Discovery of a multi-planetary system orbiting the aged Sun-like star HD 224018

TL;DR

This study reports the discovery and detailed characterization of a multi-planet system around the Sun-like star HD 224018, combining HARPS-N radial velocities with K2, CHEOPS, and TESS photometry to measure masses, radii, and densities for three inner transiting planets and to identify a distant, eccentric outer companion candidate. Through a joint modeling approach that incorporates SCALPELS decorrelation and a transit analysis pipeline, the authors derive precise masses for the inner planets (b: ~4.1 M$_\oplus$, c: ~10.4 M$_\oplus$, d: ~4.2 M$_\oplus$) and constrain the period of the outer transiting candidate (d) to ~138 days, plus an outer giant companion (e) with a long, eccentric orbit and minimum mass near 0.5 M$_{\rm Jup}$, pending confirmation. HD 224018 c emerges as a well-characterized warm sub-Neptune with a bulk density around 4.0 g cm$^{-3}$, enabling interior-structure modeling that depends on formation-location priors (inside vs outside the ice-line) and stellar-abundance priors, revealing a possible volatile envelope. Together, these results place HD 224018 among systems with inner small planets coexisting with a distant massive companion, and demonstrate the power of integrating RVs with multi-instrument transit data to illuminate planetary compositions and system architecture.

Abstract

In 2016, Kepler/K2 detected a system of two sub-Neptunes transiting the star HD 224018, one of them showing a mono-transit event. In 2017, we began a spectroscopic follow-up with HARPS-N to measure the dynamical masses of the planets using radial velocities, and collected additional transit observations using CHEOPS. We measured the fundamental physical parameters of the host star, which is an ``old Sun'' analogue. We analysed radial velocities and photometric time series, also including data by TESS, to provide precise ephemerides, radii, masses, and bulk densities of the two planets, and possibly modeling their internal structure and composition. The system turned out to be more crowded than shown by K2. Radial velocities revealed the presence of two additional bodies: a candidate cold companion on an eccentric orbit with a minimum mass nearly half that of Jupiter (eccentricity $0.60^{+0.07}_{-0.08}$; semi-major axis 8.6$^{+1.5}_{-1.6}$ au), and an innermost super-Earth (orbital period 10.6413$\pm$0.0028 d; mass 4.1$\pm$0.8 Me) for which we discovered previously undetected transit events in K2 photometry. TESS revealed a second transit of one of the two companions originally observed by K2. This allowed us to constrain its orbital period to a grid of values, the most likely being $\sim$138 days, which would imply a mass less than 9 Me, at a 3$σ$ significance level. Given the level of precision of our measurements, we were able to constrain the internal structure and composition of the second-most distant planet from the host star, a warm sub-Neptune with a bulk density of 3.9$\pm$0.5 g/cm$^{3}$. HD 224018 hosts three close-in transiting planets in the super-Earth-to-sub-Neptune regime, and a candidate cold and eccentric massive companion. Additional follow-up is needed to better characterise the physical properties of the planets and their architecture.

Figures (17)

• Figure 1: K2 light curve of HD 224018. Upper panel. Undetrended photometric time series. Middle panel. A first version of the detrended and flattened light curve, as provided in the public archive (2014PASP..126..948V; the curve analysed in this work is shown in Fig. \ref{['fig:wotandetrending']}). Naked-eye visible transit signals are marked with red dashed lines. Lower panels. Zoomed-in views showing details of the transit-like signals. The morphology of the first one looks more complex as it is the superposition of two transits.
• Figure 2: GLS periodograms (in frequency and period) of the HARPS-N original radial velocities (upper panel) and residuals. These are calculated through an iterative pre-whitening. Red triangles mark the peaks with the highest power. False alarm probability levels are indicated by horizontal dashed lines.
• Figure 3: Results from the detrending of the K2 light curve with wōtan. Upper panel. Detrended light curve, with the epochs of the transits of HD 224018 b indicated by vertical dashed lines. Lower panel. TLS periodogram after masking the 36.6 day transits and the mono-transit, and restricting the search to orbital periods shorter than 19.7 days. The main peak is found at 10.64 days. Individual transits with very low S/N transits are shown in Fig. \ref{['fig:plbsingletrans']}, demonstrating how a fine-tuned detrending was crucial for their detection.
• Figure 4: Time series and periodograms of the spectroscopic activity diagnostics BIS, CCF area (FWHM$\cdot$contrast), and S-index. First column. Time series of the activity indexes. The average values have been subtracted from each dataset. Second column. GLS periodograms. Red triangles indicate peak values. The horizontal lines correspond to levels of FAPs determined through a bootstrap analysis.
• Figure 5: Radial velocity time series selected with SCALPELS as our definitive dataset (upper panel). Radial velocities projection onto the first three basis vectors calculated with SCALPELS are shown in the other panels.