Neptune fascinates researchers with a breathtaking display of light in its atmosphere.

      Neptune is located at an incredible distance of 3 billion miles from the sun, which leaves many aspects of this planet still unknown. From shifting temperatures to the identification of small moons, and even ongoing discussions regarding the planet's actual color, Neptune always manages to intrigue us. Recently, however, scientists have discovered yet another unique characteristic: a bright auroral glow.

      Neptune often appears slightly different in various observations due to the presence of dark spots that emerge intermittently. At times, it can also exhibit brightness, as demonstrated by the observed auroras: the image above depicts these auroras in cyan.

      Identified by the James Webb Space Telescope, auroras are produced when energetic solar particles interact with a planet’s atmosphere, resulting in a luminous display. This phenomenon is similar to what we observe on Earth (often referred to as the Northern Lights) and has also been detected on planets like Mars; however, this marks the first instance of auroras being observed on Neptune.

      “Actually imaging the auroral activity on Neptune was only achievable thanks to Webb’s near-infrared sensitivity,” stated lead researcher Henrik Melin from Northumbria University. “It was astonishing to not only observe the auroras but to also see the remarkable detail and clarity of the signature, which truly took me by surprise.”

      The data was gathered using a Webb instrument known as the Near-Infrared Spectrograph, which can also analyze the components present in the planet’s atmosphere. It detected signs of a molecule called trihydrogen cation (H3+), believed to be produced by auroras.

      “H3+ has consistently been a strong indicator of auroral activity on all gas giants — Jupiter, Saturn, and Uranus — and we anticipated finding the same on Neptune during our extensive investigations utilizing the best available ground-based facilities,” explained Webb scientist Heidi Hammel. “It was only with a facility like Webb that we finally received that confirmation.”

      Nonetheless, despite the similarities with auroral activities observed on other planets within the solar system, Neptune’s auroras possess unique characteristics. Due to the planet's nearly horizontal orbit, its auroras do not align with its north and south poles as seen on other planets. During their study of this phenomenon, the researchers also discovered that Neptune’s upper atmosphere was significantly different from their expectations.

      “I was shocked — the upper atmosphere of Neptune has cooled by several hundreds of degrees,” Melin remarked. “In fact, the temperature in 2023 was just over half of what it was in 1989.”

      Through Webb’s capabilities, scientists can now delve into the intriguing mysteries of Neptune, such as the rapid cooling of its atmosphere.

      “As we envision future missions to Uranus and Neptune, it becomes clear how crucial it will be to employ instruments designed to detect infrared light to continue exploring the auroras,” said fellow researcher Leigh Fletcher from Leicester University. “This observatory has finally unveiled this previously hidden ionosphere of the giant planets.”

      The findings have been published in Nature Astronomy.