# The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b

**Authors:** Leonardo A. dos Santos, David Ehrenreich, Vincent Bourrier, Nicola Astudillo-Defru, Xavier Bonfils, François Forget, Christophe Lovis, Francesco Pepe, Stéphane Udry  
**arXiv:** [2001.04532](https://arxiv.org/abs/2001.04532)  
**Date:** 2020-01-13  
**Summary page:** https://www.summarizepaper.com/en/paper/2001.04532v1/

## Key points

- K2-18 b is a mini-Neptune exoplanet that orbits a nearby cool M3 dwarf star
- Researchers used Lyman-alpha transit spectroscopy with the STIS instrument on the HST to search for hydrogen escape from K2-18 b's atmosphere
- During one partial transit, the average blueshifted emission decreased by 67% ± 18%, tentatively indicating vigorous hydrogen atom escape being blown away by radiation pressure
- Reconstructed Lyman-alpha emission data estimated an EUV irradiation between $10^1$-$10^2$ erg s$^{-1}$ cm$^{-2}$ and a total escape rate around $10^8$ g s$^{-1}$
- K2-18 b will lose only a small fraction (<1%) of its mass and retain its volatile-rich atmosphere during its lifetime
- Further observations are necessary to confirm in-transit absorption and gain a more comprehensive understanding of K2-18 b's high energy environment

## Summary

K2-18 b is a mini-Neptune exoplanet that orbits a nearby cool M3 dwarf star located within its region of temperate irradiation. Researchers used Lyman-alpha transit spectroscopy with the Space Telescope Imaging Spectrograph (STIS) instrument on the Hubble Space Telescope (HST) to search for hydrogen escape from K2-18 b's atmosphere. Analysis of the time-series of fluxes revealed that during one partial transit, the average blueshifted emission decreased by 67% ± 18%, tentatively indicating vigorous hydrogen atom escape being blown away by radiation pressure. Reconstructed Lyman-alpha emission data estimated an EUV irradiation between $10^1$-$10^2$ erg s$^{-1}$ cm$^{-2}$ and a total escape rate around $10^8$ g s$^{-1}$. This suggests that K2-18 b will lose only a small fraction (<1%) of its mass and retain its volatile-rich atmosphere during its lifetime. Further observations are necessary to confirm in-transit absorption and gain a more comprehensive understanding of K2-18 b's high energy environment.