The Mystery of Shrinking Exoplanets

A group of planets beyond our solar system has been causing a great deal of intrigue among NASA scientists. These exoplanets appear to be shrinking, and researchers believe that radiation may be the culprit behind this perplexing phenomenon. While there is a diverse range of exoplanets in existence, ranging from gas giants to rocky planets and even fluffy “super-puffs,” there is a noticeable gap in the size distribution. Specifically, there seems to be a scarcity of planets that fall within 1.5 to 2 times the width of Earth.

According to Jessie Christiansen, a research scientist at Caltech and the science lead for the NASA Exoplanet Archive, the absence of planets within this size range is not a random occurrence. She states, “Exoplanet scientists have enough data now to say that this gap is not a fluke. There’s something going on that impedes planets from reaching and/or staying at this size.”

Intriguingly, Christiansen’s latest research suggests that some sub-Neptunes, which are planets approximately two to four times the diameter of Earth, may be shrinking. These planets are believed to lose their atmospheres, causing them to rapidly decrease in size until they resemble super-Earths. The primary driver behind this atmospheric loss is the radiation emitted by the planets’ cores, which pushes their atmospheres away into space.

The exact mechanism by which these shrinking exoplanets lose their atmospheres remains unclear. The study supports one hypothesis known as “core-powered mass loss.” This theory proposes that a planet’s core emits radiation that gradually pushes its atmosphere away, resulting in the separation of the atmosphere from the planet over time. In contrast, another hypothesis called photoevaporation suggests that a planet’s atmosphere is dissipated by the radiation emanating from its host star. However, photoevaporation is believed to occur within the first 100 million years of a planet’s life, while core-powered mass loss could transpire closer to the planet’s billionth birthday.

To test these hypotheses, Christiansen’s team analyzed data from NASA’s retired Kepler Space Telescope. They focused on star clusters that were over 100 million years old, as the age of the planets in these clusters would align with their host stars’ ages. By examining these planets, which were old enough to have potentially experienced photoevaporation but not core-powered mass loss, the scientists discovered that most of the planets retained their atmospheres. This finding suggests that core-powered mass loss is a more likely cause of eventual atmosphere loss.

While these findings provide valuable insights into the mystery of shrinking exoplanets, there is still more to unravel. Christiansen, in a statement, acknowledges that her work is far from complete. Additionally, she emphasizes that our understanding of exoplanets will continue to evolve over time. As further research is conducted and new data is gathered, perhaps new revelations will shed light on the fascinating enigma of these diminishing alien worlds.


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