Raven Cilley, Lía Corrales, and George King

University of Michigan

Atomic and molecular species in exoplanet atmospheres exhibit spectral features across the electromagnetic spectrum, meaning that atmospheres have wavelength-dependent opacities. Thus, the apparent radius of a planet, measured by transit depth, varies with wavelength. Short-wavelength transit observations provide a way to study atmospheric evolution, especially in the context of irradiation-driven atmospheric escape. For example, transits in Lyman-α can appear many times deeper than in the optical, due to neutral hydrogen absorption in extended escaping outflows (Ehrenreich et al. 2015). Atmospheric escape is one proposed explanation for interesting trends within the demographics of known planets, such as the dearth of short-period Neptune-sized planets and planets between 1.5 and 2 Earth radii. Therefore, it is important to characterize the effects of stellar irradiation on planetary atmospheres to improve our understanding of how the planets we observe evolved.
​
Most transits are observed in infrared and optical wavelengths. However, X-ray wavelengths present an especially interesting probe of exoplanet atmospheres because X-rays are absorbed efficiently by inner-shell electrons of weakly ionized materials that exist high-up in atmospheres. This allows for an estimation of the physical size of a planet’s exosphere or escaping outflow, which can be used in conjunction with the X-ray flux of the star to calculate mass-loss rates. The disadvantage of X-ray transit observations is that stars are much dimmer in X-rays than in the infrared or optical, therefore transit signals are more difficult to detect. Current X-ray observatories are not sensitive enough to reliably resolve transit signals. As of 2025, only one exoplanet has an observed X-ray transit (HD 189733 b, Poppenhaeger et al, 2013), which provided evidence for an enlarged X-ray radius, likely due to an escaping atmosphere.
​
The limitations of current observatories highlight the need for the next generation of X-ray observatories NewAthena, which will have a much larger collecting area than any previous X-ray observatory, allowing for higher stellar count rates and thus higher signal-to-noise during transit observations. Assuming X-ray radii are equal to optical, NewAthena would be capable of detecting a select few planets with bright host stars and deep transits. However, the sole observed X-ray transit so far suggests that planets appear significantly larger in the X-ray than in the optical. If this is the case generally, NewAthena may be able to observe the transits of over a dozen exoplanets, some as small as Neptune. This would provide unprecedented insight into the effects of irradiation on planetary atmospheres and how it influences the formation and survival of gas giant planets.