Now that astronomers are routinely probing the composition and evolution of exoplanet atmospheres, the fields of astrophysics, heliophysics, and exoplanetary science must devote greater attention to the particle environments of planets, i.e., space weather. Particles, although far more difficult to observe than photons, could play a significant role in stellar and planetary evolution. Particles erode and chemically modify planetary atmospheres, affecting climate, water inventories, surface radiation, biogenesis, and bioindicators, key driving science of the next flagship observatory. Meanwhile, the stellar particle wind and mass ejections alter a fundamental stellar property - rotation - in turn affecting stellar magnetic activity and associated radiation that also influences planets.
In the Solar System, space weather is monitored by a host of in situ and remote sensing means. This suite of sensors tracks variations in the solar wind and galactic cosmic rays (GCRs) and impulsive events like coronal mass ejections (CMEs) and solar energetic particle (SEP) events. CMEs can be particularly damaging to planetary atmospheres in their path. In contrast with the present-day Sun, for stars and young solar analogs observations of winds, CMEs, SEPs, and GCRs are scarce at best. This absence is striking because many nearby stars are more magnetically active than the Sun, suggesting they produce more intense space weather.
This workshop will chart a course for the new field of planetary particle environment observation and observation-guided modeling. With the confluence of new Solar System observations, renewed modeling efforts, and a significant community push to understand exoplanet atmospheres, it is time to tackle detecting and characterizing particles in the space environments of planets.
The workshop team will review existing approaches for observing particle phenomena in stellar systems and identify study areas essential to evaluate and mature new promising techniques, defining requirements for any new observatory capabilities needed to implement those that hold the greatest promise. The outcome will be a written report to serve as a central resource around which the community can organize efforts to advance the study of planetary particle environments from a sometimes speculative to a largely observational science.