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

The most important effects on an exoplanet atmosphere are driven by high-energy photons and particles from the host star, which heat and ionize the planetary atmosphere, potentially leading to its loss. The rarest, largest flares are disproportionately important. The SEEJ mission will study the Structure and Evolution of ExoJupiter atmospheres with long soft X-ray observations, with cumulative exposures of weeks, well beyond those conducted by any other mission. SEEJ will measure how often high energy flares of a given size occur, and establish, for the first time the statistics of these crucial events on exoplanet hosts. Observing transits SEEJ will measure the thermospheric scale height and thus the inflation or damage to the exoJupiter atmosphere. SEEJ will provide the data needed for theory to tie the rare flares and their effects together via improved models of exoplanet thermospheres.The X-rays emitted by an exoplanet host star is critical to the atmosphere of the planet. Specifically, X-rays can induce both life-enabling and life-threatening photochemistry in planetary atmospheres. Similarly, observed X-ray flares may be harbingers of coronal mass ejections which can aid the development of life by removing primary, hydrogen-dominated atmospheres or threaten its existence by depleting secondary atmospheres and specific critical species such as ozone. An understanding of the behavior of stellar coronae at all phases of stellar/planetary evolution is fundamental to our ability to gauge the cumulative impact of stellar X-rays on planetary atmospheres. A driving objective of SEEJ is to measure planets' exospheric and thermospheric structures as well as mass loss through the shape and extent of the upper atmosphere of exoplanets. Another driving objective of SEEJ is to measure the long term effects of host stars X-ray emission on planetary atmospheres, and possible feedback between the planet and star. SEEJ will provide detailed X-ray transit light curve profiles of suitable targets accumulating them by observing multiple exoplanet transits. SEEJ will monitor roughly 40 transits from each of as many as 7 X-ray bright exoplanet hosts in a single year.