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Arxiv: Prioritizing High-Precision Photometric Monitoring of Exoplanet and Brown Dwarf Companions with JWST -- Strategic Exoplanet Initiatives with HST and JWST White Paper Published: 5/2/2024 2:58:26 AM Updated: 5/2/2024 2:58:26 AM

Paper abstract: We advocate for the prioritization of high-precision photometric monitoringof exoplanet and brown dwarf companions to detect brightness variabilityarising from features in their atmospheres. Measurements of photometricvariability provide not only an insight into the physical appearances of thesecompanions, but are also a direct probe of their atmospheric structures anddynamics, and yield valuable estimates of their rotation periods. JWST isuniquely capable of monitoring faint exoplanet companions over their fullrotation periods, thanks to its inherent stability and powerful high-contrastcoronagraphic imaging modes. Rotation period measurements can be furthercombined with measurements of v sin i obtained using high-resolutionspectroscopy to infer the viewing angle of a companion. Photometric monitoringover multiple rotation periods and at multiple epochs will allow both short-and long-term time evolution in variability signals to be traced. Furthermore,the differences between the layers in a companion's atmosphere can be probed byobtaining simultaneous photometric monitoring at different wavelengths throughNIRCam dual-band coronagraphy. Overall, JWST will reach the highestsensitivities to variability to date and enable the light curves of substellarcompanions to be characterised with unprecedented cadence and precision at thesub-percent level.