The fundamentals of Lyman-alpha exoplanet transits

TL;DR

This paper presents a simple model for interpreting Lyman-alpha transits of exoplanets, revealing that transit duration measures outflow velocity and transit depth is influenced by stellar tidal forces, offering new insights into atmospheric escape.

Contribution

The study introduces a novel framework linking Lyman-alpha transit features to planetary outflow velocity and stellar tidal effects, refining interpretation of atmospheric escape observations.

Findings

Transit depth is mainly controlled by stellar tidal field properties.

Transit duration directly measures planetary outflow velocity.

Higher irradiation shortens and weakens transits due to faster ionization.

Abstract

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.