sunset: A database of synthetic atmospheric-escape transmission spectra for nearly every transiting planet

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, MS-16, Cambridge, MA 02138, USA

^★ Corresponding author: d.c.linssen@uva.nl

Received: 30 September 2024
Accepted: 18 April 2025

Abstract

Studying atmospheric escape from exoplanets can provide important clues about the formation and evolution of exoplanets. Observational evidence of atmospheric escape has been obtained through transit spectroscopy in strong spectral lines of various atomic species. In recent years, the number of exoplanets that have been targeted in this way has grown rapidly, mainly by observations of the metastable helium triplet. Even with this larger sample of exoplanets, many aspects of atmospheric escape are still not fully understood, such as the role of the stellar high-energy spectrum and planetary magnetic field, highlighting the need for additional observations. This work aims to identify the best targets for observations in various spectral lines. Using the atmospheric escape code sunbather, we calculated a synthetic transmission spectrum of nearly every transiting exoplanet currently known. This database of spectra, named sunset, is publicly available. We introduce metrics that predict spectral line observability, which allow for swift identification of the most favorable targets. By analyzing the complete set of spectra from a demographic perspective, we find that the atmospheric mass-loss rate does not explain all the spread in the strength of various spectral lines, suggesting that a nondetection does not immediately rule out an escaping atmosphere. Some lines also do not correlate very strongly with one another, emphasizing the benefits of observing multiple spectral lines. Our model spectra show only a modest correlation between the X-ray and extreme UV flux and the helium line strength, affirming that the absence of such a trend found by observational works is not necessarily surprising. A direct comparison between our synthetic spectra and the sample of observed metastable helium spectra shows that they are generally consistent within the large model uncertainties. This suggests, in general, that photoevaporation is able to explain the current metastable helium census.