Bright Skies on a Distant Neptune: Discovering Reflective Clouds on LTT 9779b

Exoplanets—planets orbiting stars beyond our Sun—come in a variety of sizes and temperatures. One category, the “hot Neptune,” includes planets similar in size to Neptune but orbiting much closer to their stars. LTT 9779b is one such planet, an “ultra-hot Neptune” with scorching temperatures and a puzzling atmosphere. In a recent study led by Louis-Philippe Coulombe, researchers observed this planet using the James Webb Space Telescope (JWST) to understand its unusual brightness and cloud patterns. Their findings reveal highly reflective clouds on one side of the planet, challenging our understanding of atmospheric circulation on hot Neptunes.

Observations and Methods

The researchers used JWST’s NIRISS/SOSS instrument to track LTT 9779b over a full orbit, a method called a phase curve study. This allowed them to measure how the planet’s brightness changed as it moved around its star. By observing the light reflected from the planet and its thermal emission (heat), they could determine its temperature and cloud cover at different points in its orbit.

The team analyzed the planet’s light in different wavelengths, separating the contributions from reflected starlight and its own heat emission. They divided the planet into six longitudinal slices to model variations in albedo (reflectivity) and temperature across its surface.

Key Findings: A Bright and Asymmetric Atmosphere

One of the most striking discoveries was that the planet’s western dayside is much brighter than the eastern dayside. The western side reflects about 79% of incoming starlight, while the eastern side reflects only 41%. This creates an average dayside albedo (brightness) of 50%, meaning LTT 9779b is significantly more reflective than most other hot planets.

Despite this brightness asymmetry in reflected light, the thermal emission from the planet’s dayside is symmetrical, with an average temperature of 2,260 K (about 3,600°F). The nightside, in contrast, is much cooler, staying below 1,330 K.

What’s Causing the Brightness Difference?

The researchers propose that LTT 9779b’s atmosphere is shaped by a fast-moving eastward equatorial jet, which transports heat from the star-facing side of the planet toward the nightside. This circulation pattern results in a colder western dayside, where temperatures drop enough for clouds to form. These clouds, likely made of silicates (rocky materials that condense at high temperatures), reflect a large amount of starlight, making the western dayside much brighter.

Meanwhile, the eastern dayside is hotter, preventing cloud formation and resulting in a darker, more absorbing surface. This westward cloud formation is consistent with previous observations of cooler exoplanets but is surprising for a planet as hot as LTT 9779b.

Implications for Exoplanet Research

This discovery provides a new perspective on atmospheric circulation in highly irradiated exoplanets. Most studies of hot Jupiters show different cloud distributions, but LTT 9779b’s unique reflective properties suggest that hot Neptunes might follow different atmospheric rules. The findings also support the idea that higher-metallicity atmospheres (those with a greater abundance of heavy elements) are more prone to cloud formation, influencing both the planet’s climate and appearance.

Conclusion

LTT 9779b is an exoplanet with an atmosphere dominated by bright clouds on one side and a darker, cloud-free region on the other. This asymmetry challenges traditional models of atmospheric circulation and highlights the importance of reflected light studies in understanding distant worlds. Future observations with JWST and other telescopes will help refine our understanding of cloud formation and heat transport in extreme planetary environments.

Source: Coulombe