至今，人们已发现了1781颗系外行星，其中有462个多行星系统。系外行星系统，特别是多行星系统的探测和研究是人们认识行星系统形成和演化的新窗口，为理解太阳系的形成和演化提供了可借鉴和比较的样本。从已探测到的系外行星数据来看，90%的系外行星是'热木星'，且运行在各种轨道上，这与太阳系的情形有着较大的差异。为了理解这种差异，需要更多的系外行星样本，尤其是类地的宜居行星。然而，以当今的地面视向速度测定技术及CCD测光精度，很难探测到地球大小的宜居系外行星。当多行星系统中行星处于平均运动的低阶共振时，行星间的引力作用会引起其轨道的明显变化。对于一个凌食的系外行星来讲，其明显的轨道周期变化会体现为凌食中间时刻的变化。本项目就是利用国内中、小型光学望远镜，对选定的凌食行星进行长时序的凌食事件的测光观测，精化系统的物理参数，分析凌食中间时刻变化（TTV），推断系统内是否有其它类地行星存在。

Since the first exoplanet was discovered in 1992, 1781 exoplanets have been detected in 1100 planetary systems , among which 462 ones are multiple planetary systems.The observation and study on the exoplanets and the multiple planetary systems open the new windows for us to understand how these planetary systems form and evolve. These research results can be compared to that of our solar system, and applied to address fundamental questions about our own solar system. In fact, 90% of exoplanets are 'hot Jupiters', running along kinds of orbits. The diversification of the alien planetary systems is quite different from our own solar system. In order to understand such differences, more exoplanet samples, especially super-earth and earth-like exoplanets are needed. However, the limited detection abilities of present radial velocity instruments and ground-based photometric instruments, prevent us to detect terrestrial exoplanets in habitable zone by RV and transit methods in the ground. For a planetary system with multiple planets, the gravitational interactions between planets may cause some changes on planetary orbits. Especially, when two planets are in low order mean-motion resonance, the orbital changes are significant and can be measured even using ground-based instruments. Our program will focus on mintoring the Transit Timing Variations (TTVs) of known exoplanets to detect new exoplants, maybe super-earth or earth-size ones, in the planetary systems. This program includes: (1)the long-term photometric observation for selected transiting exoplanets using Chinese middle and small size optical telescopes; (2)MCMC analysis to get the planetary parameters; (3)analysis for the mid-time of all transit events. From the periodic variation in TTV data, the orbital elements and mass of an unseen planet will be inferred.