Ultraviolet observations of atmospheric escape in exoplanets with the Habitable Worlds Observatory

TL;DR

Ultraviolet observations of exoplanet atmospheres identify atmospheric escape as a key driver of planetary evolution. The paper argues for a UV-capable spectrograph on the Habitable Worlds Observatory (HWO) to quantify mass loss, map exospheric components, and assess Earth-like atmospheres, outlining the necessary capabilities and two complementary observing programs. It specifies requirements such as resolving power $R>10^4$, wavelength coverage 1000–2000 Å, high dynamic range, and uninterrupted time-series, and presents a deep Earth-like exosphere search alongside a broad survey across many exoplanets. These efforts would extend UV exoplanet science beyond HST’s reach, advancing our understanding of habitability and atmospheric evolution in a diverse planetary population.

Abstract

Among the many recommendations of the Decadal Survey on Astronomy and Astrophysics 2020, we found that a priority area of research is to pave the pathways towards finding and characterizing habitable worlds. In this context, we aim to understand how planetary systems evolve through atmospheric escape, and develop techniques to identify potentially Earth-like worlds. Using the ultraviolet (UV) capabilities of the Habitable Worlds Observatory, we can use transit spectroscopy observations to determine what processes drive the evolution of exoplanets, how well can small exoplanets retain atmospheres, and search for Earth-like atmospheres. We advocate the development of a UV spectrograph that is capable of moderate- to high-resolution spectroscopy of point sources, access to key spectral features between 1000 and 3000 Angstrom, and UV detectors that are resilient to high count rates.

Figures (7)

• Figure 1: Schematic representation of an exoplanet atmosphere and pressure-temperature profile. Different wavelengths are absorbed at different pressure levels; hence they can be combined to probe the whole atmospheric structure VMadjar2011. Here, we focus on the upper atmosphere: the thermosphere and the exosphere. Credit: D. Ehrenreich.
• Figure 2: Spectral features detectable in transmission spectra of hot Jupiters as a function of wavelength and stellar host type Linssen2023. The wavelengths relevant for UV spectroscopy are those below 3000 Å. The line-forming radius (y-axis) is a proxy for the altitude where the feature is located in relation to the planetary surface and the symbol sizes represent the depth of the excess absorption in transmission. © D. C. Linssen & A. Oklopčić, used with permission.
• Figure 3: Effective area of different instruments capable of UV spectroscopy. According to its concept studies Bolcar2017bFrance2017, the LUVOIR/LUMOS instrument would have an effective area that is more than ten times better than the COS and STIS instruments on HST, assuming the more conservative telescope aperture size (LUVOIR-B).
• Figure 4: The Earth's Lyman-$\alpha$ geocorona as observed from a distance of 0.1 au (panel a). The Earth's exosphere is predominantly composed of neutral hydrogen, which escapes the lower atmosphere after being photodissociated into H and O atoms Kameda2017. Panel b represents the illumination of Earth by the Sun at the time of the observation. HWO may be capable of detecting this feature in Earth-like exoplanets, and we aim to make sure the its UV spectrograph is capable of doing so.
• Figure 5: Simulated Lyman-$\alpha$ transit light curve of an Earth-like planet transiting an M dwarf at a distance of 12 pc and with a radial velocity of $-55$ km s$^{-1}$DSantos2019a. The signal of the Earth-like exosphere is seen as an excess absorption of $\sim 500$ ppm (green data points) in the residuals after subtracting the transit depth of the planet's opaque disk. This signal can be detected at 4-sigma confidence within 20 transits, assuming the instrumental parameters from the concept studies of LUVOIR-B/LUMOS.