A lone transiting planet is a metronome: the dip arrives at the same interval, forever. Deviations mean something is interfering.
A second planet tugs on the first, speeding it up on one part of the orbit and slowing it on another. The transits then drift early and late by seconds to hours, in a pattern that repeats on its own longer cycle.
The pattern's shape and period encode the unseen planet's mass and orbit. Crucially, the second planet does not need to transit at all — this method sees planets whose orbits never cross the star from our angle.
The effect is strongest when the two orbits are in resonance, such as one planet completing exactly two orbits for the other's three: the tugs then line up repeatedly instead of averaging away.
The arithmetic
What is measured
Δt = tobserved − tpredicted
Simply how late or early each transit was against a fixed-period prediction. Plotted against time, this is an O−C diagram.
Rough size near resonance
Δt ∼ P · (Mperturber / M*) · f(resonance)
Amplified enormously near resonance — minutes rather than milliseconds — which is why almost every TTV detection involves resonant pairs.
Kepler-9, the first clear case
Two transiting giants
periods ~19 and ~39 days
close to a 2:1 resonance
Timing drift
tens of minutes, growing then reversing
over a ~months-long cycle
What it gave
true masses of both planets
from timing alone, no spectrograph
The value here is the true mass, not a minimum: the geometry is already known from the transits, so there is no sin i left unresolved.
What it does well
Reveals planets that never transit and are too light to find by wobble
Gives true masses, not lower bounds
Free — it reuses transit data already collected
Where it cannot help
Needs a known transiting planet to begin with
Effectively requires near-resonant orbits for a measurable signal
Solutions are often ambiguous — several configurations fit the same timings
Needs years of transits to build the pattern
Landmark discovery
Kepler-9 b and c (2010) — the first planets with masses measured purely from transit timing.
Instruments
Kepler above all; TESS and ground-based follow-up
Archive name
Transit Timing Variations — the value in NASA's discoverymethod field