Quadruple Star System with a “Planetary” Architecture

HD 91962 is an unusual quadruple star system that consists of a Sun-like star with 3 lower-mass companion stars revolving around it. The central Sun-like star is estimated to have 1.14 times the Sun’s mass. Around it, the 3 companion stars have orbital periods of 170.3 days, 8.84 years and 205 years; and they are also estimated to have 0.31, 0.64 and 0.64 times the Sun’s mass, respectively. The orbits of the inner and middle companion stars are probably locked in a 1:19 orbital resonance. Each time the middle companion star completes one orbit around the central star, the inner companion star would have completed 19 orbits. With 3 stars revolving around a central star, HD 91962 appears to have a “planetary” architecture.

In the long run, HD 91962 is dynamically stable. This unique quadruple star system probably formed from the collapse of a rotating core of gas and dust. The rotation prevented the core from collapsing directly, causing the gas and dust to form a massive and unstable disk around the central star. The disk fragmented into a number of companion stars that migrated in towards the central star. The first few companion stars probably merged with the central star, while the companion stars that formed further out stopped their inward migration when the disk of gas and dust eventually dissipated. Hence, the 3 companion stars observed today are the ones that have survived.

Reference:
Andrei Tokovinin, David W. Latham & Brian D. Mason (2015), “The unusual quadruple system HD 91962 with a "planetary" architecture”, arXiv:1504.06535 [astro-ph.SR]