What is the Most Common Type of Planet in the Galaxy? – Image for illustrative purposes only (Image credits: Unsplash)
Astronomers have long estimated that the Milky Way hosts at least one planet for every star, painting a picture of a galaxy brimming with worlds. Recent analysis of data from NASA’s Transiting Exoplanet Survey Satellite, or TESS, has uncovered a striking pattern that upends expectations. Around the small, dim red dwarfs known as mid-to-late M dwarfs – which dominate the stellar population – the most common planets found near Sun-like stars are nowhere to be seen.[1][2]
TESS Lights Up the Dim Stars
Launched in 2018, TESS scans the sky in 28-day segments, completing a full survey over 26 months. This approach has proven ideal for detecting planets transiting faint M dwarfs, which previous surveys often overlooked due to their low brightness. Researchers at McMaster University, led by PhD student Erik Gillis and supervised by Ryan Cloutier, examined TESS observations of 8,134 such stars. They identified 77 vetted transiting planet candidates, enabling a detailed look at occurrence rates.[3]
The results revealed a clear divide. These M dwarfs, which range from 8 to 40 percent the size of the Sun, teem with super-Earths – rocky worlds up to 10 times Earth’s mass. Yet sub-Neptunes, planets with thick gaseous envelopes and densities suggesting mini-versions of Neptune, appear virtually absent. This absence marks a departure from patterns around brighter, Sun-like stars, where both types thrive.[4]
Decoding Super-Earths and Sub-Neptunes
Super-Earths and sub-Neptunes represent the galaxy’s prevalent planet sizes, filling the gap between Earth and Neptune. Super-Earths tend to be dense and rocky, while sub-Neptunes feature extended atmospheres that lower their overall density. Around Sun-like stars, these two populations show a distinct “radius valley” – a scarcity of planets at intermediate sizes.[1]
Mid-to-late M dwarfs disrupt this norm. The radius valley disappears here, with super-Earths dominating and sub-Neptunes all but vanishing. “We didn’t just refine the picture – we changed it,” Gillis noted. “Around these stars, sub-Neptunes effectively vanish, which means the mechanisms shaping planets here are different.”[2]
Rethinking How Planets Form
Traditional models relied on photoevaporation to explain the radius valley. Young stars’ intense radiation would strip away the gaseous layers from smaller planets, leaving rocky super-Earths in place of would-be sub-Neptunes. M dwarfs, however, challenge this idea. Though energetically active in youth, they produce few sub-Neptunes from the outset, suggesting formation processes favor water-rich, rocky cores over gas-accreting worlds.[4]
This shift demands revisions to planet formation theories. Earlier studies focused on Sun-like stars, a minority in the Milky Way, leaving M dwarfs underrepresented. TESS data now fills that gap, showing how stellar type influences planetary architecture. “It was already astonishing to learn that the most common planets in our galaxy do not exist within our own solar system,” Cloutier observed. The new work further clarifies their origins.[2]
Examples like TOI-421 b and GJ 1214 b illustrate sub-Neptunes’ traits elsewhere, but their rarity around M dwarfs hints at diverse building blocks in protoplanetary disks. Water abundance or disk dynamics may steer outcomes toward super-Earths in these systems.
What This Means for Exoplanet Science
The findings, detailed in The Astronomical Journal, underscore TESS’s power in revealing hidden patterns. By comparing thousands of systems, scientists now grasp that planet populations vary sharply with host star. M dwarfs’ prevalence implies super-Earths may outnumber sub-Neptunes galaxy-wide, reshaping estimates of habitable worlds.[4]
Gillis emphasized the broader stakes: “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.” Future missions will probe these worlds’ atmospheres, testing if super-Earths around M dwarfs hold potential for life. This discovery not only rewrites formation models but invites a fuller census of the Milky Way’s planetary diversity.
As exoplanet hunts continue, the galaxy reveals itself as far more varied than once thought. What other surprises lurk among the stars?