transit-timing variations

When a planet transits its star, it does so like clockwork — a dip in brightness every orbit, arriving right on schedule. But what if the clock runs a little fast one time, a little slow the next? Something must be tugging the transiting planet, speeding it up here and slowing it down there. Transit-timing variations are these small deviations in when a planet's transits occur, and they betray the presence of another, unseen planet pulling on the one we can watch.

The logic is gravitational. In a system with several planets, the planets gently pull on each other, nudging their orbits. A companion planet will alternately hurry and delay the transiting planet, so its transits drift earlier and later than a perfect clock would predict — often by mere minutes, sometimes seconds, in a pattern that repeats over many orbits. By modeling those timing wiggles, astronomers can infer the mass and orbit of the unseen perturber, even if that planet never transits and so never blocks any starlight itself. The richer the timing pattern, the more the hidden planet's properties can be pinned down.

Transit-timing variations are powerful for two reasons. They can reveal non-transiting planets that no other method in a given system would catch, and they can weigh planets in tightly packed multi-planet systems where radial-velocity wobbles are too small to measure — giving masses, and with the transit radii, densities. The famous TRAPPIST-1 system's planet masses were refined largely through these timing tugs. The method's limit is that it needs a closely interacting system, ideally with planets near orbital resonances, where the mutual nudges grow large enough to detect.