A SmallSat to Study the Structure and Evolution of ExoJupiter Atmospheres (SEEJ)

The smallsat SEEJ (Structure & Evolution of ExoJupiter Atmospheres; pronounced “siege”) will measure both the fluxes of high-energy photons emanating from nearby, planet-hosting stars and the absorption depth of X-rays in the atmospheres of hot Jupiter, Saturn, and Neptune analogs. Specifically, the SEEJ investigation will determine the degree to which stellar high-energy photons inflate nearby exoplanet atmospheres and the physical characteristics of driven planetary winds. The investigation will determine the bulk composition of the inflated atmosphere and will assess the presence of dense evaporation tails resulting from this interaction. The experimental objective is to measure the high energy fluence through stellar monitoring and understand the impact of the high energy fluence by measuring the atmospheres using the X-ray transit technique.

The goal of SEEJ is to understand the relationship between stars and their planets or, in the words of the 2020 Astronomy Decadal, putting “Worlds and Suns in Context”. As the Decadal notes, stellar X-rays have a profound impact on the structure and evolution of their closely orbiting planets. X-rays and related high energy photons and particles drive unique photochemistry, but EUV photons are heavily absorbed by the interstellar medium and particles cannot be observed for the most interesting systems. That leaves X-rays as the best probes of the atmospheric layers where heating and evaporation take place. The power of X-ray observations was demonstrated by Chandra measurements of the transit of the hot Jupiter HD189733b (Poppenhaeger et al. 2013), which revealed an X-ray transit depth three times deeper than the optical transit depth. That implies that the X-ray absorbing exosphere extends nearly 100,000 km above the optical cloud tops. Such X-ray transit observations are crucial to understanding the structure and evolution of planetary atmospheres. That this is still a unique observation nearly a decade later, indicates how difficult these observations are with general purpose X-ray observatories. By devoting months of exposure to each of 11 carefully chosen, diverse, otherwise well-characterized exoplanet hosts, SEEJ will yield a 6-fold increase in the number of hot Jupiter systems with known transit light curves and a 3-4 fold increase in the number of exoplanet systems with superbly characterized X-ray fluence.