A new study reveals why some exoplanets are losing their atmospheres and shrinking.

In a recent breakthrough, scientists studying the exoplanet TOI-421 b have uncovered intriguing insights into the enigma of 'missing' planets between super-Earths and sub-Neptunes. Utilizing data from NASA's retired Kepler Space Telescope, the research indicates a potential explanation for the atmospheric loss and contraction observed in some exoplanets. Remarkably, the study proposes that these planets' cores might be driving the expulsion of their atmospheres, offering a novel perspective on the dynamics of celestial bodies beyond our solar system. This discovery contributes to unraveling the mysteries surrounding the diverse range of exoplanets, shedding light on the peculiar "size gap" that has perplexed scientists—those elusive planets falling between 1.5 to 2 times the size of Earth.

Caltech/IPAC research scientist Jessie Christiansen, serving as the science lead for the NASA Exoplanet Archive, has revealed that the detection of over 5,000 exoplanets has led to an intriguing observation. Despite expectations, there's a scarcity of planets with a diameter 1.5 to 2 times that of Earth. Christiansen asserts that this gap is not coincidental, indicating an underlying phenomenon hindering planets from reaching or maintaining this specific size range. The findings, detailed in The Astronomical Journal, highlight a significant puzzle in our understanding of exoplanetary systems.

Researchers propose that the observed gap in planet sizes may stem from sub-Neptunes gradually losing their atmospheres over time. This phenomenon occurs when a planet lacks sufficient mass and gravitational force to retain its atmosphere, causing these sub-Neptunes to shrink to the size of super-Earths. The mystery lies in the mechanism of this atmospheric loss, with scientists considering two primary possibilities: core-powered mass loss and photoevaporation. Recent findings from the study provide fresh evidence supporting the former, shedding light on the intricate processes shaping planetary atmospheres in distant solar systems.

What can understand from the "size difference" of the planets between super-Earth and sub-Neptune?

The enigma of the planetary gap appears to find its resolution in the contrasting mechanisms of core-powered mass loss and photoevaporation. Core-powered mass loss involves the gradual push of a planet's atmosphere by radiation emitted from its hot core, exerting pressure from below. On the other hand, photoevaporation occurs when a planet's atmosphere is essentially blown away by the intense radiation from its host star, akin to a hair dryer melting an ice cube. While photoevaporation is believed to transpire within a planet's initial 100 million years, core-powered mass loss unfolds much later, around the 1 billion-year mark. Regardless of the mechanism, the pivotal factor is a planet's mass—insufficient mass leads to atmospheric loss and subsequent shrinkage, as highlighted by scientist Jessie Christiansen.

In their research, Jessie Christiansen and her co-authors utilized data from NASA's K2, an extension of the Kepler Space Telescope, focusing on the star clusters Praesepe and Hyades, aged between 600 million and 800 million years. Assuming planets align in age with their host stars, the sub-Neptunes in this system should be beyond the timeframe for photoevaporation but not yet old enough for core-powered mass loss. By observing a notable presence of sub-Neptunes in Praesepe and Hyades, especially when compared to older stars in different clusters, the team could reasonably deduce that photoevaporation had not occurred. This suggests that core-powered mass loss emerges as the more plausible explanation for the fate of less massive sub-Neptunes over time in these star clusters.

Through meticulous observation of Praesepe and Hyades, researchers uncovered a striking pattern—almost 100% of stars in these clusters possess a sub-Neptune planet or planet candidate in their orbit. The size of these planets indicates that they likely retained their atmospheres. This starkly contrasts with the findings from K2's examination of older stars, aged over 800 million years, where only 25% exhibit orbiting sub-Neptunes. The disparity aligns with the anticipated timeframe for core-powered mass loss, reinforcing the notion that as stars age, the prevalence of sub-Neptunes diminishes, possibly due to the effects of this specific atmospheric loss mechanism.

Based on their observations, the research team concluded that photoevaporation likely did not occur in Praesepe and Hyades, as the sub-Neptunes still retained their atmospheres. If photoevaporation had taken place, it would have happened hundreds of millions of years earlier, resulting in planets with little to no atmosphere. Consequently, core-powered mass loss emerges as the primary explanation for the atmospheric fate of these planets. Jessie Christiansen's team dedicated over five years to construct the essential planet candidate catalog for this study.

Despite these findings, Christiansen acknowledges that the understanding of photoevaporation and core-powered mass loss may evolve, emphasizing the ongoing nature of this research. Future studies will likely scrutinize these conclusions, ensuring a comprehensive understanding before declaring the mystery of the planetary gap definitively solved. The study utilized the NASA Exoplanet Archive, operated by Caltech in Pasadena under NASA's Exoplanet Exploration Program at NASA’s Jet Propulsion Laboratory in Southern California.

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