Multi-orbital-phase and multi-band characterization of exoplanetary atmospheres with reflected light spectra

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 \umab, 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 WFIRST- or HabEx-like telescope, comparing the strategies to observe at multiple orbital phases or in multiple wavelength bands. With a $\sim20\%$ bandwidth in 600 -- 800 nm on a WFIRST-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.

天基宽分离系外行星（exoplanet）直接成像可获取其反射光光谱并测量大气特性。此前的计算表明，轨道相位变化会引发光谱信号，但尚未证实该信号可用于表征行星大气。我们借助迄今最逼真的模拟器SISTER，对搭载星罩（starshade）的行星umab开展观测模拟，以估算残余星光、太阳辉光及系外黄道光带来的不确定性。随后，我们采用贝叶斯反演算法exorelr，基于类WFIRST或类HabEx望远镜的观测数据确定大气特性的约束条件，并对比了多轨道相位观测与多波段观测的策略。在类WFIRST望远镜上采用600~800 nm波段、约20%带宽的观测设置时，反演得到一类简并情形：其气体丰度低于真实值，云系更深或完全无云。若在不同轨道相位重复观测，或在800~1000 nm波段增设第二个20%带宽的波段（保持相同积分时间，因此信噪比（S/N）有所降低），则可有效消除该简并解。而类HabEx望远镜的单次观测即可对气体丰度与云系特性实现高精度约束，且不会出现简并情形。上述结果同样普遍适用于具备类似波长覆盖范围与信噪比的日冕仪（coronagraph）高对比度光谱观测，可为未来系外行星直接成像实验的光谱带宽设计与观测计划制定提供参考。