高对比度成像：与系外行星的捉迷藏游戏
High-Contrast Imaging: Hide and Seek with Exoplanets
AI总结：
本文综述系外行星高对比度直接成像技术，介绍其方法、仪器、观测样本、成果及前景，该技术可探测宽间距低质量伴星，助力研究行星系统形成与性质。
摘要：
截至目前，已发现的约5700颗系外行星大多主要通过径向速度法和凌星法发现。这些技术对近轨道行星敏感，无法探测大的恒星-行星间距。微引力透镜（μ-lensing）是一种间接方法，可用于探测雪线及以外的行星系统，但它不属于可重复观测。与之相反，直接成像（DI）能够探测并表征宽间距（≤5-6 au）的低质量伴星。直接成像的主要挑战在于，典型的行星-恒星对比度范围从年轻木星的发射光10⁻⁶，到地球反射光的10⁻⁹不等。在过去二十年里，研究人员投入大量努力，将大口径（D≥5米）望远镜（用于降低衍射影响）与日冕仪、高阶自适应光学系统（用于校正大气湍流引起的相位误差）相结合，搭配复杂的图像后处理技术，以实现恒星与行星之间的这种对比度，从而探测并表征邻近恒星周围温度更低、距离更近的伴星。在首批开创性仪器的基础上，第二代高对比度成像仪SPHERE、GPI和SCExAO使我们能够探测数百颗恒星（例如通过SHINE和GPIES项目探测500-600颗恒星），助力人们更好地理解行星系统的人口统计学特征和形成概率。直接成像为研究气态巨行星和褐矮星的形成及物理性质提供了清晰的可行思路，未来探测极限更深的直接成像仪器（空间和地面基）将进一步拓展这一研究方向。在本文中，我们简要综述了这一新兴技术的方法、仪器、主要目标恒星样本、显著成果以及发展前景。
英文摘要：
So far, most of the about 5700 exoplanets have been discovered mainly with radial velocity and transit methods. These techniques are sensitive to planets in close orbits, not being able to probe large star--planet separations. $μ$-lensing is the indirect method that allows us to probe the planetary systems at the snow-line and beyond, but it is not a repeatable observation. On the contrary, direct imaging (DI) allows for the detection and characterization of low mass companions at wide separation (\mbox{$\leq$ 5--6 au}). The main challenge of DI is that a typical planet--star contrast ranges from $10^{-6}$, for a young Jupiter in emitted light, to $10^{-9}$ for Earth in reflected light. In the last two decades, a lot of efforts have been dedicated to combining large (D $\geq$ 5 m) telescopes (to reduce the impact of diffraction) with coronagraphs and high-order adaptive optics (to correct phase errors induced by atmospheric turbulence), with sophisticated image post-processing, to reach such a contrast between the star and the planet in order to detect and characterize cooler and closer companions to nearby stars. Building on the first pioneering instrumentation, the second generation of high-contrast imagers, SPHERE, GPI, and SCExAO, allowed us to probe hundreds of stars (e.g., 500--600 stars using SHINE and GPIES), contributing to a better understanding of the demography and the occurrence of planetary systems. The DI offers a possible clear vision for studying the formation and physical properties of gas giant planets and brown dwarfs, and the future DI (space and ground-based) instruments with deeper detection limits will enhance this vision. In this paper, we briefly review the methods, the instruments, the main sample of targeted stars, the remarkable results, and the perspective of this rising technique.
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