Three planets for Upsilon Andromedae

In 1997, a planet of roughly Jupiter's mass was discovered in orbit around the star Andromedae. Two more have now been identified.

In the 1990s, the quest to find planets outside our own Solar System finally succeeded. The first extrasolar planets to be identified are in orbit around a pulsar¹, a rapidly rotating neutron star which is quite different from our own familiar Sun. The first planetary companion to a main-sequence star — a hydrogen-burning star like the Sun — was discovered in 1995, using Doppler measurements of the reflex motion (wobble) that the planet's orbit induces in the star 51 Pegasi². Nearly 20 more main-sequence stars have since been found to possess a single companion of roughly Jupiter mass³. But now we have news of something else again — the discovery, announced last week by Paul Butler and his colleagues⁴, of a system of three massive planets orbiting the star Andromedae.