The detection of low-mass exoplanets (≤10 Earth masses (M_⊕)) yields fundamental inputs for current theories of planet formation and evolution, and supplies critical information for the planned direct-imaging missions that aim to detect and characterize Earth-like planets in the habitable zones around solar-like stars. However, the most efficient detection techniques available for low-mass exoplanets (that is, photometric transit and radial velocity methods) are heavily biased towards the detection of short-period planets (for example, ≤100 days) and strongly favour late-type stars. Here we report the discovery of Kepler-725 c, a 10 ± 3 M_⊕ exoplanet within the habitable zone of the late G-type dwarf Kepler-725. Through analysis of the transit timing variations of the relatively short-period (39.64 days) warm Jupiter Kepler-725 b, we find that Kepler-725 c has a period of 207.5 days and travels in an eccentric orbit (with an eccentricity of 0.44 ± 0.02 and an orbital semi-major axis of 0.674 ± 0.002 au), receiving a time-averaged insolation of 1.4 times the Earth’s value. This discovery demonstrates that the transit timing variation method enables the detection and accurate mass measurement of a super-Earth/mini-Neptune within a solar-like star’s habitable zone. Similar searches for such exoplanets could be conducted in other exoplanetary systems in the era of the Transiting Exoplanet Survey Satellite mission and upcoming PLAnetary Transits and Oscillations of stars and Earth 2.0 missions.

对低质量系外行星（≤10 个地球质量 （M_⊕））的探测为当前的行星形成和演化理论提供了基本输入，并为计划的直接成像任务提供了关键信息，这些任务旨在检测和表征类太阳恒星周围宜居带中的类地行星。然而，可用于低质量系外行星的最有效探测技术（即光度凌日和径向速度方法）严重偏向于短周期行星的探测（例如，≤100 天），并且强烈偏爱晚期恒星。在这里，我们报告了开普勒-725 c 的发现，这是一颗 10 ± 3 米 _⊕ 系外行星，位于晚期 G 型矮星开普勒-725 的宜居带内。通过分析相对较短周期（39.64 天）温暖的木星开普勒-725 b 的凌日时间变化，我们发现开普勒-725 c 的周期为 207.5 天，在偏心轨道上行进（偏心率为 0.44 ± 0.02，轨道半长轴为 0.674 ± 0.002 au），接收到地球值的 1.4 倍的时间平均日照。这一发现表明，凌日时间变化方法能够检测和精确测量类太阳恒星宜居带内的超级地球/迷你海王星。在凌日系外行星勘测卫星任务和即将到来的恒星凌日和振荡以及地球 2.0 任务的时代，可以在其他系外行星系统中对此类系外行星进行类似的搜索。