No Hydrogen Detected in SuperEarth HD97658b's Exosphere

No hydrogen exosphere detected around the super-Earth HD97658 b

Authors:

Bourrier et al

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 (less than 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

HD 97658b: a 7.55 Earth Mass Superearth Confirmed

TRANSIT CONFIRMATION AND IMPROVED STELLAR AND PLANET PARAMETERS FOR THE SUPER-EARTH HD 97658 b AND ITS HOST STAR

Authors:

Van Grootel et al

Abstract:

Super-Earths transiting nearby bright stars are key objects that simultaneously allow for accurate measurements of both their mass and radius, providing essential constraints on their internal composition. We present here the confirmation, based on Spitzer transit observations, that the super-Earth HD 97658 b transits its host star. HD 97658 is a low-mass (M * = 0.77 ± 0.05 M ☉) K1 dwarf, as determined from the Hipparcos parallax and stellar evolution modeling. To constrain the planet parameters, we carry out Bayesian global analyses of Keck-High Resolution Echelle Spectrometer (Keck-HIRES) radial velocities and Microvariability and Oscillations of STars (MOST) and Spitzer photometry. HD 97658 b is a massive ($M_P=7.55^{+0.83}_{-0.79}\, M_{\oplus }$) and large ($R_{P} = 2.247^{+0.098}_{-0.095} R_{\oplus }$ at 4.5 μm) super-Earth. We investigate the possible internal compositions for HD 97658 b. Our results indicate a large rocky component, of at least 60% by mass, and very little H-He components, at most 2% by mass. We also discuss how future asteroseismic observations can improve the knowledge of the HD 97658 system, in particular by constraining its age. Orbiting a bright host star, HD 97658 b will be a key target for upcoming space missions such as the Transiting Exoplanet Survey Satellite (TESS), the Characterizing Exoplanet Satellite (CHEOPS), the Planetary Transits and Oscillations of stars (PLATO), and the James Webb Space Telescope to characterize thoroughly its structure and atmosphere.

The Chemistry of Exoatmospheres for SuperEarths and Mini Neptunes

PHOTOCHEMISTRY IN TERRESTRIAL EXOPLANET ATMOSPHERES. III. PHOTOCHEMISTRY AND THERMOCHEMISTRY IN THICK ATMOSPHERES ON SUPER EARTHS AND MINI NEPTUNES

Authors:

Hu et al


Abstract:

Some super Earths and mini Neptunes will likely have thick atmospheres that are not H2-dominated. We have developed a photochemistry-thermochemistry kinetic-transport model for exploring the compositions of thick atmospheres on super Earths and mini Neptunes, applicable for both H2-dominated atmospheres and non-H2-dominated atmospheres. Using this model to study thick atmospheres for wide ranges of temperatures and elemental abundances, we classify them into hydrogen-rich atmospheres, water-rich atmospheres, oxygen-rich atmospheres, and hydrocarbon-rich atmospheres. We find that carbon has to be in the form of CO2 rather than CH4 or CO in a H2-depleted water-dominated thick atmosphere and that the preferred loss of light elements from an oxygen-poor carbon-rich atmosphere leads to the formation of unsaturated hydrocarbons (C2H2 and C2H4). We apply our self-consistent atmosphere models to compute spectra and diagnostic features for known transiting low-mass exoplanets GJ 1214 b, HD 97658 b, and 55 Cnc e. For GJ 1214 b, we find that (1) C2H2 features at 1.0 and 1.5 μm in transmission and C2H2 and C2H4 features at 9-14 μm in thermal emission are diagnostic for hydrocarbon-rich atmospheres; (2) a detection of water-vapor features and a confirmation of the nonexistence of methane features would provide sufficient evidence for a water-dominated atmosphere. In general, our simulations show that chemical stability has to be taken into account when interpreting the spectrum of a super Earth/mini Neptune. Water-dominated atmospheres only exist for carbon to oxygen ratios much lower than the solar ratio, suggesting that this kind of atmospheres could be rare.