Neptune Auroras (Hubble and Webb Image)

At the left, an enhanced-color image of Neptune from NASA’s Hubble Space Telescope. At the right, that image is combined with data from NASA’s James Webb Space Telescope. The cyan splotches, which represent auroral activity, and white clouds, are data from Webb’s Near-Infrared Spectrograph (NIRSpec), overlayed on top of the full image of the planet from Hubble’s Wide Field Camera 3.

Auroras occur when energetic particles, often originating from the Sun, become trapped in a planet’s magnetic field and eventually strike the upper atmosphere. The energy released during these collisions creates the signature glow.

Webb’s detection of auroras on Neptune is the first time astronomers have captured direct evidence of this phenomenon on the planet most distant from the Sun. In addition to the visible glow in the imagery, the spectrum from Webb also found an extremely prominent emission line signifying the presence of the trihydrogen cation (H₃⁺), which can be created in auroras. 

Neptune’s auroras do not occur at the northern and southern poles of the planet, where we see auroras on planets like Earth and Jupiter, because of the strange nature of Neptune’s magnetic field, which is tilted by 47 degrees from the planet’s rotational axis.

Webb’s study of Neptune also revealed that the planet’s upper atmosphere has cooled by several hundred degrees, likely the reason that Neptune’s auroras have remained undetected for so long.