Astronomers have discovered something surprising about planets beyond our solar system.
Even though planets are incredibly common—scientists now estimate there is at least one planet for every star in our galaxy—the types of planets we see depend strongly on the type of star they orbit.
And the most common planets in the Milky Way don’t seem to form around the most common stars.
These distant worlds, called exoplanets, orbit stars outside our solar system.
Around stars like our Sun, astronomers often find two main types of planets: super-Earths and sub-Neptunes.
Super-Earths are rocky planets that can be up to ten times heavier than Earth, while sub-Neptunes are slightly larger planets with thick gas atmospheres, similar to a smaller version of Neptune.
Over the past decade, scientists have learned that these planets are very common—but mostly around Sun-like stars.
However, stars like our Sun are actually in the minority. Most stars in the Milky Way are much smaller and dimmer, known as M dwarfs. These stars are only about 8 to 40 percent the size of the Sun.
Because they are faint, they have been difficult to study—until recently.
Thanks to NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers can now observe these small stars more effectively.
TESS scans different parts of the sky over time, allowing scientists to detect planets by watching for tiny dips in starlight as planets pass in front of their stars.
Using data from TESS, researchers at McMaster University made an unexpected discovery. Around mid-to-late M dwarfs, sub-Neptune planets are almost completely missing. Instead, these stars are surrounded mostly by super-Earths. This challenges previous ideas about how planets form.
For years, scientists believed the difference between super-Earths and sub-Neptunes was mainly due to a process called photoevaporation. This happens when strong radiation from a star strips away a planet’s atmosphere over time.
Since M dwarfs are known to be very active, they should be capable of removing atmospheres from planets. But the new findings suggest that this process alone cannot explain why sub-Neptunes are so rare around these stars.
Instead, the researchers think that planets around M dwarfs may form differently from the beginning.
These planets might be richer in water and less likely to develop thick gas atmospheres. In other words, the conditions around these small stars may naturally favor the formation of super-Earths rather than sub-Neptunes.
This discovery, published in The Astronomical Journal, highlights how much we still have to learn about the universe. Just 30 years ago, scientists had not confirmed the existence of any exoplanets. Today, thousands have been discovered, revealing patterns that continue to surprise researchers.
By studying more types of stars and planetary systems, astronomers are slowly building a clearer picture of how planets form and evolve. These findings may even help us understand the conditions needed for life, both in our galaxy and beyond.
Source: KSR.
