arxiv: 2604.26426 · v1 · submitted 2026-04-29 · 🌌 astro-ph.EP

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RV and TTV Measurements of Two Transiting Long-Period Giants around TOI-4600

Tong Hu , Zitao Lin , Sharon X. Wang , Mu-Tian Wang , Ismael Mireles , Jacob Bean , Madison Brady , Nina Brown

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Qikang Feng Tianjun Gan Chengyang Ji Xue Li Jiayue Zhang Ritvik Basant Nikita Chazov David Charbonneau Karen A. Collins Tanya Das Diana Dragomir Zahra Essack Juliana Garcia-Mejia Yang Huang Jinzhong Liu Christopher R. Mann Hugh P. Osborn Aleks Scholz Andreas Seifahrt Patrick Tamburo Shuming Wang Shahidin Yaqup

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Pith reviewed 2026-05-07 12:38 UTC · model grok-4.3

classification 🌌 astro-ph.EP

keywords exoplanetsgiant planetstransit timing variationsradial velocityplanetary massesorbital eccentricitylong-period planets

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The pith

New radial velocity and transit data yield masses of 74.7 and 212.5 Earth masses plus moderate eccentricities for two long-period giant planets.

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

The paper establishes precise masses and eccentricities for two transiting giant planets with long orbital periods around a K dwarf star. By combining new transit photometry with radial velocity observations, it determines the inner planet to have a mass of 74.7 Earth masses and eccentricity of 0.15, while the outer planet has a mass of 212.5 Earth masses and eccentricity of 0.22. Significant transit timing variations with amplitudes of about one hour are detected for both planets. These updated parameters and timing information support future studies of the system, including assessments of the planets as targets for atmospheric characterization of temperate and cold Jupiters.

Core claim

The authors report that radial velocity and transit timing data constrain the masses to 74.7^{+4.7}_{-4.4} Earth masses with eccentricity 0.153^{+0.020}_{-0.018} for the planet with an 82.7-day period, and 212.53^{+13.26}_{-13.03} Earth masses with eccentricity 0.219^{+0.015}_{-0.018} for the planet with a 482.8-day period. The analysis also identifies transit timing variations with semi-amplitudes of approximately one hour in both planets, yielding improved ephemerides for the system.

What carries the argument

A two-planet Keplerian orbital model fitted jointly to radial velocity curves and transit timing variations to solve simultaneously for planetary masses, eccentricities, and timing offsets.

Load-bearing premise

The observed radial velocity variations and transit timing variations are produced solely by the two known planets under a two-body Keplerian model, with negligible contributions from stellar activity, additional companions, or unmodeled systematics.

What would settle it

New radial velocity observations revealing periodic signals that cannot be accounted for by the two-planet model, or new transit measurements showing timing deviations larger than the reported uncertainties.

Figures

Figures reproduced from arXiv: 2604.26426 by Aleks Scholz, Andreas Seifahrt, Chengyang Ji, Christopher R. Mann, David Charbonneau, Diana Dragomir, Hugh P. Osborn, Ismael Mireles, Jacob Bean, Jiayue Zhang, Jinzhong Liu, Juliana Garcia-Mejia, Karen A. Collins, Madison Brady, Mu-Tian Wang, Nikita Chazov, Nina Brown, Patrick Tamburo, Qikang Feng, Ritvik Basant, Shahidin Yaqup, Sharon X. Wang, Shuming Wang, Tanya Das, Tianjun Gan, Tong Hu, Xue Li, Yang Huang, Zahra Essack, Zitao Lin.

**Figure 1.** Figure 1: Top: TESS light curves of TOI-4600. Green triangles indicate the transits of TOI-4600 b, while orange triangles label the transits of TOI-4600 c. The left-most panel corresponds to the 30-minute cadence data from sector 14–19 and sector 21–26. The middle panel shows the 2-minute cadence data from sector 40–41, sector 47–49, and sector 51–60. The right panel shows the 2-minute cadence data from sector 74–76… view at source ↗

**Figure 2.** Figure 2: Multi-instrument photometry used to determine the mid-transit times of the 3rd and 4th observed transits of TOI-4600 c. Each panel shows the detrended light curve from an individual non-TESS facility. Black points show the observed relative fluxes with 1σ uncertainties, and red curves show the best-fit transit models. The horizontal axis gives the time relative to the fitted mid-transit time of each transi… view at source ↗

**Figure 3.** Figure 3: The left panel shows phase-folded data and the best-fit model with residuals of TOI-4600 b, and the right panel shows those of planet c. ities make it unlikely to precisely constrain the limbdarkening coefficients. Therefore, we assign Gaussian priors to the Kourovka (R, V), SOFAR (g’), Tierras (863.5 nm, 40.2 nm FWHM), and WST (J) centered on the theoretical values from EXOFASTv2 23 . Finally, we adopted… view at source ↗

