Science potential of characterizing gas giant exoplanets with HWO

With the ability to directly image Earth-like exoplanets and search their atmospheres for biosignatures, the upcoming Habitable Worlds Observatory (HWO) will also collect high signal-to-noise ratio (S/N) reflected-light photometry and spectra of nearby gas giant exoplanets. Such high-quality data would allow investigations into the formation and evolution of low-mass, low-temperature gas giants and could enable the detection of exomoons around these planets. We use the EXOSIMS direct imaging mission simulator to model HWO observations of Jupiter-radius gas giants at Earth-like and Jupiter-like instellations around the 164 stars in the ExEP target list (1.3 pc≤d≤23 pc). We find that HWO should be able to achieve S/N≥5 broadband visible-light detections of gas giants in this instellation range within 5 min of integration. At Jupiter-like instellations, 10 h of R=1000 near-IR spectroscopy with HWO should reveal water, methane, and ammonia absorption features in the atmospheres of gas giants orbiting any ExEP star, offering unique insight into the formation conditions and histories of gas giants at a wide range of semi-major axes and masses. HWO optical time-series photometry should exceed 1% flux precision for gas giants at Jupiter’s instellation level at d≤7 parsecs and for Earth-instellation gas giants around any ExEP target star. Time-series light curves at this cadence and precision could, over tens of hours, reveal rotation-induced variability comparable to Jupiter’s. Eclipses of Mars-sized exomoons may be detectable in high-cadence light curves (∼5 min exposures) of Jupiter-sized planets in the habitable zones of ExEP stars at d≤10 parsecs. For any hypothetical Earth-like exomoons with oxygen-rich atmospheres at d≤7 parsecs from the Solar System, HWO might be able to detect the spectral signature of molecular oxygen amid the parent planet’s photon noise in deep (∼400 h integration) spectroscopic HWO observations at R=1000. Such moons, if they exist, represent additional habitable worlds that HWO could investigate for biosignatures.