Evidence for Multiple Companions to υ Andromedae

TLDR

The bright F8 V star υ Andromedae shows a 4.6‑day Doppler periodicity indicating a Jupiter‑mass companion at 0.059 AU, and the outer two companions, if confirmed, would be the first multiple‑planet system around a main‑sequence star, offering insights into planet formation and orbital eccentricities. Follow‑up Lick and AFOE spectrometer observations confirm the 4.6‑day period and reveal additional 241‑ and 1267‑day periodicities, while non‑Keplerian stellar activity explanations are ruled out by photometric and chromospheric stability. The periodicities are consistent with Keplerian motion, implying two additional companions at 0.83 and 2.5 AU with minimum masses of 2.0 and 4.6 MJUP, and the three‑planet system is dynamically stable according to analytic and numerical simulations.

Abstract

The bright F8 V star υ Andromedae was previously reported to have a 4.6 day Doppler velocity periodicity, consistent with having a Jupiter-mass companion orbiting at 0.059 AU. Follow-up observations by both the Lick and Advanced Fiber-Optic Echelle spectrometer (AFOE) planet survey programs confirm this periodicity and reveal additional periodicities at 241 and 1267 days. These periodicities are consistent with Keplerian orbital motion and imply two additional companions orbiting at 0.83 and 2.5 AU, with minimum (M sin i) masses of 2.0 and 4.6 MJUP, respectively. Non-Keplerian explanations for the observed Doppler velocity variations, including radial and nonradial pulsations, rotational modulation of surface features, and stellar magnetic cycles, are examined. These explanations seem unlikely based on the observed photometric and chromospheric stability of the star. This putative three-planet system is found to be dynamically stable by both analytic techniques and numerical simulations. The outer two companions both reside in eccentric orbits, as do all nine known extrasolar planet candidates in distant orbits. If real, this multiple-planet system is the first around a main-sequence star, and its study should offer insights into planet formation, planet-planet interactions, and the observed eccentricities of planetary orbits.