Direct imaging of widely separated exoplanets from space will obtain their
reflected light spectra and measure atmospheric properties. Previous
calculations have shown that a change in the orbital phase would cause a
spectral signal, but whether this signal may be used to characterize the
atmosphere has not been shown. We simulate starshade-enabled observations of
the planet 47 Uma b, using the to-date most realistic simulator SISTER to
estimate the uncertainties due to residual starlight, solar glint, and
exozodiacal light. We then use the Bayesian retrieval algorithm ExoReLℜ to
determine the constraints on the atmospheric properties from observations using
a Roman- or HabEx-like telescope, comparing the strategies to observe at
multiple orbital phases or in multiple wavelength bands. With a ∼20%
bandwidth in 600 - 800 nm on a Roman-like telescope, the retrieval finds a
degenerate scenario with a lower gas abundance and a deeper or absent cloud
than the truth. Repeating the observation at a different orbital phase or at a
second 20% wavelength band in 800 - 1000 nm, with the same integration time
and thus degraded S/N, would effectively eliminate this degenerate solution.
Single observation with a HabEx-like telescope would yield high-precision
constraints on the gas abundances and cloud properties, without the degenerate
scenario. These results are also generally applicable to high-contrast
spectroscopy with a coronagraph with a similar wavelength coverage and S/N, and
can help design the wavelength bandwidth and the observation plan of exoplanet
direct imaging experiments in the future.Comment: 11 pages, 4 figures, 2 tables, accepted for publication in A