Compact Multi-planet Systems are more Common around Metal-poor Hosts

John M. Brewer1,2, Songhu Wang1, Debra A. Fischer1, and Daniel Foreman-Mackey3
Astrophysical Journal Letters 867, L3 Link to Article [DOI: 10.3847/2041-8213/aae710]
1Department of Astronomy, Yale University, 52 Hillhouse Avenue, New Haven, CT 06511, USA
2Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA
3Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA

In systems with detected planets, hot Jupiters and compact systems of multiple planets are nearly mutually exclusive. We compare the relative occurrence of these two architectures as a fraction of detected planetary systems to determine the role that metallicity plays in planet formation. We show that compact multi-planet systems occur more frequently around stars of increasingly lower metallicities using spectroscopically derived abundances for more than 700 planet hosts. At higher metallicities, compact multi-planet systems comprise a nearly constant fraction of the planet hosts despite the steep rise in the fraction of hosts containing hot and cool Jupiters. Since metal-poor stars have been underrepresented in planet searches, this implies that the occurrence rate of compact multis is higher than previously reported. Due to observational limits, radial velocity planet searches have focused mainly on high-metallicity stars, where they have a higher chance of finding giant planets. New extreme-precision radial velocity instruments coming online that can detect these compact multi-planet systems can target lower-metallicity stars to find them.

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