HARPS-N, TESS, and CHEOPS discover a transiting sub-Neptune and two outer companions around the bright solar analogue HD 85426

Abstract

We provide a detailed characterisation of the planetary system orbiting HD 85426 (TOI-1774). This bright G-type star ($M_{\ast}$: 0.99 $\text{M}_{\odot}$; $R_{\ast}$: 1.13 $\text{R}_{\odot}$; age: 7.4 Gyr; V mag: 8.25) hosts a transiting sub-Neptune, HD 85426 b, with an orbital period of 16.71 days and a blackbody equilibrium temperature of $824^{+11}_{-11}$ K. By jointly analysing HARPS-N RVs, TESS, and CHEOPS photometric data and using two different stellar activity mitigation techniques, we constrain planet b's mass to $6.0^{+1.5}_{-1.6}$ $\text{M}_{\oplus}$ and $8.5^{+1.3}_{-1.4} $ $\text{M}_{\oplus}$, depending on the mitigation technique. We investigate the dependence of these results on the priors, data selection, and inclusion of other Keplerians in the modelling. Using this approach, we identify the presence of two non-transiting planetary companions with minimum masses near 10 $\text{M}_{\oplus}$ and orbital periods of 35.7 and 89 days. Additionally, we reject the initial hypothesis that the 35.7-day periodic signal was due to stellar activity. We also determine HD 85426 b's radius to be $2.78^{+0.05}_{-0.04}$ $\text{R}_{\oplus}$ and compute a transmission spectroscopy metric in the range of 82 to 115, making this planet a highly valuable target for atmospheric characterisation.

Figures (20)

• Figure 1: Photometric 2-minute cadence PDCSAP time series of TIC 4897275 (HD 85426) in TESS sectors 21 (top panel) and 48 (bottom panel) are shown in grey, with 30-minute binned data overlaid in black. The transits of HD 85426 b are indicated by the blue box transits. The rise in flux shortly after BJD 2,459,630 is caused by a passing asteroid. The predicted conjunction time windows for planet c and planet candidate d are shaded in orange and violet, respectively. The width of these conjunction time windows was set to twice the predicted uncertainty.
• Figure 2: In the top panel, the HARPS-N DRS 3.0.1 RVs (light blue) and YARARA RVs (violet) are shown. The observations marked in black were rejected due to instrumental issues or rejection by either activity mitigation algorithm. The difference between the two RV sets are shown in the bottom panel.
• Figure 3: GLS periodograms from DRS (solid red lines) and YARARA (dashed blue lines) for set 1. The solid vertical line indicates the periods of HD 85426 b (16.71 d), and the dashed vertical lines show the periods of the most dominant Keplerian signals at 35.7 and 90 d. The False Alarm Probability of 1 (0.1) per cent is indicated by the light-grey (dark-grey) horizontal line. The top panel shows the periodograms of the two RV sets, whereas the second panel shows the periodograms after removing the most dominant sinusoidal signal at 35.8 d. From the third to the ninth panel from the top, we show the periodograms of the activity indicators, and in the last panel, we show the window function.
• Figure 4: Time series of the DRS RVs and the CCF contrast and best-fitting sinusoidal model. Both time series were normalised independently to match in scatter.
• Figure 5: GLS periodograms of the YARARA RVs. The grey vertical line indicates the period of HD 85426 b. The blue vertical line shows the period of planet candidate HD 85426 c. The grey horizontal line indicates the level of 1 per cent False Alarm Probability.