A new study could explain the “missing” exoplanets between super-Earths and sub-Neptunes.
Some exoplanets appear to be losing their atmospheres and shrinking. In a new study using
The difference in the size of the exoplanet
Exoplanets (planets outside our solar system) come in a range of sizes, from small, rocky planets to colossal gas giants. In between are rocky super-Earths and larger sub-Neptunes with fluffy atmospheres. But there is a notable absence – a “size gap” – of planets that are between 1.5 and 2 times the size of Earth (or between super-Earths and sub-Neptunes) that scientists have been working to better understand.
“Scientists have now confirmed the detection of more than 5,000 exoplanets, but there are fewer planets than expected, with a diameter between 1.5 and 2 times that of Earth,” said Caltech/IPAC research scientist Jessie Christiansen, scientific lead of the NASA Exoplanet Archive and lead author of the new study in The Astronomical Journal. “Exoplanet scientists now have enough data to say this gap is not a fluke. There is something going on that prevents planets from reaching and/or staying this size.”
Researchers think this gap could be explained by the fact that certain sub-Neptunes have lost their atmospheres over time. This loss would happen if the planet did not have enough mass, and therefore gravitational force, to maintain its atmosphere. Therefore, sub-Neptunes that are not massive enough would shrink to approximately the size of super-Earths, leaving a gap between the two planet sizes.
But exactly how these planets are losing their atmospheres remains a mystery. Scientists have established two likely mechanisms: one is called nuclear-fueled mass loss; and the other, 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/
The other main explanation for the planetary gap, photoevaporation, happens when a planet’s atmosphere is essentially destroyed by hot radiation from its host star. In this scenario, “the star’s high-energy radiation acts like a hairdryer on an ice cube,” she said.
Although photoevaporation is thought to occur during a planet’s first 100 million years, core-driven mass loss is thought to occur much later – close to a billion years into a planet’s life. But with either mechanism, “if we don’t have enough mass, we can’t hold on and we lose atmosphere and shrink,” Christiansen added.
Discovering evidence through observation
For this study, Chistiansen and his co-authors used data from NASA’s K2, an extended mission of the Kepler Space Telescope, to observe the Praesepe and Hyades star clusters, which are between 600 million and 800 million years old. Because planets are generally thought to be the same age as their host star, the sub-Neptunes in this system would have been past the age at which photoevaporation could have occurred, but not far enough to have experienced core-fueled mass loss.
Therefore, if the team saw that there were many sub-Neptunes in Praesepe and Hyades (compared to older stars in other clusters), they could conclude that photoevaporation did not occur. In this case, core-fueled mass loss would be the most likely explanation of what happens to the less massive sub-Neptunes over time.
By observing Praesepe and Hyades, researchers discovered that almost 100% of the stars in these clusters still have a subregion.
This differs from the other oldest stars observed by K2 (stars over 800 million years old), of which only 25% have sub-Neptunes in orbit. The older age of these stars is closer to the time period in which core-fueled mass loss is thought to occur.
From these observations, the team concluded that photoevaporation could not have occurred at Praesepe and Hyades. If it had happened, it would have happened hundreds of millions of years earlier, and these planets would have little or no atmosphere left. This leaves core-fueled mass loss as the main explanation for what likely happens to the atmospheres of these planets.
Ongoing research and Kepler’s legacy
Christiansen’s team spent more than five years building the catalog of planet candidates needed for the study. But the research is far from complete, she said, and it’s possible that the current understanding of photoevaporation and/or core-fueled mass loss could evolve. The findings will likely be put to the test by future studies before anyone can declare the mystery of this planetary gap solved once and for all.
This study was conducted using the NASA Exoplanet Archive, which is operated by Caltech in Pasadena under contract with NASA as part of the Exoplanet Exploration Program, located at NASA’s Jet Propulsion Laboratory in Southern California. JPL is a division of Caltech.
Reference: “K2 Escalation. VII. Evidence for a High Occurrence Rate of Hot Sub-Neptunes at Intermediate Ages” by Jessie L. Christiansen, Jon K. Zink, Kevin K. Hardegree-Ullman, Rachel B. Fernandes, Philip F. Hopkins, Luisa M. Rebull, Kiersten M Boley , Galen J. Bergsten and Sakhee Bhure, November 15, 2023, The Astronomical Journal.
NASA’s Kepler Mission
On October 30, 2018, Kepler ran out of fuel and ended its mission after nine years, during which it discovered more than 2,600 confirmed planets around other stars, along with thousands of additional candidates that astronomers are working to confirm. .
NASA’s Ames Research Center in Silicon Valley, California, manages the Kepler and K2 missions for NASA’s Science Mission Directorate. JPL managed the development of the Kepler mission. Ball Aerospace & Technologies Corporation operated the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder.