The realm of exoplanets has always been a source of fascination for NASA scientists. With various types of planets existing beyond our solar system, each exhibiting unique characteristics, there is still much to be learned. However, one perplexing phenomenon has grabbed their attention – the enigmatic absence of planets that should fall within a specific size range. In this article, we delve into the research conducted by Jessie Christiansen, a research scientist at Caltech and science lead for the NASA Exoplanet Archive, to uncover the potential cause behind these missing exoplanets.

A Mysterious Gap

Among the vast assemblage of over 5,000 discovered exoplanets, there is an intriguing gap where planets measuring between 1.5 to two times the width of Earth should be. Instead, we find a surplus of super-Earths and sub-Neptunes, leaving experts to question the absence of this intermediate planetary class. Christiansen states, “There’s something going on that impedes planets from reaching and/or staying at this size.” This conundrum has led scientists to investigate the intriguing possibility that radiation is the culprit behind the shrinking of these planets.

Radiation emitted from the cores of planets may play a vital role in their shrinkage and eventual disappearance from the size spectrum. According to Christiansen’s recent research, the radiation emitted by the planets’ cores pushes their atmospheres away, causing them to dwindle in size. Consequently, the shrinking exoplanets lose the mass and gravity required to keep their atmospheres close, resulting in their transformation into super-Earths. However, despite this groundbreaking revelation, the precise mechanism responsible for the loss of atmosphere remains elusive.

Scientists contemplating this conundrum have proposed two distinct hypotheses to explain this phenomenon: core-powered mass loss and photoevaporation. Core-powered mass loss posits that a planet’s core emits radiation that gradually separates its atmosphere, while photoevaporation suggests that a planet’s atmosphere dissipates due to the radiation emitted by its host star. The latter hypothesis, however, is believed to occur within the first 100 million years of a planet’s life, whereas core-powered mass loss could span closer to a planet’s one billionth birthday.

To shed light on these hypotheses, Christiansen’s team analyzed data from NASA’s retired Kepler Space Telescope. By examining star clusters over 100 million years old, which are believed to share similar ages with their hosted planets, scientists could determine whether the planets had experienced photoevaporation or core-powered mass loss. Their findings revealed that the majority of the planets within these clusters retained their atmospheres, providing greater plausibility to the core-powered mass loss hypothesis. However, it is worth noting that recent research suggests a potential ongoing sequence where both processes contribute to the phenomenon.

Continued Exploration and Unanswered Questions

Though this research offers crucial insights into the mystery of shrinking exoplanets, many questions remain unanswered. Christiansen herself acknowledges that her work is far from complete and highlights the continuous development of our understanding of exoplanets over time. As scientists tirelessly strive to unravel the complexities of these distant worlds, the enigma of missing exoplanets continues to captivate and challenge their intellectual curiosity.

The hypothesis that radiation is responsible for the shrinking of exoplanets represents a significant breakthrough in the field of exoplanet research. While core-powered mass loss appears to be a more plausible explanation based on recent findings, the intricacies of this phenomenon remain unsolved. As our knowledge of exoplanets expands and technology progresses, future studies hold promise in elucidating the true nature of these missing planets and paving the way for a deeper comprehension of the broader exoplanetary landscape.


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