McMaster researchers find most common exoplanets vanish around most common stars

For thirty years, astronomers have been cataloging the planets that orbit distant stars. The numbers are staggering: at least one planet for every star in the Milky Way, billions upon billions of worlds we will never visit. But a team at McMaster University has uncovered something that doesn't add up. The most abundant planets in the galaxy are not orbiting the most abundant stars.

Around Sun-like stars, astronomers have long known that sub-Neptunes and super-Earths dominate. Sub-Neptunes are smaller cousins of Neptune, gas-shrouded worlds with thick atmospheres. Super-Earths are rocky planets up to ten times heavier than Earth. For nearly a decade, researchers have watched these two types proliferate around stars similar to our own. The problem is that Sun-like stars are rare. They represent only a minority of the stellar population. The vast majority of stars in the galaxy are M dwarfs—small, dim, cool stars that range from eight to forty percent the size of our Sun. If the same planetary patterns held everywhere, M dwarfs should be teeming with sub-Neptunes and super-Earths. But no one had been able to look closely enough to check.

The obstacle was observational. M dwarfs are faint. They don't shine brightly enough for traditional telescopes to study their planetary systems with ease. That changed when NASA launched the Transiting Exoplanet Survey Satellite, known as TESS. The spacecraft sweeps a new section of the sky every 28 days, completing a full survey of the heavens in just over two years. For the first time, researchers had a systematic, detailed view of planetary systems around the galaxy's most common stars.

Erik Gillis, a PhD student in McMaster's Department of Physics and Astronomy, led the analysis of TESS data focused on mid-to-late M dwarfs. What he and his supervisor, Ryan Cloutier, found was stark: these stars host plenty of super-Earths but virtually no sub-Neptunes. The absence was not marginal or ambiguous. Sub-Neptunes had effectively vanished. "We didn't just refine the picture—we changed it," Gillis said. The discovery, published in the Astronomical Journal, suggests that the mechanisms shaping planets around M dwarfs operate fundamentally differently from those around Sun-like stars.

Astronomers have long explained the distinction between super-Earths and sub-Neptunes through a process called photoevaporation. Intense radiation from a star strips away a planet's atmosphere over time. M dwarfs are extremely active and should be capable of this atmospheric erosion. Yet the data showed something more extreme: not just erosion, but wholesale absence. Gillis proposed an alternative explanation. Perhaps planets forming around M dwarfs simply don't develop into sub-Neptunes in the first place. The conditions in these stellar systems may favor the formation of water-rich, rocky worlds rather than gas-laden planets. If true, it would mean that the pathways to planetary diversity are not universal but depend heavily on the type of star at the center of a system.

The implications reach beyond planetary science. Understanding how planets form and what they are made of touches on deeper questions about the origins of life itself. Our own solar system, once humanity's only reference point, now appears to be just one example among countless variations. "Our solar system was once the only example we had," Cloutier reflected. "Now, thanks to missions like TESS, we can compare thousands of systems and uncover patterns that rewrite our assumptions." The discovery that the most common planets in the galaxy do not orbit the most common stars is itself humbling. But it is also an invitation: the universe is more diverse, more complex, and less predictable than the models suggested. The work of understanding it has only begun.