The Densities of Planets in Multiple Stellar Systems

Abstract

We analyze the effect of companion stars on the bulk density of 29 planets orbiting 15 stars in the Kepler field. These stars have at least one stellar companion within 2″, and the planets have measured masses and radii, allowing an estimate of their bulk density. The transit dilution by the companion star requires the planet radii to be revised upward, even if the planet orbits the primary star; as a consequence, the planetary bulk density decreases. We find that if planets orbited a faint companion star, they would be more volatile-rich, and in several cases their densities would become unrealistically low, requiring large, inflated atmospheres or unusually large mass fractions in an H/He envelope. In addition, for planets detected in radial velocity data, the primary star has to be the host. We can exclude 14 planets from orbiting the companion star; the remaining 15 planets in seven planetary systems could orbit either the primary or the secondary star, and for five of these planets the decrease in density would be substantial even if they orbited the primary, since the companion is of almost equal brightness as the primary. Substantial follow-up work is required in order to accurately determine the radii of transiting planets. Of particular interest are small, rocky planets that may be habitable; a lower mean density might imply a more volatile-rich composition. Reliable radii, masses, and thus bulk densities will allow us to identify which small planets are truly Earth-like.

Planetary Candidates Observed by Kepler. VIII. A Fully Automated Catalog with Measured Completeness and Reliability Based on Data Release 25

“…transit, causing the radii to be significantly underestimated (Ciardi et al 2015;Furlan & Howell 2017). As a result, we fully expect that once follow-up observations are obtained for these stars, several of the PCs in this catalog, including those listed in the eta-Earth sample, will be found to have radii larger than reported in this catalog.…”

The L 98-59 System: Three Transiting, Terrestrial-size Planets Orbiting a Nearby M Dwarf

“…Photometric contamination from nearby sources can result in various false-positive scenarios (e.g., background eclipsing binaries) and can bias the measured planetary radius from photometric analysis (see, e.g., Ciardi et al 2015;Furlan & Howell 2017;Ziegler et al 2018). In this work, we use several high-resolution images to tightly constrain the possible background sources and companion stars present near L 98-59.…”

Three Red Suns in the Sky: A Transiting, Terrestrial Planet in a Triple M-dwarf System at 6.9 pc

“…The planet is 6.9pc away, and orbits one member of a stellar triplet. Multi-star systems present numerous challenges which sometimes deter planet hunters: astrometric perturbations from stellar companions at small separations can hinder the measurement of the trigonometric parallax of the system; the presence of bound companions can result in trends in the radial velocities (RVs) of a star that can mask the signals of planets; and light contamination from close stellar companions in the photometry of a host star can result in an underestimated planet radius (Ciardi et al 2015;Furlan & Howell 2017;Hirsch et al 2017). Yet these complications are also opportunities to measure the stellar orbits and investigate the potential formation scenarios for the planets that are found within; indeed all of these features are present in the system that is the subject of our study.…”