Gravitational perturbation
Small tugs from other bodies that pull a planet or comet slightly off its simple orbit; measuring them can reveal unseen masses.
If the Sun and a single planet were alone, the planet would trace a perfect ellipse forever. In reality every planet, moon and comet also feels the gravity of all the others. These extra, smaller pulls are called perturbations. They nudge orbits slightly out of shape, make them slowly turn, and speed up or slow down bodies passing near a giant planet.
Astronomers can calculate the perturbations caused by known bodies and allow for them. If something is still left over, there may be a mass nobody has seen. That is how Neptune was found. In the early 19th century Uranus kept straying from its predicted path in ways that Jupiter and Saturn could not explain. John Couch Adams in England and Urbain Le Verrier in France each calculated where an unseen outer planet would have to be. In September 1846 Johann Galle at the Berlin Observatory received Le Verrier's prediction, searched that part of the sky and found an eighth-magnitude "star" that was not on his chart. It was the new planet.
Comets are perturbed strongly when they pass near Jupiter or Saturn, which is why predicting the return of Halley's Comet means allowing for the giant planets.
The method does not always point to a new object. An unexplained drift in Mercury's orbit led some astronomers to propose another planet, Vulcan, closer to the Sun. It was never found; the explanation turned out to be Einstein's general relativity.
See it in action
Coming up in future stories
- The Comet That Kept Its Appointment 1758 · Prohlis near Dresden, Saxony
- The Planet Found With a Pen 1846 · Paris and Berlin Observatory