Modeling tails of escaping gas in exoplanet atmospheres with Harmonica

TL;DR

This paper introduces a modeling method using Harmonica to analyze the tails of escaping gas in exoplanet atmospheres, demonstrated on JWST data of HAT-P-18b, revealing a significant helium tail and validating the approach.

Contribution

The paper presents a novel application of Harmonica software to model and infer the shape and size of escaping gas tails in exoplanet atmospheres from transit light curves.

Findings

HAT-P-18b has a helium tail of approximately 15.8 planetary radii.

The method successfully fits observed helium tail features in JWST data.

Injection-recovery tests confirm the methodology's effectiveness.

Abstract

Exoplanets that reside close to their host stars, and therefore receive substantial amounts of X-ray and ultraviolet radiation, are prone to suffer from strong atmospheric escape. This can lead to the creation of an envelope of escaping gas along the planet's orbital trajectory, often referred to as a tail. When transiting in front of their host star, these tails can not only produce larger depths in the transit light curves, but also introduce significant asymmetries between ingress and egress. Using the publicly available software Harmonica, we present a method to model the light curves of transiting planets surrounded by extended envelopes of escaping gas, and subsequently infer the shape and size of the latter. We apply this method to the JWST NIRISS/SOSS observations of HAT-P-18b, which show pronounced helium tail features in its spectroscopic light curve of the metastable helium triplet at 10830 \r{A}. Our model reveals that, in order to fit the observed light curve of HAT-P-18b, the planet must possess a trailing helium tail of $15.79^{+1.14}_{-1.05}$ planetary radii. We carry out injection-recovery tests to validate the effectiveness of the proposed methodology. We demonstrate that, with sufficient precision, we would be able to fit a multi-layer envelope to the data, which would provide insight into the relative radial variations in the opacity profile.