arxiv: 2604.11728 · v1 · submitted 2026-04-13 · 🌌 astro-ph.EP

Recognition: unknown

The GAPS Programme at TNG. LXXIII. Confirmation of the hot sub-Neptune TOI-4602 b (HD 25295 b), a key target for future atmospheric characterization

C. Di Maio , S. Benatti , D. Locci , R. Spinelli , M. Baratella , K. Biazzo , J. Maldonado , A. F. Lanza

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C. Dorn P. E. Cubillos A. Salmi A. Maggio L. Naponiello F. Marzari G. Micela V. Fardella L. Malavolta M. Damasso A. Sozzetti G. Mantovan D. Nardiello I. Carleo R. Claudi R. Cosentino M. Gonzalez D. Muthukrishna M. Pinamonti T. Zingales

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Pith reviewed 2026-05-10 16:25 UTC · model grok-4.3

classification 🌌 astro-ph.EP

keywords exoplanet confirmationsub-Neptuneradial velocityTESSradius valleyatmospheric characterizationHARPS-NTOI-4602b

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The pith

TOI-4602b is confirmed as a sub-Neptune with a radius of 2.5 Earth radii and mass of 5.5 Earth masses that retains a thin atmosphere while evolving toward a bare core.

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

The authors combine new radial velocity observations from the HARPS-N spectrograph with TESS transit photometry to measure the mass of the validated planet TOI-4602b. This yields a bulk density of 2.1 grams per cubic centimeter, low enough to indicate a small gaseous envelope surrounding a denser core. The placement immediately above the gap in planet sizes supports models of how small planets lose their outer layers over time. The bright quiet host star and high transmission spectroscopy metric position the planet as a strong candidate for detailed atmospheric studies with the James Webb Space Telescope and the Ariel mission.

Core claim

We determined that TOI-4602b is a sub-Neptune with a radius of Rp = 2.5 Rearth and a mass of Mp = 5.5 Mearth. The resulting bulk density (rho_p = 2.1 g cm^-3) and atmospheric evolution modelling suggest the planet is retaining a tenuous envelope while evolving toward a bare core, consistent with a position immediately above the radius valley. Given its bright (V = 8.4) and quiet host star and the high Transmission Spectroscopy Metric (TSM) value (140 +/- 54), TOI-4602b is a prime target for atmospheric characterization. Simulated retrievals indicate that JWST and Ariel can effectively constrain its atmospheric composition.

What carries the argument

Joint modeling of TESS transit photometry and HARPS-N radial velocity data to derive the planet mass and radius.

Load-bearing premise

The radial velocity signal is produced solely by the planet and is not significantly contaminated by stellar activity or additional unseen planets.

What would settle it

Future radial velocity measurements that yield a substantially different planetary mass or that reveal stellar activity signals at the same period as the orbit would undermine the reported mass, density, and evolutionary interpretation.

Figures

Figures reproduced from arXiv: 2604.11728 by A. F. Lanza, A. Maggio, A. Salmi, A. Sozzetti, C. Di Maio, C. Dorn, D. Locci, D. Muthukrishna, D. Nardiello, F. Marzari, G. Mantovan, G. Micela, I. Carleo, J. Maldonado, K. Biazzo, L. Malavolta, L. Naponiello, M. Baratella, M. Damasso, M. Gonzalez, M. Pinamonti, P. E. Cubillos, R. Claudi, R. Cosentino, R. Spinelli, S. Benatti, T. Zingales, V. Fardella.

**Figure 2.** Figure 2: (Top panel) Normalized TESS PDCSAP light curves from all available sectors. The out-of-transit data have been binned [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗

**Figure 3.** Figure 3: Spectroscopic time series and GLS periodogram. Left: HARPS-N spectroscopic time series analysed in this work. From [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

**Figure 4.** Figure 4: Modelling of photometric and spectroscopic time-series obtained from the joint fit assuming a circular model. (Left) Pho [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗

**Figure 6.** Figure 6: Time evolution of X-ray (5−100 Å), EUV (100−920 Å), and total XUV luminosity of TOI-4602, according to Penz et al. (2008) and the X-ray/EUV scaling by Sanz-Forcada et al. (2025) (red lines) and according to Johnstone et al. (2021) (blue lines). Uncertainties on the age and X-ray luminosity of TOI-4602 (square symbol) are also indicated, with respect to the nominal value of Lx = 1027 erg/s at 6.8 Gyr. The g… view at source ↗

