Starshade Rendezvous: Exoplanet Orbit Constraints from Multi-Epoch Direct Imaging

The addition of an external starshade to the Nancy Grace Roman Space Telescope will enable the direct imaging of Earth-radius planets orbiting at ~1 AU. Classification of any detected planets as Earth-like requires both spectroscopy to characterize their atmospheres and multi-epoch imaging to trace their orbits. We consider here the ability of the Starshade Rendezvous Probe to constrain the orbits of directly imaged Earth-like planets. The target list for this proposed mission consists of the 16 nearby stars best suited for direct imaging, around which ~10-15 planets are expected to be discovered. Of these planets, ~1-2 will be Earth-like in mass and temperature. The field of regard for the starshade mission is constrained by solar exclusion angles, resulting in four observing windows during a two-year mission. We find that for Earth-like planets that are detected at least three times during the four viewing opportunities, their semi-major axes are measured with a median precision of 7 mas, or a median fractional precision of 3%. Habitable-zone planets can be correctly identified as such 96.7% of the time, with a false positive rate of 2.8%. If a more conservative criteria is used for habitable-zone classification (95% probability), the false positive rate drops close to zero, but with only 81% of the truly Earth-like planets correctly classified as residing in the habitable zone.

在南希·格蕾丝·罗曼太空望远镜（Nancy Grace Roman Space Telescope）上加装外部星冕仪（starshade），将能够直接成像在约1天文单位（AU）轨道上运行的地球半径行星。要将探测到的行星归类为类地行星，既需要通过光谱学（spectroscopy）表征其大气，也需要通过多历元成像（multi-epoch imaging）追踪其轨道。本文探讨了星冕仪交会探测器（Starshade Rendezvous Probe）对直接成像的类地行星轨道进行约束的能力。这项拟议任务的目标列表包含16颗最适合直接成像的邻近恒星，预计将在这些恒星周围发现约10-15颗行星。在这些行星中，约1-2颗将具有类地的质量和温度。星冕仪任务的视场受太阳排除角（solar exclusion angles）限制，因此在两年任务期内将有四个观测窗口。研究发现，对于在四次观测机会中至少被探测到三次的类地行星，其半长轴（semi-major axes）的测量中值精度为7毫角秒（mas），或中值相对精度为3%。宜居带行星的正确识别率可达96.7%，假阳性率为2.8%。如果采用更保守的宜居带分类标准（95%概率），假阳性率将降至接近零，但真正类地行星中被正确归类为位于宜居带的比例仅为81%。