In the past decade it has become clear that atmospheric escape plays a significant role in the evolution of exoplanets, particularly for small planets. Lyman-Alpha transit spectroscopy is one of the most successful methods used to observe atmospheric escape. However difficulties in interpreting the resulting spectrum means that it has not been utilised to conclusively constrain atmospheric escape models. Contamination of the spectrum by absorption of Lyman-Alpha radiation in the interstellar medium and geo-coronal emission from the Earth’s exosphere makes meaningful transit detections only available in the line wings of the spectrum. 3D hydrodynamics simulations show that the outflow that is being observed in these line wings is determined by the interaction of the escaping planetary gas and the circumstellar environment. Crucially, this means that these observations cannot be sensibly compared to traditional atmospheric escape models. Instead it requires bespoke 3D hydrodynamic simulations. These have been somewhat successful in reproducing the features seen in observations, however due to computational constraints it is not possible to run these over a large range of parameters. This is especially important as key parameters such as the stellar wind and EUV output of stars is poorly constrained. Therefore so far, these have not been successful in quantitatively testing atmospheric escape models.
In this talk, I will present a semi-analytic model of the interaction between the escaping gas and the circumstellar environment motivated by the results of 3D hydrodynamics simulations which (1) approximates the outflow to the required level of accuracy to compare to observations and (2) is computationally light enough to perform a rigorous parameter study. Finally, I will argue that this framework will finally allow for the testing of atmospheric escape models using Lyman-Alpha transits.