Separating a planet from its star

Adapted from IOAA 2013 (Volos), theory problem 2. We have restated it in our own words and kept the original data. The solution was last checked on 16 October 2026.

A giant planet orbits a star at an average distance of 5 au, and the system is 250 pc from us. What is the smallest diameter of a telescope that could separate the planet from the star? Assume that the telescope works in visible light of wavelength about 500 nm, outside Earth's atmosphere, and that its optics are perfect.

AnswerThe telescope must be at least about 6.3 m across. In practice the star outshines the planet so strongly that much more is needed to see the planet, as Chapter 9 explains.