Constraining the Densities of the Three Kepler-289 Planets with Transit Timing Variations

TL;DR

This study refines the masses and densities of Kepler-289's three planets using extended TTV data, revealing their compositions and migration histories, with implications for planet formation theories.

Contribution

It provides improved mass and density measurements for all three planets, including the first constraints on Kepler-289c's heavy element content, and insights into their formation and migration.

Findings

All three planets have more precise mass and density estimates.

Inner planets likely have hydrogen-rich atmospheres.

Kepler-289c contains about 30.5 Earth masses of metals and formed beyond 1 au.

Abstract

Kepler-289 is a three-planet system containing two sub-Neptunes and one cool giant planet orbiting a young, Sun-like star. All three planets exhibit transit timing variations (TTVs), with both adjacent planet pairs having orbital periods close to the 2:1 orbital resonance. We observe two transits of Kepler-289c with the Wide-field InfraRed Camera (WIRC) on the 200" Hale Telescope at Palomar Observatory, using diffuser-assisted photometry to achieve space-like photometric precision from the ground. These new transit observations extend the original four-year Kepler TTV baseline by an additional 7.5 years. We re-reduce the archival Kepler data with an improved stellar activity correction and carry out a joint fit with the Palomar data to constrain the transit shapes and derive updated transit times. We then model the TTVs to determine the masses of the three planets and constrain their densities and bulk compositions. Our new analysis improves on previous mass and density constraints by a factor of two or more for all three planets, with the innermost planet showing the largest improvement. Our updated atmospheric mass fractions for the inner two planets indicate that they likely have hydrogen-rich envelopes, consistent with their location on the upper side of the radius valley. We also constrain the heavy element composition of the outer saturn-mass planet, Kepler-289c, for the first time, finding that it contains 30.5 $\pm$ 6.9 $M_{\oplus}$ of metals. We use dust evolution models to show that Kepler-289c must have formed beyond 1~au, and likely beyond 3~au, and then migrated inward.