Exoplanets transiting bright nearby stars are key objects for advancing our knowledge of planetary formation and evolution. The wealth of photons from the host star gives detailed access to the atmospheric, interior and orbital properties of the planetary companions. ν² Lupi (HD 136352) is a naked-eye (V = 5.78) Sun-like star that was discovered to host three low-mass planets with orbital periods of 11.6, 27.6 and 107.6 d via radial-velocity monitoring¹. The two inner planets (b and c) were recently found to transit², prompting a photometric follow-up by the brand new Characterising Exoplanets Satellite (CHEOPS). Here, we report that the outer planet d is also transiting, and measure its radius and mass to be 2.56 ± 0.09 R_⊕ and 8.82 ± 0.94 M_⊕, respectively. With its bright Sun-like star, long period and mild irradiation (~5.7 times the irradiation of Earth), ν² Lupi d unlocks a completely new region in the parameter space of exoplanets amenable to detailed characterization. We refine the properties of all three planets: planet b probably has a rocky mostly dry composition, while planets c and d seem to have retained small hydrogen–helium envelopes and a possibly large water fraction. This diversity of planetary compositions makes the ν² Lupi system an excellent laboratory for testing formation and evolution models of low-mass planets.

穿过附近明亮恒星的系外行星是推进我们对行星形成和演化知识的关键对象。来自主星的大量光子可以详细了解行星伴星的大气、内部和轨道特性。ν ² Lupi (HD 136352) 是一颗肉眼 ( V  = 5.78) 的类太阳恒星，通过径向速度监测发现它拥有三颗轨道周期分别为 11.6、27.6 和 107.6 d 的低质量行星¹。两颗内行星（b 和 c）最近被发现凌日²，促使全新的外行星表征卫星 (CHEOPS) 进行光度测量。在这里，我们报告外行星 d 也在凌日，并测量其半径和质量为 2.56 ± 0.09 R _⊕和 8.82 ± 0.94  M _⊕，分别。凭借其明亮的类太阳恒星、长周期和温和的辐射（大约是地球辐射的 5.7 倍），ν ² Lupi d 在系外行星的参数空间中开启了一个全新的区域，可以进行详细的表征。我们改进了所有三颗行星的特性：行星 b 可能具有大部分干燥的岩石成分，而行星 c 和 d 似乎保留了少量的氢氦包层和可能大量的水部分。这种行星成分的多样性使^ν2 Lupi 系统成为测试低质量行星形成和演化模型的绝佳实验室。