What are Ultrahot Jupiters? Insights from the JWST

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In a recent study, a team of researchers developed a three-dimensional (3D) spectroscopic map of a distant exoplanet’s atmosphere, revealing surprising details about its structure and chemistry.

The findings, published in the Journal of Nature Astronomy, come from a study led by Navjot Kumar and colleagues at the Council of Scientific and Industrial Research in India (1). Using the powerful capabilities of the National Aeronautics and Space Administration (NASA)’s James Webb Space Telescope (JWST), the team investigated the properties of exoplanet WASP-18b, which is an “ultrahot Jupiter” orbiting so close to its host star that its surface temperatures soar beyond 2,000 °C (1).

“Here, we present a spectroscopic eclipse map of an extrasolar planet, resolving the atmosphere in multiple dimensions simultaneously,” the authors wrote in their study (1). “The mapping reveals weaker longitudinal temperature gradients than were predicted by theoretical models, indicating the importance of hydrogen dissociation and/or nightside clouds in shaping global thermal emission.”

What are Ultrahot Jupiters?

Ultrahot Jupiters are exoplanet gas giants that are susceptible to extreme stellar radiation, creating dramatic temperature and chemical variations across the atmospheres (1,2). Although scientists have long suspected such variability, previous observations offered only broad, hemisphere-averaged data. By using the Near-Infrared Infrared Imager and Slitless Spectrograph (NIRISS) instrument aboard JWST, the researchers captured the most detailed eclipse map of an exoplanet ever produced, resolving the planet’s atmosphere by longitude, latitude, and altitude (1).

What did their analysis reveal?

The analysis revealed that WASP-18b’s temperature gradients were weaker than predicted by leading theoretical models. This suggests that complex atmospheric processes, such as hydrogen dissociation and the presence of nightside clouds, may play key roles in redistributing heat across the planet (1). The researchers identified two distinct thermal regions: a blazing “hotspot” near the substellar point directly facing the star, and a cooler “ring” encircling the dayside limbs (1).

What was detected in the hotspot?

Within the hotspot, the team detected a strong thermal inversion, where temperature increases with altitude, which is driven by optical absorbers such as titanium oxide (TiO), vanadium oxide (VO), and hydrogen anions (H⁻) (1). The hotspot’s water vapor abundance was slightly lower than the global average, which is consistent with predictions that extreme heat causes water molecules to break apart.

Meanwhile, the ring region exhibited colder temperatures and less certain chemical signatures, hinting at complex atmospheric mixing and chemical gradients (1). Together, these insights provide a spatially resolved glimpse into how radiation, chemistry, and dynamics interact on a planetary scale beyond our solar system.

References

1. Challener, R. C.; Mansfield, M. W.; Cubillos, P. E.; et al. Horizontal and Vertical Exoplanet Thermal Structure from a JWST Spectroscopic Eclipse Map. Nat. Astron. 2025, ASAP. DOI: 10.1038/s41550-025-02666-9
2. Stangret, M.; Palle, E.; Casasayas-Barris, N.; et al. The Obliquity and Atmosphere of the Ultra-hot Jupiter TOI-1431b (MASCARA-5b): A Misaligned Orbit and No Signs of Atomic or Molecular Absorptions. A & A 2021, 654, A73. DOI: 10.1051/0004-6361/202040100