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Eclipse timing variations

Two stars eclipse each other on a clock, and a planet nudges the clock.

17 planets found this way → Minimum mass Orbital period of the circumbinary planet
Two stars eclipse each other on a strict schedule; an outer planet shifts the eclipse times binary pair + outer planet eclipses: the clock arrival time drifts by seconds

How the measurement works

In an eclipsing binary, two stars pass in front of one another on a strict schedule — a natural clock, often good to seconds.

A planet orbiting the pair from outside moves the whole binary around their common centre of mass. When the binary is displaced toward us the eclipse light arrives fractionally early; when displaced away, fractionally late.

The measurement is therefore of light travel time across the binary's own small orbit, and the drift repeats with the planet's period.

This is the shakiest method in the catalogue. Binaries have their own reasons for drifting — magnetic cycles in the stars change their shape and their orbit — and several announced planets were later withdrawn once longer baselines showed a non-planetary pattern.

The arithmetic

Light travel time
Δt = (abin sin i / c) · (Mp / (Mbin + Mp))
The displacement of the binary, divided by the speed of light. Seconds of timing for a Jupiter at a few AU.

Order of magnitude

Binary displaced by ~0.01 AU by a Jupiter-mass planet a few AU out
Light crossing that ~5 seconds the entire signal
Baseline needed a decade or more to see the pattern repeat convincingly

Seventeen planets in the catalogue rest on this, and it is fair to say some remain debated.

What it does well
  • Reaches circumbinary planets that other methods struggle with
  • Sensitive to wide orbits
  • Can reuse decades of archival eclipse observations
Where it cannot help
  • Stellar magnetic activity mimics the same drift — a documented source of retracted claims
  • Needs very long baselines
  • Only an approximate mass
Landmark discovery NN Serpentis b and c — a much-studied and long-debated case.
Instruments Ground-based long-term photometry, amateur networks included
Archive name Eclipse Timing Variations — the value in NASA's discoverymethod field

Browse the 17 planets found by this method →

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