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Pulsar timing

The most precise clocks in the universe, nudged by orbiting rock.

8 planets found this way → Mass Orbital period Eccentricity — all to extraordinary precision
Pulses from a neutron star arrive early and late as an orbiting planet moves it neutron star + planet pulses: a clock accurate to nanoseconds tiny, repeating shifts in arrival time

How the measurement works

A pulsar is a neutron star spinning up to hundreds of times a second, sweeping a radio beam past us like a lighthouse. The pulses arrive with a regularity that rivals atomic clocks.

Anything orbiting the pulsar moves it slightly toward and away from us, and the pulses arrive correspondingly early and late — by microseconds or less, against a clock stable to nanoseconds.

The precision is so extreme that bodies far lighter than the Moon are detectable. Nothing else in astronomy comes close.

It is also how exoplanets were first found at all: two planets around PSR 1257+12 in 1992, three years before the first around a normal star. They orbit the corpse of a star that exploded, and how they got there is still argued over.

The arithmetic

Timing residual
Δt = (apsr sin i / c) · sin(2πt / P)
The pulsar's own orbital displacement expressed as light travel time. Fitting this sine wave out of the arrival times yields the planet.

PSR 1257+12, the first exoplanets known

Planet masses 0.02, 4.3 and 3.9 Earth masses the lightest ever found, by a wide margin
Timing precision ~microseconds over years on a clock stable to nanoseconds
Found in 1992 three years before 51 Pegasi b

Eight planets in the whole catalogue came this way, and they remain the most precisely characterised — and the most alien, orbiting a stellar remnant rather than a star.

What it does well
  • Unmatched precision — sub-lunar masses are detectable
  • Reveals masses and orbits to many decimal places
  • Historically first
Where it cannot help
  • Only works around pulsars, which are rare and nothing like the Sun
  • Says nothing about ordinary planetary systems
  • Requires large radio telescopes and years of monitoring
Landmark discovery PSR 1257+12 b, c, d (1992) — the first confirmed exoplanets, full stop.
Instruments Arecibo (historically), Green Bank, Parkes, FAST
Archive name Pulsar Timing — the value in NASA's discoverymethod field

Browse the 8 planets found by this method →

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