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

Abstract

We confirm the planetary nature of Kepler-539 b (aka Kepler object of interest K00372.01), a giant transiting exoplanet orbiting a solar-analogue G2 V star. The mass of Kepler-539 b was accurately derived thanks to a series of precise radial velocity measurements obtained with the CAFE spectrograph mounted on the CAHA 2.2-m telescope. A simultaneous fit of the radial-velocity data and Kepler photometry revealed that Kepler-539 b is a dense Jupiter-like planet with a mass of M p = 0.97 ± 0.29 M Jup and a radius of R p = 0.747 ± 0.018 R Jup , 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 ≈0.45 au from the habitable zone. By analysing the mid-transit times of the 12 transit events of Kepler-539 b 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-539 c and our analysis suggests that it has a mass between 1.2 and 3.6 M Jup , 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 ± 0.12 dex, which, together with both a gyrochronological and isochronal analysis, suggests that the parent star is relatively young.

Dynamical masses of two infant giant planets

“…For planet e we obtain a higher, but less precise, density. It this result were confirmed, it could be one of the densest giant planets known to date, similar to HATS-17 b 12 and Kepler-539 b 13 . For the two smaller planets, c and d, we calculated 3σ upper limits on their masses of 0.27 M Jup and 0.32 M Jup , respectively, which set upper limits to the densities of 4.4 g cm −3 and 2.8 g cm −3 , respectively.…”

“…With an estimated age of 20 million years, V1298 Tau is one of the youngest solar-type stars known to host transiting planets: it harbours a multiple system composed of two Neptune-sized, one Saturn-sized, and one Jupiter-sized planets 10,11 .Here we report the dynamical masses of two of the four planets. We find that planet b, with an orbital period of 24 days, has a mass of 0.60 Jupiter masses and a density similar to the giant planets of the Solar System and other known giant exoplanets with significantly older ages 12,13 . Planet e, with an orbital period of 40 days, has a mass of 1.21 Jupiter masses and a density larger than most giant exoplanets.…”

“…Here we report the dynamical masses of two of the four planets. We find that planet b, with an orbital period of 24 days, has a mass of 0.60 Jupiter masses and a density similar to the giant planets of the Solar System and other known giant exoplanets with significantly older ages 12,13 . Planet e, with an orbital period of 40 days, has a mass of 1.21 Jupiter masses and a density larger than most giant exoplanets.…”

Using HARPS-N to characterize the long-period planets in the PH-2 and Kepler-103 systems

“…calculated using precise mass and radius measurements) 9 . Of these planets, 3 have densities comparable to Kepler-103c: Kepler-1657b (Hébrard et al 2019), Kepler-539b (Mancini et al 2016) and HD80606b (Naef et al 2001;Moutou et al 2009). These three planets have many parameter values in common: they all have masses similar, if not multiple times larger, than Jupiter, with Jupiter-like radii, and are at an orbital distance of approximately 0.5 AU.…”

The TROY project

“…Photometric variability at the 1.5 mmag level appears in the CAFOS and WiFSIP light curves around the L 5 region, which might be due to the moderate activity of the host star (log R = −4.65 dex; Mancini et al 2016). No significant dimming is detected down to 6 R ⊕ at L 5 in the CAFOS light curve; the WiFSIP data are of slightly worse quality in this case.…”