When JWST observed what was thought to be a hot Jupiter, it uncovered something unexpected: a fluffy, shrinking planet more similar to Neptune.

"Nearly three decades after the discovery of the first exoplanet, astronomers remain uncertain about how these diverse worlds form. Each young transiting planet offers crucial insights into developing models of planetary formation. This is why, in 2023, the James Webb Space Telescope (JWST) focused its attention on a young planet named HIP 67522b for a few hours.

At just 17 million years old, HIP 67522b was initially thought to be a typical close-in, Jupiter-sized planet. However, JWST’s observations revealed an unexpected surprise: HIP 67522b is nothing like Jupiter. Instead, it has a puffy, extended atmosphere filled with water vapor and carbon dioxide, suggesting it belongs to a completely different category of planets.

The planet’s inflated atmosphere puzzled researchers, led by Pa Chia Thao (University of North Carolina at Chapel Hill). If HIP 67522b had Jupiter’s mass, its strong gravity would have kept its atmosphere compact. But the atmosphere’s far-reaching extension indicates the planet is much less massive than expected. Calculations suggest it weighs about 14 times Earth’s mass, making it one of the lightest giants ever discovered — more akin to sub-Neptunes than Jupiters.

Sub-Neptunes are peculiar planets larger than Earth but smaller than ice giants, a type absent in our solar system but common across the galaxy. Scientists believe these planets form with large atmospheres, which are later stripped away through processes like star-induced boil-off. HIP 67522b appears to be undergoing this very process. Its low mass and proximity to its star make it incapable of holding onto its gases, meaning its atmosphere is likely evaporating. Over the next billion years, the planet may lose most of its atmosphere, leaving behind only a shrunken core.

This discovery provides a rare opportunity for astronomers to observe atmospheric escape in action, shedding light on how sub-Neptunes reach their final forms. “Measuring the atmospheric properties of young planets offers unique insights into their formation and evolution,” says Munazza Alam (Space Telescope Science Institute), who was not part of the study. Early constraints on atmospheric escape processes help refine our understanding of how planetary atmospheres are shaped.

In addition to its implications for sub-Neptune formation, the study achieved another first: astronomers measured the mass of HIP 67522b using the spectrum of starlight passing through its atmosphere, bypassing the usual challenges of observing young, active stars. This innovative approach could revolutionize how astronomers weigh young exoplanets.

“It’s notoriously challenging to weigh young exoplanets,” notes Shreyas Vissapragada (Carnegie Observatories), who was also not involved in the research. “This comprehensive study shows just how powerful JWST is for measuring the masses of young exoplanets, opening new doors for future discoveries.”