But many stars are partnered in binary star systems.

ECONOMIST: The dizziest, giddiest stars

Most stars in the galaxy are, in fact, in binary systems, and astronomers have been wondering whether the gravity of a second star nearby might disrupt a dust disc too much for planets to coalesce.

ECONOMIST: Dusty genesis

Secondly, the detection of multiple planets demolishes once and for all the lingering doubts of some astronomers that the single planets found around other stars are not planets at all, but small, faint components in otherwise conventional binary star systems.

ECONOMIST: Planet hunting