This massive exoplanet has an atmosphere that can appear luminous under certain conditions.

Imagine Jupiter, the gas giant, and then expand it to more than five times its size. Now, place it close to a sun-like star so that its atmospheric surface is heated to around 4,600 degrees Fahrenheit.

This is WASP-121b, a type of exoplanet known as a “hot Jupiter.” It’s located about 900 light-years from Earth and, like Earth, it has a stratosphere—a layer of the atmosphere previously difficult to observe in exoplanets.

A recent study published in *Nature* used the Hubble Space Telescope to observe WASP-121b’s stratosphere. This atmospheric layer is located above the troposphere (the layer where we live) and below the outermost layers of an atmosphere. On Earth, the stratosphere is characterized by increasing temperatures with altitude, due to ozone absorbing UV radiation. This is in contrast to the cooler upper layers of the atmosphere.

Researchers, led by Thomas Evans from the University of Exeter, used Hubble to detect infrared light from water molecules in WASP-121b’s atmosphere. Infrared light, which is invisible to the human eye, reveals the presence of high-energy water vapor, indicating a significant amount of heat being released.

Hubble’s observations showed that WASP-121b’s stratosphere is glowing due to high temperatures, with the temperature rising by about 1,000 degrees Fahrenheit from the lower to the upper part of the stratosphere. This is surprising because, despite the cold temperatures of space, WASP-121b’s stratosphere gets hotter with altitude, similar to Earth’s stratosphere.

The researchers suspect that compounds such as titanium oxide and vanadium oxide in WASP-121b’s atmosphere might be contributing to this heating effect, but further research is needed to confirm this.

Understanding the atmospheres of exoplanets like WASP-121b may not directly impact our understanding of the solar system, but it opens doors to studying a wide range of planetary atmospheres. Evans notes that as technology advances, such as with the upcoming James Webb Space Telescope, scientists will be able to explore smaller exoplanets and their potential for supporting life. Exploring a broad range of exoplanets is essential for a comprehensive understanding of planetary atmospheres.