Valley Around Mid-to-late M Dwarfs
M Dwarfs Dominate the Galaxy’s Stellar Landscape
Mid-to-late M dwarfs represent the galaxy’s most abundant stars. These diminutive bodies, ranging from 8 to 40 percent the size of the Sun, outnumber all other stellar types combined in the Milky Way. Their cool temperatures, around 3,300 Kelvin on average, and low masses – typically 0.1 to 0.4 times the Sun’s – make them faint and challenging to observe.
Historically, studies focused on brighter Sun-like stars, leaving a gap in knowledge about planets around these prevalent hosts. Missions like TESS changed that equation. By scanning nearly the entire sky, TESS captured dips in starlight from transiting planets, offering unprecedented access to M dwarf systems.[1]
TESS’s Deep Dive into 8,000 M Dwarf Systems
The McMaster team, led by PhD student Erik Gillis under the supervision of Assistant Professor Ryan Cloutier, scoured TESS observations of 8,134 mid-to-late M dwarfs. They employed a custom pipeline to process light curves from 86 sectors spanning 2018 to 2024. This involved cleaning data for flares and stellar rotation, searching for transits with the Transit Least Squares algorithm, and rigorously vetting candidates.
From this effort emerged 77 vetted transiting planet candidates orbiting 65 stars. Injection-recovery tests quantified detection sensitivity, enabling precise occurrence rate calculations. The result: a cumulative rate of 1.10 ± 0.16 planets larger than Earth orbiting within 30 days per star.[2] This matches rates around slightly larger early M dwarfs, confirming these tiny stars as prolific planet hosts.
Super-Earths Thrive While Sub-Neptunes Vanish
Around Sun-like stars, planets cluster into two main size groups: rocky super-Earths (1 to 1.6 times Earth’s radius) and gaseous sub-Neptunes (1.8 to 3 times Earth’s radius). This creates a “radius valley” – a gap in the distribution – often explained by photoevaporation, where stellar radiation strips atmospheres from smaller worlds.
Yet around mid-to-late M dwarfs, the picture flips dramatically. The radius distribution becomes unimodal, peaking sharply at 1.25 ± 0.05 Earth radii. Super-Earths abound at 0.954 ± 0.147 per star, while sub-Neptunes dwindle to 0.148 ± 0.045 per star – a ratio of 5.5 to 1.[2]
| Planet Type | Occurrence Rate (per mid-to-late M dwarf, <30 days) | Comparison to Sun-like Stars |
|---|---|---|
| Super-Earths (1-1.6 R⊕) | 0.954 ± 0.147 | Common |
| Sub-Neptunes (1.8-3 R⊕) | 0.148 ± 0.045 | Widespread |
Gillis noted, “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.”[1]
Planet Formation Theories Face a Reckoning
These active M dwarfs should excel at photoevaporation, yet sub-Neptunes are nearly absent. This suggests formation processes differ fundamentally. Models of water-rich pebble accretion predict a fading radius valley toward lower stellar masses, aligning with the observed unimodal peak.
Sub-Neptunes around M dwarfs may represent water worlds rather than hydrogen-helium enveloped giants. Cloutier remarked, “It was already astonishing to learn that the most common planets in our galaxy do not exist within our own solar system. Now with this recent work we’re developing a clearer picture of where these super-Earths and sub-Neptunes come from.”[1] No hot Jupiters appeared either, with an upper limit of 0.012 per star within 10 days.
- The radius valley disappears around the lowest-mass stars.
- Super-Earths outnumber larger planets by wide margins.
- Water-rich formation may dominate for close-in worlds.
Broader Implications for Habitability Searches
M dwarfs hold about half the galaxy’s stars, so their planet populations matter greatly for life detection. Super-Earths in habitable zones – receiving 0.2 to 2 times Earth’s insolation – occur at 0.178_{-0.115}^{+0.116} per star, including potential terrestrials. Conservative zones yield upper limits below 0.19.
Gillis emphasized, “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. This research brings us closer to that.”[1] Future missions will build on TESS, probing compositions and atmospheres.
This McMaster study underscores the diversity of planetary systems. While our solar system lacks these common types, the galaxy’s dominant architecture favors compact super-Earths around tiny stars. Such insights guide telescopes toward promising targets, reminding us that cosmic real estate defies easy expectations.