这里涉及的搭配是 discover sth doing sth（doing 为宾补）→(discover 变成被动式) be discovered doing sth（doing 为补语）。你的句子中的 orbiting 不是定语，而是补语，补充说明exoplanet；不定式短语 to be discovered orbiting a star like our own 才是定语，修饰  exoplanet。

例句如：

He was discovered hiding in the shed. 人们发现他原来藏在棚屋里。

The fault was discovered during a routine test. 这一缺陷在常规检测中被发现。