Emissions from the upper-atmospheric molecular ion \({{\rm{H}}}_{3}^{+}\) have been used to study the global-scale interactions of Jupiter, Saturn and Uranus with their surrounding space environments for over 30 years, revealing the processes shaping the aurorae. However, despite repeated attempts, and contrary to models that predict it should be present, this ion has proven elusive at Neptune. Here, using observations from the James Webb Space Telescope, we detect \({{\rm{H}}}_{3}^{+}\) at Neptune, as well as distinct infrared southern auroral emissions. The average upper-atmosphere temperature is a factor of two cooler than those derived 34 years ago by Voyager 2, showing that the energy balance of this region is regulated by physical processes acting on a timescale shorter than both Neptunian seasons (40 yr) and the solar cycle.
