An exoplanet is a planet that orbits a star other than the Sun. The nearest star beyond the Sun, Proxima Centauri, is about 4.24 light years away, so every exoplanet is far beyond anything a spacecraft has visited. Everything we know about them comes from light that has travelled for years.

Why not simply photograph them? A planet makes no light of its own that matters in visible colours. It shines only by reflecting its star’s light, and it reflects only a tiny fraction of it.