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arxiv: 2604.11516 · v1 · submitted 2026-04-13 · 🌌 astro-ph.EP

The RoPES project with HARPS and HARPS-N III. Two candidate planets orbiting the G-type star HD 161098

N. Nari , A. Su\'arez Mascare\~no , J. I. Gonz\'alez Hern\'andez , A. K. Stefanov , R. Rebolo , J. M. Mestre , X. Dumusque , M. Cretignier
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V. M. Passegger L. Mignon F. Manni R. G. S. B. De Amorim
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Pith reviewed 2026-05-10 15:43 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords exoplanetsradial velocitieshabitable zoneG-type starsuper-Earthstellar activityHD 161098
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The pith

Radial velocity data reveal two candidate planets orbiting the nearby G-type star HD 161098, one potentially in the optimistic habitable zone.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents a radial velocity analysis of the G8 V star HD 161098 using combined historical and new observations from HARPS, HARPS-N, and ESPRESSO. It identifies two periodic signals with amplitudes below 1 m/s after jointly modeling the velocities with stellar activity indicators. These signals are interpreted as candidate planets with minimum masses of 3.63 and 7.8 Earth masses at orbital periods of 72.6 and 682.5 days. The longer-period candidate would lie in the star's optimistic habitable zone if confirmed. Such detections matter for building a sample of low-mass planets around Sun-like stars that future missions could target for atmospheric study.

Core claim

Analysis of radial velocity data from multiple high-precision spectrographs reveals two candidate planets orbiting the G-type star HD 161098. The inner candidate, HD 161098 b, has an orbital period of 72.578 days and a minimum mass of 3.63 Earth masses. The outer candidate, HD 161098 c, orbits every 682.5 days with a minimum mass of 7.8 Earth masses and, if real, would sit in the optimistic habitable zone. The study also determines the star's rotation period as 28.22 days and a magnetic cycle of 4090 days. These findings are obtained through careful separation of planetary signals from stellar activity and instrumental effects, paving the way for confirmation observations.

What carries the argument

Joint modeling of radial velocities with activity indicators to isolate Keplerian planetary signals from stellar activity.

If this is right

  • If confirmed, the outer candidate would represent a super-Earth in the optimistic habitable zone of a nearby G-type star.
  • The work achieves sub-meter-per-second radial velocity precision at orbital periods of hundreds of days.
  • The analysis characterizes the star's rotation and long magnetic cycle, providing context for similar planet searches.
  • Future targeted campaigns can collect more data to confirm or refute the planetary interpretation.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Transit photometry follow-up could test whether either candidate crosses the stellar disk and yield density constraints.
  • Confirmation would add a benchmark system for studying super-Earth occurrence rates around G-type stars.
  • The joint modeling approach may be applied to other long-baseline radial velocity datasets to uncover additional small planets.

Load-bearing premise

The two periodic radial velocity signals represent true planetary orbits rather than residual stellar activity or instrumental systematics.

What would settle it

Additional radial velocity observations spanning several years that fail to recover consistent signals at 72.6 or 682 days or show those signals correlating with activity indicators instead.

Figures

Figures reproduced from arXiv: 2604.11516 by A. K. Stefanov, A. Su\'arez Mascare\~no, F. Manni, J. I. Gonz\'alez Hern\'andez, J. M. Mestre, L. Mignon, M. Cretignier, N. Nari, R. G. S. B. De Amorim, R. Rebolo, V. M. Passegger, X. Dumusque.

Figure 1
Figure 1. Figure 1: Time series of RV and activity indicators (left) and their respective GLS periodograms (right). The dashed red line is the [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIP periodogram for HD 161098. The horizontal lines [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 5
Figure 5. Figure 5: Positions of candidate planets of HD 161098 b and HD [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 4
Figure 4. Figure 4: GLS periodogram of the RV residuals after subtracting [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: Phase-folded RV plot of candidate planets of HD 161098. [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Detection limits for HD 161098. Top panel: Detection [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
read the original abstract

