No hydrogen exosphere detected around the super-Earth HD97658 b

TL;DR

This study finds no hydrogen exosphere around super-Earth HD97658b, suggesting its atmosphere is stable due to low stellar irradiation, and demonstrates the reliability of the observational methodology for detecting atmospheric escape.

Contribution

It provides the first detailed analysis of HD97658b's atmosphere, showing the absence of hydrogen escape and validating the methodology for studying exoplanetary atmospheres.

Findings

No hydrogen exosphere detected around HD97658b

HD97658 is a weak, soft X-ray source with chromospheric variability

Low XUV irradiation likely prevents atmospheric escape

Abstract

The exoplanet HD97658b provides a rare opportunity to probe the atmospheric composition and evolution of moderately irradiated super-Earths. It transits a bright K star at a moderate orbital distance of 0.08 au. Its low density is compatible with a massive steam envelope that could photodissociate at high altitudes and become observable as escaping hydrogen. Our analysis of 3 transits with HST/STIS at Ly-alpha reveals no such signature, suggesting that the thermosphere is not hydrodynamically expanding and is subjected to a low escape of neutral hydrogen (<10^8 g/s at 3 sigma). Using HST Ly-alpha and Chandra & XMM-Newton observations at different epochs, we find that HD97658 is a weak and soft X-ray source with signs of chromospheric variability in the Ly-alpha line core. We determine an average reference for the intrinsic Ly-alpha line and XUV spectrum of the star, and show that HD97658 b is in mild conditions of irradiation compared to other known evaporating exoplanets with an XUV irradiation about 3 times lower than the evaporating warm Neptune GJ436 b. This could be why the thermosphere of HD97658b is not expanding: the low XUV irradiation prevents an efficient photodissociation of any putative steam envelope. Alternatively, it could be linked to a low hydrogen content or inefficient conversion of the stellar energy input. The HD97658 system provides clues for understanding the stability of low-mass planet atmospheres. Our study of HD97658 b can be seen as a control experiment of our methodology, confirming that it does not bias detections of atmospheric escape and underlining its strength and reliability. Our results show that stellar activity can be efficiently discriminated from absorption signatures by a transiting exospheric cloud. They also highlight the potential of observing the upper atmosphere of small transiting planets to probe their physical and chemical properties