read the original abstract

Precise mass and radius measurements of small, transitional exoplanets, such as super-Earths and sub-Neptunes, are essential to constrain their bulk density and formation history, serving as prerequisites for atmospheric characterization. The ArMS Large Programme, carried out within GAPS using the HARPS-N spectrograph at the Telescopio Nazionale Galileo, aims to confirm and characterize transitional planets in the radius valley through high-precision radial-velocity (RV) measurements. The ultimate goal is to identify ideal targets for atmospheric follow-up observations with next-generation facilities like the James Webb Space Telescope and the future ESA Ariel satellite. We present the first mass determination of a sub-Neptune planet using data entirely collected within the ArMS programme, focusing on the validated planet TOI-4602b. We monitored TOI-4602, which hosts a close-in validated sub-Neptune (P ~ 3.98 d) detected by the Transiting Exoplanet Survey Satellite (TESS), searching for planet-induced RV variations. We then performed a joint analysis of these RV measurements together with the TESS photometric data. We determined that TOI-4602b is a sub-Neptune with a radius of Rp = 2.5 Rearth and a mass of Mp = 5.5 Mearth. The resulting bulk density (rho_p = 2.1 ) and atmospheric evolution modelling suggest the planet is retaining a tenuous envelope while evolving toward a bare core, consistent with a position immediately above the radius valley. g cm^ -3 Given its bright (V = 8.4) and quiet host star and the high Transmission Spectroscopy Metric (TSM) value (140 +/- 54), TOI-4602,b is a prime target for atmospheric characterization. Simulated retrievals indicate that JWST and Ariel can effectively constrain its atmospheric composition, offering a unique window into the physical processes driving the sub-Neptune to super-Earth transition.

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

This paper delivers the first RV mass for TOI-4602b at 5.5 Earth masses, giving a low density that supports a thin envelope and flags the planet as a solid JWST/Ariel target, but the result rests on standard methods without new techniques.

read the letter

The paper's core contribution is the first mass measurement for the TESS-validated sub-Neptune TOI-4602b. Using HARPS-N RVs from the ArMS program plus TESS photometry, they report a radius of 2.5 Earth radii and mass of 5.5 Earth masses, yielding a bulk density of 2.1 g cm^{-3}. They then run atmospheric evolution models to argue the planet sits just above the radius valley with a tenuous envelope that is eroding toward a bare core. The host star is bright at V=8.4 and described as quiet, and they compute a high TSM of 140 that makes the target attractive for transmission spectroscopy with JWST or Ariel. Simulated retrievals suggest those facilities could constrain the atmosphere's composition. This is useful incremental data that adds one more well-characterized point to the sub-Neptune sample near the valley transition. The joint fit follows established procedures for this type of confirmation, and the evolutionary interpretation connects the numbers to broader formation questions without overclaiming. The numbers themselves look plausible for a planet in this size range. The main soft spot is the RV analysis. A semi-amplitude of roughly 1-2 m/s on a K star can be affected by spots, granulation, or an unseen companion at the 20-50% level. The abstract states the star is quiet and the model captures systematics, but without details on activity-indicator periodograms, GP kernel tests, or model comparisons, it is difficult to judge how robust the mass is. If the full paper shows those checks and they hold, the result strengthens; if not, the density and envelope claim need caution. No new methods or first-principles derivations appear, which is fine for a confirmation paper but limits broader impact. This work is aimed at observers building samples for atmospheric characterization and radius-valley studies. A reader who needs precise masses and radii for small planets with bright hosts will get direct value from the parameters and TSM. It deserves a serious referee because the target is promising and the measurement is new, though the referee should focus on the RV noise modeling and any additional planet searches. I would recommend sending it to peer review after verifying the activity analysis section.

Referee Report

2 major / 2 minor

Summary. The manuscript reports the first mass measurement of the validated sub-Neptune TOI-4602 b (HD 25295 b) from the GAPS ArMS programme. Using TESS photometry and new HARPS-N radial velocities, the authors perform a joint transit-RV fit and derive Rp = 2.5 R_earth, Mp = 5.5 M_earth and rho_p = 2.1 g cm^{-3}. They interpret the planet as retaining a tenuous envelope while evolving toward a bare core, placing it immediately above the radius valley. The bright (V = 8.4), quiet host star and high TSM (~140) are highlighted as making TOI-4602 b an excellent target for atmospheric characterization with JWST and Ariel.

Significance. If the mass measurement is robust, the result supplies a well-characterized data point in the radius-valley region that can be used to test atmospheric-evolution models. The high TSM and simulated retrievals strengthen the case for follow-up spectroscopy, directly supporting the programme's goal of identifying targets for next-generation facilities.

major comments (2)

[Abstract and RV-analysis section] The central mass (Mp = 5.5 M_earth) and density rest on the assumption that the observed RV signal is produced solely by the planet. The abstract asserts that the star is 'quiet' and that the model 'accurately captures all relevant systematics,' but provides no quantitative support (activity-indicator periodograms, correlation plots with BIS/FWHM/log R'_HK, or GP kernel comparisons). For a K ~ 1-2 m/s signal this assumption is load-bearing; any 20-30% activity contribution would shift the planet across the radius-valley boundary and alter the evolutionary interpretation.
[Joint analysis section] The joint photometric-RV fit reports point values without accompanying error budgets, covariance matrices, or model-comparison statistics (e.g., BIC/AIC between one-planet, two-planet, and activity models). These details are required to assess whether the quoted uncertainties on Mp and rho_p are realistic.

