Detection of an Earth-sized exoplanet orbiting the nearby ultracool dwarf star SPECULOOS-3

Located at the bottom of the main-sequence, ultracool dwarf stars are widespread in the solar neighborhood. Nevertheless, their extremely low luminosity has left their planetary population largely unexplored, and only one of them, TRAPPIST- 1, has so far been found to host a transiting planetary system. In this context, we present the SPECULOOS project’s detection of an Earth-sized planet in a 17- hour orbit around an ultracool dwarf of M6.5 spectral type located 16.8 parsecs away. The planet’s high irradiation (16 times that of Earth) combined with the infrared luminosity and Jupiter-like size of its host star make it one of the most promising rocky exoplanets for detailed emission spectroscopy characterization with JWST. Indeed, our sensitivity study shows that just 10 secondary eclipse observations with the MIRI/LRS instrument on board JWST should bring strong constraints on its atmospheric composition and/or surface mineralogy.

处于主序星底端的超冷矮星（ultracool dwarf star）在太阳邻域中广泛分布。尽管此类天体光度极低，其行星系统群体仍几乎未被探索，迄今为止仅发现TRAPPIST-1一例拥有凌星行星系统的超冷矮星。在此研究背景下，本工作报道了SPECULOOS项目的一项探测成果：在距离地球16.8秒差距、光谱型为M6.5的超冷矮星周围，发现一颗轨道周期为17小时的地球大小行星。该行星受到的恒星辐照强度为地球的16倍，结合其宿主恒星的红外光度与类木（Jupiter-like）尺寸，使其成为可通过詹姆斯·韦布空间望远镜（James Webb Space Telescope，JWST）开展高精度发射光谱表征的最具潜力的岩质系外行星之一。灵敏度分析结果显示，仅需利用韦布空间望远镜搭载的中红外仪器/低分辨率光谱仪（Mid-Infrared Instrument/Low-Resolution Spectrometer，MIRI/LRS）开展10次二次凌食观测，即可对该行星的大气成分及/或表面矿物学特征施加强有力的约束。