Analyzing Exoplanet Phase Curve Information Content: Toward Optimized Observing Strategies

Secondary eclipses and phase curves reveal information about the reflectivity and heat distribution in exoplanet
atmospheres. The phase curve is composed of a combination of reflected and thermally emitted light from the
planet, and for circular orbits the phase curve peaks during the secondary eclipse or at an orbital phase of 0.5.
Physical mechanisms have been discovered that shift the phase curve maximum of tidally locked close-in planets
to the right, or left, of the secondary eclipse. These mechanisms include cloud formations and atmospheric
superrotation, both of which serve to shift the thermally bright hot-spot or highly reflective bright spot of the
atmosphere away from the sub-stellar point. Here, we present a methodology for optimizing observing strategies
for both secondary eclipses and phase curves with the goal of maximizing the information gained about the
planetary atmosphere while minimizing the (assumed) continuous observation time. We show that we can increase
the duty cycle of observations aimed at the measurements of phase curve characteristics (amplitude, phase offset)
by up to 50% for future platforms such as CHaracterising ExOPlanets Satellite (CHEOPS) and JWST. We apply
this methodology to the test cases of the Spitzer phase curve of 55-Cancri-e, which displays an eastward shift in its
phase curve maximum as well as model-generated observations of an ultra-short period planet observed with
CHEOPS.