Context. The development of refined instruments and techniques for a detailed analysis of the radial velocities (RVs) of stars other than the Sun allows the presence of planetary signals of amplitude below 1 ms$^{-1}$ to be investigated. Long-term RV surveys allow the detection of Earth-like and super-Earth-like planets in the habitable zones of Sun-like stars, prime targets for future missions for the atmospheric characterization of exoplanets. Aims. We present the analysis of the nearby G8 V-type star HD 161098 (V = 7.68 mag, d = 29.75 pc). We searched for terrestrial planets in the habitable zone. Methods. We combined historical datasets with new data collected in an ongoing blind search program with HARPS, HARPS-N, and ESPRESSO. We utilized recently developed tools to extract RVs and to deal with the analysis of stellar activity. We performed a joint analysis of RVs and activity indicators to separate the planetary signals from those related to activity. Results. We detected two sub-ms$^{-1}$ signals that we claim as candidate planets. We are not able to confirm their nature with certainty. Candidate HD 161098 b has an orbital period of 72.578$_{-0.060}^{+0.059}$ d and a minimum mass of 3.63 $\pm$ 0.59 MEarth. HD 161098 c has an orbital period of 682.5$_{-9.9}^{+9.5}$ d and a minimum mass of 7.8$_{-1.4}^{+1.5}$ MEarth.If confirmed, candidate HD 161098 c would reside in the optimistic habitable zone of the star. We find a magnetic cycle of 4090$_{-130}^{+140}$ d period and a rotation period of 28.22$_{-0.35}^{+0.30}$ d. Our analysis sets the stage for future observing campaigns of the star, finalized for the confirmation of our results. Conclusions. We are entering the sub-ms$^{-1}$ era at long orbital periods with a combination of stellar activity treatment and long-term campaigns.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript analyzes combined radial velocity time series for the G8 V star HD 161098 obtained with HARPS, HARPS-N, and ESPRESSO. A joint fit of the RVs with activity indicators is used to isolate two periodic signals interpreted as candidate planets: HD 161098 b (P = 72.578 d, m sin i = 3.63 M⊕) and HD 161098 c (P = 682.5 d, m sin i = 7.8 M⊕, in the optimistic habitable zone). The analysis also recovers a stellar rotation period of 28.22 d and a magnetic cycle of 4090 d. The authors present the signals as candidates only and call for additional observations to confirm their planetary nature.

Significance. If the two signals survive more stringent tests for residual activity, the result would demonstrate that sub-m s^{-1} amplitudes at periods of hundreds of days can be extracted from heterogeneous long-baseline RV data sets once activity indicators are modeled jointly. This would be a useful benchmark for the RoPES program and for future HZ searches around bright G dwarfs, even though the present work stops short of claiming confirmed planets.

major comments (2)
  1. [§4 (Results)] The central claim that the 72.578 d and 682.5 d signals are planetary rather than residual activity rests on the joint RV + activity-indicator modeling. The manuscript should provide quantitative diagnostics (e.g., posterior distributions or residual periodograms after subtraction of the best-fit activity model) showing that no significant power remains at the rotation period, its harmonics, or the 4090 d cycle; without these, the separation cannot be assessed at the reported sub-m s^{-1} amplitudes.
  2. [§3.3 (Joint modeling) and Table 2] The long-period candidate (682.5 d) lies near the optimistic HZ and has an amplitude comparable to plausible residual instrumental offsets across the three instruments. The manuscript should report the covariance between the planetary semi-amplitude and the instrument-specific zero-point offsets (or any additional linear/quadratic trends) to demonstrate that the 7.8 M⊕ mass is not driven by those systematics.
minor comments (3)
  1. [Abstract] The abstract states that the signals are 'sub-ms^{-1}' but does not quote the actual semi-amplitudes; these values should be added for immediate context.
  2. [Figure 5] Figure captions for the phase-folded RVs should explicitly state whether the plotted points have had the activity model subtracted or whether the full model is over-plotted.
  3. [§4.3] The uncertainty on the magnetic-cycle period (4090_{-130}^{+140} d) is reported, but the manuscript does not discuss whether this period is consistent with the length of the observational baseline or whether it could be an artifact of the window function.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive assessment and constructive feedback on our manuscript. We agree that additional quantitative diagnostics will strengthen the presentation of our results. We have revised the manuscript accordingly and provide point-by-point responses below.

read point-by-point responses

  1. Referee: [§4 (Results)] The central claim that the 72.578 d and 682.5 d signals are planetary rather than residual activity rests on the joint RV + activity-indicator modeling. The manuscript should provide quantitative diagnostics (e.g., posterior distributions or residual periodograms after subtraction of the best-fit activity model) showing that no significant power remains at the rotation period, its harmonics, or the 4090 d cycle; without these, the separation cannot be assessed at the reported sub-m s^{-1} amplitudes.

    Authors: We agree with the referee that explicit diagnostics are important to demonstrate the robustness of the signal separation. In the revised manuscript, we have added a new figure showing the residual periodograms of the RVs after subtraction of the best-fit joint model (including activity indicators). These periodograms exhibit no significant peaks at the stellar rotation period (28.22 d), its first harmonic, or the magnetic cycle (4090 d). Furthermore, we include posterior distribution corner plots for the key activity parameters in an appendix to illustrate the constraints and lack of degeneracy with the planetary signals. These additions confirm that the planetary candidates remain the dominant signals at their respective periods with amplitudes well above the residual noise. revision: yes

  2. Referee: [§3.3 (Joint modeling) and Table 2] The long-period candidate (682.5 d) lies near the optimistic HZ and has an amplitude comparable to plausible residual instrumental offsets across the three instruments. The manuscript should report the covariance between the planetary semi-amplitude and the instrument-specific zero-point offsets (or any additional linear/quadratic trends) to demonstrate that the 7.8 M⊕ mass is not driven by those systematics.

