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.
