Kepler-539: a young extrasolar system with two giant planets on wide orbits and in gravitational interaction

TL;DR

This paper confirms Kepler-539b as a dense Jupiter-like exoplanet with a wide orbit, and suggests the presence of a more massive, eccentric companion planet c causing transit time variations, indicating a young, dynamically interacting system.

Contribution

It provides the first detailed characterization of Kepler-539b and evidence for a second, eccentric planet c influencing its transit timings, revealing a young, gravitationally interacting system.

Findings

Kepler-539b is a dense Jupiter-like planet with a 125.6-day orbit.

Transit time variations suggest a second planet c with 1.2-3.6 Mjup and high eccentricity.

The host star is relatively young, indicated by high lithium content.

Abstract

We confirm the planetary nature of Kepler-539b (aka Kepler object of interest K00372.01), a giant transiting exoplanet orbiting a solar-analogue G2 V star. The mass of Kepler-539b was accurately derived thanks to a series of precise radial velocity measurements obtained with the CAFE spectrograph mounted on the CAHA 2.2m telescope. A simultaneous fit of the radial-velocity data and Kepler photometry revealed that Kepler-539b is a dense Jupiter-like planet with a mass of Mp = 0.97 Mjup and a radius of Rp = 0.747 Rjup, making a complete circular revolution around its parent star in 125.6 days. The semi-major axis of the orbit is roughly 0.5 au, implying that the planet is at roughly 0.45 au from the habitable zone. By analysing the mid-transit times of the 12 transit events of Kepler-539b recorded by the Kepler spacecraft, we found a clear modulated transit time variation (TTV), which is attributable to the presence of a planet c in a wider orbit. The few timings available do not allow us to precisely estimate the properties of Kepler-539c and our analysis suggests that it has a mass between 1.2 and 3.6 Mjup, revolving on a very eccentric orbit (0.4<e<0.6) with a period larger than 1000 days. The high eccentricity of planet c is the probable cause of the TTV modulation of planet b. The analysis of the CAFE spectra revealed a relatively high photospheric lithium content, A(Li)=2.48 dex, which, together with both a gyrochronological and isochronal analysis, suggests that the parent star is relatively young.