If the mass estimates are correct, the HR 8799 system is the first multiple-planet extrasolar system to be directly imaged.

Also, astrometric observations and dynamical considerations in multiple-planet systems can sometimes provide an upper limit to the planet's true mass.

Boötes is also home to multiple-planet systems.

This and subsequent solitary planet discoveries prepared astronomers for the possibility that the first multiple-planet system they observed would differ considerably from the solar system.

The reason astronomers have not sighted more multiple-planet systems, they say, is simply that they have not looked long enough.

This cache brims with multiple-planet systems: 115 doubles, 45 triples, and 10 with at least four.

She was part of the team to discover the first known multiple-planet system.

That is a tool, they say, that is becoming increasingly valuable for determining the masses of small planets in multiple-planet systems.

Planets in multiple-planet systems are highlighted in yellow.

The first observations of multiple-planet systems around other stars have deepened the consternation of astronomers and theorists of planetary dynamics.