New research may explain ‘missing’ exoplanets between super-Earths and sub-Neptune. Some exoplanets appear to be losing their atmospheres and shrinking. In a new study using NASA’s decommissioned Kepler space telescope, astronomers found evidence of a possible cause: The cores of these planets are pushing their atmospheres from the inside out. The study was published on: Astronomy Magazine.

Exoplanets (planets outside our solar system) range in size from small rocky planets to huge gas giants. It lies between rocky super-Earths and larger sub-Neptunes with loose atmospheres. But there is a conspicuous absence of planets 1.5 to 2 times the size of Earth (or between super-Earth and sub-Neptune) – a “size gap” – that scientists are trying to better understand.

“So far, scientists have confirmed the discovery of more than 5,000 exoplanets, but fewer than expected are planets 1.5 to 2 times the diameter of Earth,” said Jesse Christiansen, a California Institute of Technology/IPAC investigator and scientific director of NASA’s Exoplanet Archive. ” said. . author of the new study. “Exoplanet scientists now have enough data to say that this gap is no coincidence. Something is happening to prevent planets from reaching and/or remaining this size.”

Researchers believe this gap can be explained by certain sub-Neptunes losing their atmospheres over time. This loss will occur if the planet does not have enough mass, and therefore gravity, to retain its atmosphere. So sub-Neptunian planets that are not large enough will shrink to approximately the size of a super-Earth, leaving a gap between the two planet sizes.

But exactly how these planets lost their atmospheres remains a mystery. Scientists have settled on two possible mechanisms: one is called nucleus-fed mass loss; The other is photoevaporation. The study found new evidence supporting the former.

This video explains the differences between the main types of exoplanets, or planets outside our solar system. Credit: NASA/JPL-Caltech

solve the mystery

Christiansen said that core-induced mass loss occurs when the radiation emitted from a planet’s hot core pushes the atmosphere away from the planet over time and this radiation pushes the atmosphere from below. Photoevaporation, another important explanation for planetary emptiness, occurs when a planet’s atmosphere is essentially blown away by the hot radiation of its host star. In this scenario, “the high-energy radiation from the star acts like a hair dryer on an ice cube,” he said.

While photoevaporation is thought to have occurred within the first 100 million years of the planet’s existence, core-induced mass loss is thought to have occurred much later, closer to 1 billion years of the planet’s life. But with any mechanism “if you don’t have enough mass, you can’t hold it, you lose your atmosphere and you get stuck,” Christiansen added.

For this study, Chistiansen and his co-authors used data from NASA’s K2, an extended mission of the Kepler Space Telescope, to look at the Praesepe and Hyades star clusters between 600 and 800 million years ago. Since planets are generally thought to be the same age as their stars, sub-Neptunians in this system may have survived into the age when photoevaporation could occur, but they are not old enough to experience core-induced mass loss.

So if the team sees that there are a large number of sub-Neptunian stars in Presep and the Hyades (compared to older stars in other clusters), they can conclude that photoevaporation is not occurring. In such a case, core-fed mass loss would be the most likely explanation for what will happen to less massive sub-Neptunes over time.

During observations of Praesepe and the Hyades, researchers found that almost 100% of the stars in these clusters still had planets or planet candidates in their orbits below Neptune. Judging by the size of these planets, researchers believe they retain their atmospheres.

This is in contrast to other old stars observed by K2 (stars older than 800 million years), only 25% of which orbit below Neptune. The older ages of these stars are closer to the timescales on which core-induced mass loss is thought to occur.

From these observations, the team concluded that photoevaporation cannot occur in Praesepe and the Hyades. If this were the case, it would have formed hundreds of millions of years ago and these planets would have very little atmosphere left. This makes core-induced mass loss the main explanation for what will happen to the atmospheres of these planets.

Christiansen’s team spent more than five years creating the catalog of candidate planets needed for the study. However, he said the research is not over yet and it is possible to improve the current understanding of photoevaporation and/or nuclear-induced mass loss. The findings will likely be tested in future studies before anyone declares that the mystery of this planetary abyss has been definitively solved.

This work was performed using the NASA Exoplanet Archive, managed by Caltech Pasadena under contract with NASA as part of NASA’s Exoplanet Research Program located at NASA’s Jet Propulsion Laboratory in Southern California. JPL is a division of Caltech.