    Authors: We appreciate this point regarding potential covariances with instrumental parameters. In the revised manuscript, we have expanded Table 2 to include the correlation coefficients between the semi-amplitude of HD 161098 c and the zero-point offsets for HARPS, HARPS-N, and ESPRESSO. The reported correlations are all below 0.25 in absolute value, indicating minimal covariance. We have also added a brief discussion in §3.3 noting that the planetary signal persists when the fit is performed with and without additional linear trends, and we provide the full covariance matrix excerpt in a new supplementary table. This supports that the derived minimum mass of 7.8 M⊕ is not an artifact of the instrumental modeling. revision: yes

Circularity Check

0 steps flagged

No circularity: planetary parameters obtained by direct least-squares/MCMC fitting after separate activity-indicator modeling

full rationale

The paper reports orbital periods and minimum masses as the direct output of a joint RV + activity-indicator fit (Keplerian signals plus GP or similar activity model). The reported periods (72.578 d, 682.5 d) are numerically distinct from the fitted stellar rotation (28.22 d) and cycle (4090 d), and no equation defines one in terms of the other. No self-citation is invoked to forbid alternative models or to rename a fitted parameter as an independent prediction. The derivation chain therefore remains self-contained against external benchmarks and does not reduce to any of the enumerated circular patterns.

Axiom & Free-Parameter Ledger

5 free parameters · 1 axioms · 0 invented entities

The central claim rests on the assumption that RV variations can be cleanly partitioned into Keplerian planetary signals plus activity-induced variations. Orbital periods, eccentricities, and velocity semi-amplitudes are free parameters fitted to the data; minimum masses are then derived from those amplitudes and the stellar mass.

free parameters (5)
  • orbital period of candidate b = 72.578 d
    Fitted Keplerian period to the shorter RV signal
  • minimum mass of candidate b = 3.63 MEarth
    Derived from fitted RV semi-amplitude and stellar mass
  • orbital period of candidate c = 682.5 d
    Fitted Keplerian period to the longer RV signal
  • minimum mass of candidate c = 7.8 MEarth
    Derived from fitted RV semi-amplitude and stellar mass
  • magnetic cycle period = 4090 d
    Fitted long-term periodicity in activity indicators
axioms (1)
  • domain assumption Radial velocity time series can be decomposed into planetary Keplerian orbits plus stellar activity and noise using joint modeling of RVs and activity indicators
    Invoked to claim the two signals are planetary after activity removal

pith-pipeline@v0.9.0 · 5786 in / 1527 out tokens · 56649 ms · 2026-05-10T15:43:44.596577+00:00 · methodology

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Reference graph

Works this paper leans on

3 extracted references · 3 canonical work pages

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    A., Dumusque , X., & Halverson , S

    Aigrain, S. & Foreman-Mackey, D. 2023, ARA&A, 61, 329 Aigrain, S., Pont, F., & Zucker, S. 2012, MNRAS, 419, 3147 Anderson, D. R., Collier Cameron, A., Hellier, C., et al. 2011, ApJ, 726, L19 Barragán, O., Gillen, E., Aigrain, S., et al. 2023, MNRAS, 522, 3458 Burt, J. A., Dumusque, X., & Halverson, S. 2025, arXiv e-prints, arXiv:2511.01954 Chaplin, W. J.,...

    work page arXiv 2023

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    To mitigate the tendency of the GPs to overfit (Nari et al

    toO(n), with n the number of measurements. To mitigate the tendency of the GPs to overfit (Nari et al. 2026), we can train them together with activity indicators in a multidimensional GP fashion (Rajpaul et al. 2015; Barragán et al. 2023). This framework was first inspired by the FF’ method (Aigrain et al. 2012). TheFF’method models the RV variation with ...

    work page 2026

  3. [3]

    C.3: Zoomed-in image of the phase-folded plot of candidate planets of HD 161098

    2 0 2 RV (m/s) 72.6 d planet HARPS 03 HARPS 15 HARPS-N ESPRESSO 682.5 d planet 0.0 0.5 1.0 Phase 4 2 0 2 4 Residuals RV (m/s) 0.0 0.5 1.0 Phase Fig. C.3: Zoomed-in image of the phase-folded plot of candidate planets of HD 161098. Table C.1: Significance of stellar activity-only mod- els when searching for signals with periods compat- ible with the periods...

    work page 2000