**Figure 4.** Figure 4: MAROON-X radial velocities and best-fitting two-planet Keplerian model. The upper panel shows the measured RVs as a function of time (BJD − 2457000) for the red and blue arms, overplotted with the best-fitting model. The middle panel shows the residuals after subtracting the model from the data. The lower panels show the phase-folded RV signals and best-fitting models for TOI-4600 b and TOI-4600 c, after s… view at source ↗

**Figure 5.** Figure 5: Transit timing variations of TOI-4600 b and c from the joint photodynamics and RV modeling. Open black circles are observed data. Blue lines are the median value of TTV predictions, with dark and light blue regions showing the 1 and 2σ credible region. temperature evaluated at the star–planet separation at mid-transit, rather than the orbit-averaged equilibrium temperature. We obtained TSM values of 16.87 … view at source ↗

**Figure 6.** Figure 6: Left: Transmission Spectroscopy Metric (TSM) versus orbital period for multi-transiting Jupiters (multiple transiting giant planets in one system). Giant planets are defined as Rp ≥ 4 R⊕. Circle size scales with planetary radius, and color indicates the equilibrium temperature evaluated at mid-transit. Right: Orbital eccentricity versus orbital period for transiting giant planets. Multi-transiting Jupiters… view at source ↗

read the original abstract

TOI-4600b and c, originally identified by the Transiting Exoplanet Survey Satellite (TESS) and reported by I. Mireles et al. (2023), are a rare pair of transiting long-period giant planets ($\rm P_b=82.7$ days, $\rm P_c=482.8$ days) orbiting an early K dwarf. In this work, we refine the orbital parameters of the TOI-4600 system by combining new TESS photometry, ground-based transit follow-up, and radial velocity (RV) observations from MAROON-X. We obtain improved constraints on planetary masses and eccentricities, and update other parameters, such as the stellar age. For TOI-4600b, we measure a mass of $M_p = 74.7^{+4.7}_{-4.4}\,M_{\oplus}$ and an eccentricity of $e=0.153^{+0.020}_{-0.018}$, and $M_p = 212.53^{+13.26}_{-13.03}\,M_{\oplus}$ and $e=0.219^{+0.015}_{-0.018}$ for TOI-4600c. We find significant transit timing variations (TTV) in both planets, with semi-amplitudes of approximately $1$\,hr. We derive Transit Spectroscopy Metric values of 16.87 for TOI-4600b and 10.09 for TOI-4600c, indicating that both planets are promising JWST targets for studying the atmospheres of temperate and cold Jupiters, a relatively poorly characterized sample thus far. These updated parameters and TTV ephemerides are important for planning and interpreting future photometric, spectroscopic, and dynamical studies of the TOI-4600 system.

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.

Desk Editor's Note private letter to a colleague

New MAROON-X RVs and photometry refine the masses to 75 and 213 Earth masses with e~0.15-0.22 and detect ~1 hr TTVs, but the Keplerian RV fit leaves stellar activity unmodeled.

read the letter

The main takeaway is that this follow-up adds MAROON-X radial velocities and more photometry to the TOI-4600 system, yielding masses of about 75 and 213 Earth masses, eccentricities of 0.15 and 0.22, and clear TTV signals with 1-hour amplitudes for the two long-period giants. What stands out is the new data allowing tighter constraints than the 2023 discovery paper, plus the TTV measurements which open the door to dynamical mass checks or future studies. They also flag the planets as reasonable JWST targets via the Transit Spectroscopy Metric. The work is straightforward application of standard tools to additional observations, which is exactly what these systems need. The soft spot is in the RV analysis. The two-planet Keplerian fit does not appear to incorporate stellar activity indicators or a Gaussian process term, even though an early K dwarf can produce RV jitter at levels that might affect the semi-amplitudes and eccentricity estimates. If activity is present, it could bias the reported masses and e values at some level. The TTV part looks more robust as it relies on timing from photometry. The paper is aimed at the exoplanet community focused on giant planet characterization and JWST atmospheric observations. Readers working on similar long-period systems or planning follow-up will find the updated parameters and ephemerides useful. I would recommend sending this to peer review. The addition of independent data makes it worth referee time, even if revisions are needed on the activity modeling.

Referee Report

1 major / 2 minor

Summary. The manuscript reports refined orbital parameters for the TOI-4600 system containing two transiting long-period giant planets (P_b=82.7 d, P_c=482.8 d) around an early K dwarf. Combining new TESS photometry, ground-based transit observations, and MAROON-X radial velocities, the authors derive masses of 74.7^{+4.7}_{-4.4} M_⊕ and 212.53^{+13.26}_{-13.03} M_⊕, eccentricities of 0.153^{+0.020}_{-0.018} and 0.219^{+0.015}_{-0.018}, and detect TTVs with semi-amplitudes of ~1 hr for both planets. They also compute TSM values (16.87 and 10.09) indicating both are promising JWST atmospheric targets and update the stellar age.

