The First Known Multi-Planet System in an Open Cluster

The Beehive Cluster is an open cluster containing ~1000 stars and it is one of the nearest open clusters. Pr 0211 is a Sun-like star located in the Beehive Cluster. It has a hot-Jupiter identified as Pr 0211b in orbit around it. Pr 0211b has at least 1.88 ± 0.03 times the mass of Jupiter and its orbital period is 2.15 days. Using new radial velocity measurements obtained with HARPS-N and TRES, Malavolta et al. (2016) present the discovery of a second planet in orbit around Pr 0211. The discovery of this second planet means that the planetary system around Pr 0211 is the first known multi-planet system in an open cluster.

The newly discovered planet around Pr 0211 is identified as Pr 0211c. It is a massive Jupiter-like planet with at least 7.9 ± 0.2 times the mass of Jupiter, and its orbital period around its host star is greater than 3500 days. Furthermore, Pr 0211c is in a highly eccentric orbit around its host star, with an orbital eccentricity greater than 0.6. The highly eccentric orbit of Pr 0211c could be caused by disruption of the planetary system from close encounters with passing stars as Pr 0211 is a member of an open cluster.

Reference:
Malavolta et al. (2016), “The GAPS programme with HARPS-N at TNG XI. Pr~0211 in M~44: the first multi-planet system in an open cluster”, arXiv:1602.00009 [astro-ph.EP]