Scientists believe they can finally explain the mysterious absence of planets 1.5 to 2 times larger than Earth

There is a surprising lack of medium-sized orbs. Some exoplanets appear to lose their atmospheres and then shrink through a process called “nuclear-induced mass loss.”

Exoplanets come in different shapes and sizes: from small rocky worlds to massive gas giants. But what is striking is that researchers have encountered only a few medium-sized exoplanets, which are planets with radii between 1.5 and 2 times that of Earth. This is a gap between the rocky super-Earths and the larger, gas-covered planets called sub-Neptunes. Astronomers have been trying to figure out the reasons for this observed gap for some time. And New study Now finally shedding more light on the issue.

It’s no coincidence
Researchers have now discovered more than 5,000 exoplanets. “Surprisingly, there are fewer planets with a diameter of 1.5 to 2 times the diameter of Earth than originally expected,” says researcher Jessie Christiansen. “However, we have now collected enough data to conclude that this gap is not a coincidence. There is clearly something going on that prevents planets from reaching and maintaining this specific size.”

shrinkage
Scientists suspect that this gap can be explained by the gradual loss of the atmosphere of some sub-Neptunian planets. In this case, the atmosphere “leaks away” when the planet doesn’t have enough mass – and therefore gravity – to maintain its atmosphere. As a result, sub-Neptunian planets that are not massive enough will shrink to about the size of super-Earths. This is said to create a striking gap between these two classes of planets.

how?
But this is not the whole story. How exactly these planets lost their atmospheres remains a mystery. The researchers hypothesize two possible mechanisms: one called nuclear-induced mass loss, and the other called photoevaporation (see box). Now, recent research has provided new evidence that the first mechanism, mass loss due to nuclear weapons, is more likely.

More on nuclear-induced mass loss and photoevaporation
Core-induced mass loss occurs when radiation from a planet’s hot core pushes the planet’s atmosphere apart. This happens because radiation is pressuring the atmosphere from below, so to speak. Photoevaporation occurs when a planet’s atmosphere is blasted by hot radiation from its parent star. In this scenario, the star’s high-energy radiation acts like a “hairdryer on an ice block.” Photoevaporation likely occurred within the first 100 million years of the planet’s life. In contrast, mass loss caused by nuclear weapons is thought to occur significantly later, about a billion years after the planet’s age.

In this study, the researchers used data from NASA’s K2, an extension mission of the Kepler Space Telescope, which searched for exoplanets in part of our Milky Way Galaxy between 2009 and 2018. They focused on the Praesepe and Hyades star clusters, which are between 600 million and 800 million years old. Because planets are generally considered to be the same age as their parent star, subplanets in this system will have passed the age at which photoevaporation could potentially occur, but are not old enough to experience mass loss due to nuclear energy.

Pricebe and Hyades
When researchers observed Prisebe and Hyades, they discovered that almost all of the stars contained a sub-Neptunian planet. Given the age of these groups, the team concluded that photoevaporation likely did not occur. This means that these planets were able to maintain their atmosphere. This is in contrast to other older stars studied by K2 (stars older than 800 million years), of which only 25% contain sub-Neptunian stars. The oldest ages of these stars coincide with the time frame in which mass loss due to nucleation is thought to occur.

Loss of mass due to nuclear weapons
In short, this means that photoevaporation most likely did not occur at Praesepe and Hyades – and is unlikely to happen again. If that had happened, it would have happened hundreds of millions of years ago, and the planets would have little or no atmosphere left. This suggests that over time, less massive subplanets may continue to lose their atmospheres due to nuclear-induced mass loss.

Mysterious absence
The findings of the study are interesting. It explains the mysterious absence of planets 1.5 to 2 times the size of Earth. So it does appear that some exoplanets are losing their atmospheres and shrinking. Meanwhile, astronomers have now found evidence of a possible cause: that the cores of these planets are pushing their atmospheres inward.

Despite these new ideas, the team stresses that research into planetary rifts is not yet fully completed. In fact, our current understanding of photoevaporation and/or nuclear-induced mass loss may continue to evolve. So we will have to wait for more studies before we get definitive answers.