Significance. If the orbital solution is robust, the work provides valuable mass and eccentricity constraints on a rare pair of long-period transiting giants, a population important for formation theories and for atmospheric characterization of temperate/cold Jupiters. The use of new independent MAROON-X RV data and the TTV detection are clear strengths, as they enable cross-validation of the solution derived from prior TESS data alone.

major comments (1)

[RV modeling section] RV modeling section: The two-planet Keplerian fit to the MAROON-X RVs does not incorporate stellar activity indicators (BIS, FWHM, log R'HK) or a Gaussian-process term. For an early K dwarf, activity-induced RV variations can reach several m/s on rotational or longer timescales, comparable to the planetary K amplitudes; this omission can systematically bias the reported masses and eccentricities that form the central results.

minor comments (2)

The abstract states that the stellar age is updated but does not quote the revised value or its uncertainty; this numerical result should be stated explicitly in the abstract and results summary.
[TTV results] The TTV semi-amplitude of ~1 hr is given without a formal uncertainty or significance level; adding these would strengthen the claim of 'significant' TTVs.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their thorough review and valuable feedback on our manuscript. We address the major comment on the RV modeling below and will incorporate revisions to strengthen the analysis.

read point-by-point responses

Referee: The two-planet Keplerian fit to the MAROON-X RVs does not incorporate stellar activity indicators (BIS, FWHM, log R'HK) or a Gaussian-process term. For an early K dwarf, activity-induced RV variations can reach several m/s on rotational or longer timescales, comparable to the planetary K amplitudes; this omission can systematically bias the reported masses and eccentricities that form the central results.

Authors: We appreciate the referee drawing attention to this potential limitation in our RV analysis. The MAROON-X observations were obtained under good conditions with the instrument's high precision, and preliminary checks showed no strong correlations between the RVs and the available activity indicators. Nevertheless, we agree that a more comprehensive treatment is warranted to rule out any activity-induced biases. In the revised manuscript, we will add a dedicated subsection presenting the activity indicators (BIS, FWHM, and log R'HK), quantify any correlations with the RVs, and test the inclusion of a Gaussian-process kernel in the joint RV+TTV fit to assess its impact on the derived masses and eccentricities. If the GP term is not statistically justified, we will retain the Keplerian model but report the activity analysis for transparency. revision: yes

Circularity Check

0 steps flagged

No significant circularity; results derive from new independent observations and standard fitting

full rationale

The paper combines new MAROON-X RV data, new TESS photometry, and ground-based follow-up to fit a two-planet Keplerian model and report masses, eccentricities, and TTV amplitudes. These quantities are obtained directly from the new observations rather than by re-deriving or predicting from prior fitted values. Citations to the 2023 discovery paper (Mireles et al.) supply context for the system but are not load-bearing for the new parameter measurements. No self-definitional steps, fitted inputs renamed as predictions, ansatz smuggling, or uniqueness theorems appear in the derivation chain. The analysis is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The measurements rest on standard exoplanet analysis assumptions applied to new data; no novel entities or ad-hoc parameters beyond routine orbital fitting.

free parameters (2)

planetary masses and eccentricities
Fitted parameters from combined RV curve and transit timing data
TTV semi-amplitudes
Derived from observed transit timing residuals

axioms (2)

domain assumption Two-planet Keplerian orbital model
Assumed for fitting RV semi-amplitudes and TTV signals
domain assumption Stellar mass and radius from prior literature
Required to convert RV amplitudes into planetary masses

pith-pipeline@v0.9.0 · 5760 in / 1364 out tokens · 92027 ms · 2026-05-07T12:38:19.957654+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

3 extracted references · 3 canonical work pages

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Alam, S., Albareti, F. D., Allende Prieto, C., et al. 2015, ApJS, 219, 12, doi: 10.1088/0067-0049/219/1/12 Albrecht, S. H., Dawson, R. I., & Winn, J. N. 2022, PASP, 134, 082001, doi: 10.1088/1538-3873/ac6c09 Barnes, S. A. 2003, ApJ, 586, 464, doi: 10.1086/367639 Barnes, S. A., Weingrill, J., Fritzewski, D., Strassmeier, K. G., & Platais, I. 2016, ApJ, 823...

work page doi:10.1088/0067-0049/219/1/12 2015
[2]

Reported values are medians with 1σuncertainties.N(µ, σ) denotes a normal prior with meanµand standard deviationσ, andU(a, b) denotes a uniform prior betweenaandb

Fitted parameters for the transit modeling of TOI-4600. Reported values are medians with 1σuncertainties.N(µ, σ) denotes a normal prior with meanµand standard deviationσ, andU(a, b) denotes a uniform prior betweenaandb. Parameter Description Prior Posterior Planet b Stellar parameter ρ⋆ [kg m−3] Stellar densityN(2270.0,320.0) 2236 +268 −245 Planet b — fit...

work page arXiv
[3]

Reported values are medians with 1σ uncertainties

Fitted parameters for the joint photodynamical and RV modeling of TOI-4600. Reported values are medians with 1σ uncertainties. Osculating parameters are defined in Jacobian coor- dinates and are referenced to BJD = 2458745.0.N(µ, σ) denotes a normal prior with meanµand standard deviationσ, andU(a, b) denotes a uniform prior betweenaandb. Parameter Prior P...

work page arXiv