Hamilton, On Apr 30: Astronomers now estimate there is at least one planet for every star in our galaxy. These worlds, called exoplanets, are planets that orbit stars outside our solar system. But new research from McMaster University reveals a surprising twist: the most common planets in our galaxy don’t exist around the most common stars.
Around stars like our Sun, the most common planets are sub-Neptunes worlds thought to resemble Neptune but smaller in size and super-Earths, rocky planets that are up to ten times more massive than Earth. For nearly a decade, astronomers have known that these two types of planets are widespread around Sun-like stars across the galaxy. But Sun-like stars make up only a minority of the stars in our galaxy, leaving a gap in our understanding of how planets form.
To fill that gap, McMaster researchers examined planets orbiting mid-to-late M dwarfs. These small stars, just eight to 40 per cent the size of our Sun, make up most of the stars in the Milky Way. Because of their faintness, they have historically been difficult to study.
NASA’s Transiting Exoplanet Survey Satellite (TESS) changed that, providing an unparalleled view of these stars and their planetary systems. By observing a new patch of sky every 28 days, the satellite surveys the entire sky over 26 months.
Using the TESS data, the McMaster team discovered that mid-to-late M dwarfs host many super-Earths but virtually no sub-Neptunes, a finding that challenges existing theories of planet formation.
“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,” says Erik Gillis, a PhD student in the Department of Physics and Astronomy.
Gillis conducted the work under the supervision of Ryan Cloutier, assistant professor and Canada Research Chair in Exoplanetary Astronomy.
Astronomers have long attributed the distinction between super-Earths and sub-Neptunes to photoevaporation, a process where intense starlight strips away a planet’s atmosphere. Mid-to-late M dwarfs are extremely active and should be capable of evaporating planetary atmospheres efficiently, but not to the extent we’re seeing here, explains Gillis. The fact that sub-Neptunes exist in such small numbers around these stars suggests that planet formation here may favour water-rich worlds rather than gas-shrouded sub-Neptunes.
“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,” says Gillis.
The findings, published today in The Astronomical Journal, come at a time when exoplanet science is growing rapidly. The first exoplanets were discovered just 30 years ago – a blink of an eye compared to some other astronomical fields.
Since then, researchers have studied only a small fraction of planetary systems, often assuming the same patterns hold everywhere because the same physical processes shape planets across the galaxy.
“Our solar system was once the only example we had. Now, thanks to missions like TESS, we can compare thousands of systems and uncover patterns that rewrite our assumptions,” says Cloutier.
“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.”
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