  Atmospheric escape is key to explaining the long-term evolution of planets in
our Solar System and beyond, and in the interpretation of atmospheric
measurements. Hydrodynamic escape is generally thought to be driven by the flux
of extreme ultraviolet photons that the planet receives from its host star.
Here, we show that the escape from planets orbiting hot stars proceeds through
a different yet complementary process: drawing its energy from the intense near
ultraviolet emission of the star that is deposited within an optically thin,
high-altitude atmospheric layer of hydrogen excited into the lower state of the
Balmer series. The ultra-hot exoplanet KELT-9b likely represents the first
known instance of this Balmer-driven escape. In this regime of hydrodynamic
escape, the near ultraviolet emission from the star is more important at
determining the planet mass loss than the extreme ultraviolet emission, and
uncertainties in the latter become less critical. Further, we predict that gas
exoplanets around hot stars may experience catastrophic mass loss when they are
less massive than 1--2 Jupiter masses and closer-in than KELT-9b, thereby
challenging the paradigm that all large exoplanets are stable to atmospheric
escape. We argue that extreme escape will affect the demographics of close-in
exoplanets orbiting hot stars.
