Madrid, 16 years old (European Press)
This could explain why there are no planets with sizes between super-Earths and sub-Neptunes, according to the study published in The Astronomical Journal.
Exoplanets (planets outside our solar system) come in a variety of sizes, from small rocky planets to massive gas giants. In between are giant, rocky terrestrial planets and sub-planets larger than Neptune with puffy atmospheres. But there is a distinct absence (“size gap”) of planets between 1.5 and 2 times the size of Earth (or between super-Earths and sub-Neptunian planets) that scientists are working to better understand.
“Scientists have now confirmed the discovery of more than 5,000 exoplanets, but there are fewer planets than expected, with diameters ranging between 1.5 and 2 times the diameter of Earth,” Jesse Christiansen, a research scientist at Caltech/AIPAC, said in a statement. NASA’s Exoplanet Archive and first author of the new study. “Exoplanet scientists now have enough data to say that this gap is not a coincidence. There is something going on that prevents planets from reaching and/or remaining this large.”
Researchers believe that this gap can be explained by some sub-Neptunian planets losing their atmosphere over time. This loss would occur if the planet did not have enough mass and therefore gravitational force to retain its atmosphere. Therefore, sub-Neptunian planets that are not massive enough will shrink to about the size of the terrestrial planets, leaving a gap between the sizes of the two planets.
But how these planets lose their atmosphere is still a mystery. Scientists have settled on two possible mechanisms: one is called nucleation-induced mass loss; The other, photoevaporation. The study revealed new evidence supporting the former.
The core-induced mass loss occurs when radiation from a planet’s hot core pushes the atmosphere away from the planet over time, “and that radiation pushes the atmosphere from below,” Christiansen said.
The other main explanation for a planetary gap is photoevaporation, which occurs when a planet’s atmosphere is washed away by hot radiation from its host star. In this scenario, “the star’s high-energy radiation acts like a hairdryer on an ice cube,” he said.
While photoevaporation is thought to occur within the first 100 million years of the planet’s life, mass loss caused by nucleation is thought to occur much later, about a billion years after the planet’s life. But with either mechanism, “if you don’t have enough mass, you can’t hold it, you lose the atmosphere and you shrink,” Christiansen added.
In this study, Chittiansen and his colleagues used data from NASA’s K2, an extension mission of the Kepler space telescope, to observe the Praesepe and Hyades star clusters, which are between 600 and 800 million years old. Because planets are generally thought to be the same age as their host star, the sub-Neptunian planets in this system are well past the age at which photoevaporation can occur, but are not old enough to undergo core-driven mass loss.
So, if the team sees that there are many sub-Neptunian planets in Prasepe and Hyades (compared to older stars in other clusters), they can conclude that photoevaporation did not occur. In this case, core-induced mass loss would be the most likely explanation for what happens to sub-Neptune’s less massive mass over time.
By observing Prisebe and Haades, researchers discovered that nearly 100% of the stars in these clusters still have a sub-Neptunian planet or planet candidate in their orbit. Judging by the size of these planets, researchers believe that they have preserved their atmospheres.
This differs from other older stars observed by K2 (stars older than 800 million years), of which only 25% have sub-Neptune orbits. The oldest age of these stars is closer to the time period in which mass loss due to nucleation is thought to occur.
From these observations, the team concluded that photoevaporation could not have occurred at Praesepe and Hyades. If so, it would have happened hundreds of millions of years ago, and these planets would have little or no atmosphere. This makes core-induced mass loss the main explanation for what is likely to happen to these planets’ atmospheres.
Christiansen’s team spent more than five years building the list of planet candidates needed for study. But he said the research is still far from complete, and current understanding of photoevaporation and/or nucleation-induced mass loss could evolve. The results will likely have to be tested in future studies before anyone can announce that the mystery of this planetary gap has been solved once and for all.