  Lyman-$\alpha$ transits have been detected from several nearby exoplanets and
are one of our best insights into the atmospheric escape process. However, due
to ISM absorption, we typically only observe the transit signature in the
blue-wing, making them challenging to interpret. This challenge has been
recently highlighted by non-detections from planets thought to be undergoing
vigorous escape. Pioneering 3D simulations have shown that escaping hydrogen is
shaped into a cometary tail receding from the planet. Motivated by this work,
we develop a simple model to interpret Lyman-$\alpha$ transits. Using this
framework, we show that the Lyman-$\alpha$ transit depth is primarily
controlled by the properties of the stellar tidal field rather than details of
the escape process. Instead, the transit duration provides a direct measurement
of the velocity of the planetary outflow. This result arises because the
underlying physics is the distance a neutral hydrogen atom can travel before it
is photoionized in the outflow. Thus, higher irradiation levels, expected to
drive more powerful outflows, produce weaker, shorter Lyman-$\alpha$ transits
because the outflowing gas is ionized more quickly. Our framework suggests that
the generation of energetic neutral atoms may dominate the transit signature
early, but the acceleration of planetary material produces long tails. Thus,
Lyman-$\alpha$ transits do not primarily probe the mass-loss rates. Instead,
they inform us about the velocity at which the escape mechanism is ejecting
material from the planet, providing a clean test of predictions from
atmospheric escape models.

Earth and Planetary Astrophysics