Transit timing variation

Finding an unseen planet by the way it makes a visible one run late.

If a transiting planet were alone, its transits would arrive like clockwork. Another planet tugging on it makes the timing drift by seconds or minutes, and the pattern of that drift reveals the second planet's mass and period even if it never transits at all. The technique confirmed the TRAPPIST-1 system's architecture and weighed its planets, which no other method could have done for a star that faint. The effect is strongest where planets are near a resonance — orbital periods in a simple ratio, so the tugs accumulate in the same direction rather than averaging out. TRAPPIST-1's seven planets are in a resonant chain, which is why a technique that usually yields a rough mass gave that system some of the best-measured planets outside our own.