Ultra-hot Neptune exoplanets represent a fascinating class of exoplanets characterised, not surprisingly, by their Neptune-like mass. This, combined with an extremely high temperature due to their close proximity to the host stars make them an intriguing subject. These worlds typically orbit their star at distances much closer than Mercury is to our Sun. As a result they are bathed in intense radiation that heats their atmospheres to temperatures in excess of 1,700°C. The extreme environment causes significant atmospheric alterations which can often lead to the evaporation of lighter elements, leaving behind the heavier.

Artist impression of surface of LTT 9779 b (Credit : CC0 Public Domain)

The James Webb Space Telescope has provided amazing observations of one such hot-neptune exoplanet known as LTT 9779 b.  The images, which were part of a research project led by Louis-Philippe Coulombe from the Université de Montréal’s Trottier Institute for Research on Exoplanets has been published in Nature Astronomy. The study reveals extreme conditions on this exoplanet that is tidally locked, completing an orbit in less than one day, the same time it takes to complete a revolution. On the star facing side temperatures reach a staggering 2,000°C while it is permanently bathed in light from the star while the other hemisphere is in constant darkness.

The study revealed a strong atmospheric asymmetry, with reflective clouds composed of silicate minerals on the western hemisphere of its daytime side yet by contrast they are absent from the eastern hemisphere. This discovery by Coulombe’s team reveals how powerful eastward winds transport heat around the planet, creating this unique cloud formations that reflect significant amounts of radiation. By analysing thermal emissions and reflected light at different orbital positions, the team detected water vapour and developed a new atmospheric model. It showed how there is a delicate balance between the intense incoming stellar radiation and redistribution of heat.

The study wouldn’t have been possible without the power of the James Webb Space Telescope and in particular, the Near Infrared Imager and Slitless Spectrograph (NIRISS.) The powerful combination was used to observe LTT 9779 b for nearly 22 hours which allowed data to be captured during a complete orbit which included two secondary eclipses and a primary transit.

The James Webb Space Telescope (Credit : NASA)

The wonderful celestial dance between the star and its planet meant the team could study how the planet’s light emissions and reflections changed throughout its rotation and orbit. Since LTT 9779 b is tidally locked, different portions of its surface were observable at different orbital positions, with the hot dayside reflecting and emitting significantly more light than the cooler nightside. By analysing spectral data collected at these different phases, the team were able to create detailed maps of the planet’s temperature variations, atmospheric composition, and cloud patterns across its surface.

The study offers a valuable new insight into the planet’s atmospheric composition and weather patterns, advancing our understanding of this strange class of exoplanets that exist in environments far more extreme than anything in our solar system.

Source : Today’s forecast: partially cloudy skies on an “ultra-hot Neptune”