In the universe beyond our solar system, the diversity of planets, known as exoplanets, is vast and fascinating.
Interestingly, scientists at NASA observed an intriguing trend: some of these planets appear to be shrinking in size, a phenomenon that may be related to radiation exposure.
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Additionally, they noted an absence of planets between 1.5 and twice the width of Earth, a gap that challenges current understanding of planetary formation.
The mysterious lack of medium-sized planets
Jessie Christiansen, a Caltech researcher and science lead for the NASA Exoplanet Archive, offered an explanation to the mysterious gap in the size of exoplanets, which was observed in recent research published in The Astronomical Journal.
Among the more than 5,000 exoplanets already discovered, most fall into the category of super-Earths or sub-Neptunes, but there is a notable absence of planets with intermediate sizes.
Mass loss vs. photoevaporation: hypotheses under debate
In the study, Christiansen and his team suggest that sub-Neptunes may be shrinking due to radiation from their cores, losing their atmospheres and shrinking to the size of super-Earths. This process may provide an answer to the observed gap.
The recent study explored the possibility that exoplanets are shrinking due to the loss of atmosphere, caused by insufficient mass to retain it. The research considered two main hypotheses:
- ‘Core-fueled mass wasting’, where radiation from the planetary core expels the atmosphere;
- ‘Photoevaporation’, in which radiation from the host star plays a role.
Using data from the Kepler Space Telescope, Christiansen's team analyzed ancient star clusters and found that most of them planets retained their atmospheres, suggesting that core-fueled mass loss is more likely, although both processes can coexist.
Christiansen emphasizes that the mystery still persists and the understanding of exoplanets will continue to evolve.