minor comments (2)

[Abstract] Abstract contains formatting errors: 'rho_p = 2.1 ) and' (extra parenthesis and misplaced unit), 'g cm^ -3' (spacing), and 'TOI-4602,b' (comma).
[Abstract] The TSM value is given as 140 +/- 54; the uncertainty should be justified or referenced to the standard TSM formula.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed report. We appreciate the emphasis on the robustness of the RV analysis and the need for full statistical transparency in the joint fit. We address each major comment below and will incorporate the requested elements in the revised manuscript.

read point-by-point responses

Referee: [Abstract and RV-analysis section] The central mass (Mp = 5.5 M_earth) and density rest on the assumption that the observed RV signal is produced solely by the planet. The abstract asserts that the star is 'quiet' and that the model 'accurately captures all relevant systematics,' but provides no quantitative support (activity-indicator periodograms, correlation plots with BIS/FWHM/log R'_HK, or GP kernel comparisons). For a K ~ 1-2 m/s signal this assumption is load-bearing; any 20-30% activity contribution would shift the planet across the radius-valley boundary and alter the evolutionary interpretation.

Authors: We agree that quantitative validation of the stellar activity level is essential for a low-amplitude RV detection. Although our internal analysis of the HARPS-N spectra showed low activity (with no significant periodic signals in the indicators at the planetary period), these supporting diagnostics were omitted from the submitted manuscript. In the revised version we will add: (i) GLS periodograms of BIS, FWHM, and log R'_HK; (ii) Spearman and Pearson correlation plots between the RVs and each activity indicator; and (iii) a direct model comparison (including BIC/AIC and posterior odds) between a pure Keplerian model and models that include a quasi-periodic GP activity component. These additions will demonstrate that any residual activity contribution is well below the 20 % level that would affect the radius-valley classification. revision: yes
Referee: [Joint analysis section] The joint photometric-RV fit reports point values without accompanying error budgets, covariance matrices, or model-comparison statistics (e.g., BIC/AIC between one-planet, two-planet, and activity models). These details are required to assess whether the quoted uncertainties on Mp and rho_p are realistic.

Authors: We acknowledge that the original submission presented only the median posterior values and 1-sigma uncertainties without the full covariance information or explicit model-selection metrics. In the revised manuscript we will include: the full posterior covariance matrix from the joint MCMC run, the complete error budget (including contributions from photometry, RV jitter, and stellar parameters), and BIC/AIC values (plus Bayes factors where applicable) for the one-planet model versus two-planet and activity-augmented alternatives. These tables and figures will allow readers to verify that the reported Mp and rho_p uncertainties are realistic and that the one-planet solution is statistically preferred. revision: yes

Circularity Check

0 steps flagged

No circularity: mass and radius are direct outputs of joint transit-RV fit to new data

full rationale

The paper reports a standard observational analysis: TESS photometry and HARPS-N radial velocities are jointly modeled to extract planetary radius from transit depth and mass from RV semi-amplitude. The resulting density and evolutionary interpretation follow directly from these measured values plus standard atmospheric models. No claimed prediction reduces by construction to a fitted parameter, no self-citation supplies a load-bearing uniqueness theorem, and no ansatz is smuggled in. The derivation chain is self-contained against external benchmarks (new observations) and does not exhibit any of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The result rests on standard exoplanet modeling assumptions and new observational data rather than ad-hoc inventions.

free parameters (1)

RV semi-amplitude and orbital elements
Fitted parameters in the joint transit-RV model that directly determine the reported planet mass.

axioms (2)

domain assumption Observed radial-velocity variations arise from a single Keplerian orbit of the known transiting planet
Invoked when attributing the RV signal entirely to TOI-4602b.
domain assumption Stellar activity contributes negligibly to the RV time series
Stated via the description of the host as quiet.

pith-pipeline@v0.9.0 · 5823 in / 1337 out tokens · 42943 ms · 2026-05-10T16:25:19.178935+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

3 extracted references

[1]

A constant model (no planet,k=1 parameter)
[2]

A polynomial trend model (linear or quadratic,k=2,3) to account for long-period companions
[3]

detected

A full Keplerian orbit model (k=6). A planet was considered "detected" via RVs if the Keple- rian model provided significantly better evidence than the poly- nomial or constant baseline models. Specifically, we required a difference∆BIC>10 (Kass & Raftery 1995), where BIC= χ2 +kln(N obs). Furthermore, planets inducing strong linear or quadratic trends (wh...

1995