The effect of stellar magnetic activity on measurements of morning and evening asymmetry of planetary terminator

A.I. Shapiro; Ch. Helling; J. de Wit; L. Carone; L. Gizon; N. Kostogryz; S. Kiefer; S.K. Solanki; S. Mercier; S. Seager

REVIEW 3 major objections 4 minor 31 references

The effect of stellar magnetic activity on measurements of morning and evening asymmetry of planetary terminator

T0 review · 3 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read Magnetic activity on a host star can create up to 600 ppm of false asymmetry in a 10,000 ppm transit light curve.

desk verdict The effect is real and would matter if the assumed limb magnetisation contrast holds, but the 600 ppm headline is not yet backed by data. read the letter →

arxiv 2507.16451 v1 pith:CEWZ6GST submitted 2025-07-22 astro-ph.SR

N. Kostogryz , A.I. Shapiro , L. Carone , L. Gizon , Ch. Helling , S. Kiefer , S. Mercier , S. Seager

show 4 more authors

S.K. Solanki Y. Unruh J. de Wit V. Witzke

This is my paper · ORCID

classification astro-ph.SR

keywords stellarmagneticactivitytransitlightcurveasymmetrymorning-eveningterminatorcontrastlimbdarkeningfaculaeexoplanettransmissionspectroscopybrighteningWASP-39b

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper identifies a stellar mechanism that can masquerade as morning-evening asymmetry in exoplanet transit light curves: small-scale magnetic fields brighten the stellar limb relative to the non-magnetic case, and when the star's eastern and western limbs are magnetized differently, the ingress and egress depths differ. Using three-dimensional radiative magnetohydrodynamic simulations of solar-like atmospheres, the authors find the effect can reach about 600 parts per million (ppm) for a 10,000 ppm transit at 0.6 µm, commensurate with the planetary signals observers currently seek. Because the stellar asymmetry falls toward infrared wavelengths while planetary asymmetries have a different spectral shape, the paper argues the two can be separated by comparing transit depths across wavelengths. The conclusion that even photometrically quiet stars can produce such asymmetries changes what can be safely assumed about a host star when interpreting transit observations.

What carries the argument

The load-bearing object is the limb-brightness contrast of magnetized small-scale surface concentrations, computed from 3D radiative magnetohydrodynamic simulations of stellar atmospheres with ray-by-ray spectral synthesis. Magnetic fields corrugate the stellar surface and expose bright walls near the limb, making magnetic regions brighter there than the quiet photosphere, which flattens the limb darkening; the paper then combines half-disk light curves with different magnetization levels to obtain asymmetric transits. The second element of the machinery is the wavelength dependence: because the Planck function's sensitivity to temperature changes decreases toward the infrared, the magnetic asymmetry shrinks at longer wavelengths, and this spectral signature is the paper's proposed tool for separating stellar from planetary asymmetry in observations.

What would settle it

Measure ingress-egress depth differences over several transits of a solar-like star while simultaneously imaging its surface magnetic field (for example with Zeeman Doppler imaging), and check whether the sign and amplitude of the asymmetry at about 0.6 µm track the inferred difference in magnetic flux between the eastern and western near-limb regions; a clear absence of correlation would falsify the mechanism.

Watch

Extended reading notes

Core claim

The paper's central claim is that stellar magnetic activity, through its effect on limb darkening, can generate an ingress-egress asymmetry in transit light curves that is independent of any planetary atmosphere. The asymmetry is driven by small-scale magnetic field concentrations, which appear as bright patches in intergranular lanes and whose brightness contrast grows toward the limb; a difference in magnetization between the two sides of the star near the transit chord therefore makes one half of the transit deeper than the other. In a 10,000 ppm transit at 0.6 µm, the computed ingress-egress depth difference reaches up to 600 ppm, grows nearly linearly with the east-west magnetization difference, and does not depend strongly on the absolute magnetization level. The signal peaks in the visible and weakens toward the infrared, following the temperature sensitivity of the Planck function, and the paper shows for the WASP-39b case that this wavelength dependence is distinct enough from the planetary morning-evening signal to allow disentangling the two. The authors also note that the relevant small-scale fields often form without spots and without detectable photometric variability, so a photometrically quiet star is not automatically safe from this contamination.

Load-bearing premise

The quantitative amplitude rests on assuming that the simulated magnetized patches, drawn from atmosphere boxes roughly nine million meters across with imposed field strengths of 100 to 300 gauss, faithfully represent real stellar limb regions, and that the two sides of the star can differ in near-limb magnetization by several hundred gauss.

Editorial extensions

If this is right

An ingress-egress asymmetry in a transit light curve can no longer be treated as purely planetary; a stellar contribution of hundreds of ppm must be considered in the visible.
Photometrically quiet host stars are not exempt, since the relevant small-scale magnetic features often appear without spots and without detectable broad-band variability.
The stellar contamination can either amplify or mute the apparent planetary morning-evening temperature contrast, depending on which stellar limb is more magnetized during a given transit.
For the WASP-39b system, the observed 4.4 µm asymmetry is unlikely to be explained by stellar magnetic activity alone, but the stellar signal can be comparable to the planetary signal at visible and near-infrared wavelengths.
Averaging over many transits reduces the activity signal roughly as the inverse square root of the number of transits, except for tilted non-equatorial orbits, where the chord crosses different stellar latitudes at ingress and egress and the asymmetry persists.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If the computed amplitudes hold, retrieval codes for high-precision transmission spectroscopy should fit a stellar limb-asymmetry component jointly with the planetary signal rather than subtracting a disk-averaged stellar baseline.
The same mechanism should affect any measurement that compares stellar flux on either side of a transit chord, so combined analysis of ingress-only and egress-only spectra could constrain the east-west magnetization difference of the host star.
A natural extension is to map the effect across spectral type and activity level, since the paper's models are solar-like; M dwarfs, with their different facular contrasts, could show a different wavelength dependence that either helps or complicates the disentangling.
Observing the same planet at multiple wavelengths and fitting the two components simultaneously would test the proposed disentangling directly, and transit timing variations could provide an additional constraint on the limb magnetization asymmetry.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Two things to know. First, this is the first paper to argue that an east-west difference in stellar limb magnetisation produces an asymmetry between transit ingress and egress, distinct from the usual unocculted-feature contamination. That is a real and useful idea, and the wavelength-dependence plot against WASP-39b gives observers a concrete way to separate stellar from planetary asymmetry. Second, the quantitative headline—up to 600 ppm at 0.6 µm for a 10,000 ppm transit—does not yet have solid ground under it. The stress-test note has it right: the model assumes an east-west near-limb flux-density difference of a few hundred gauss and asserts this is realistic without providing magnetogram statistics or a solar-calibrated distribution. If the real difference is a few tens of gauss, the effect drops to a few tens of ppm, which would not 'significantly interfere' with the WASP-39b signal.

What is new: prior contamination work treated spots and faculae as broadband modulations of the whole transit; here the asymmetry is between the two limbs, and it uses 3D MHD limb-darkening models from Kostogryz et al. 2024, not 1D models. The forward model is clean: MURaM boxes with imposed vertical fields, MPS-ATLAS spectra, and a geometric transit. No free parameters are fitted to the target result, and the code is established in previous work. The paper also honestly flags that a global hemispheric contrast may be rare and shifts the argument to near-limb regions.

The weak point is exactly where the stress-test sits. Section 2 says the near-limb difference 'can readily reach several hundred Gauss' and refers to Sect. 3, but Sect. 3 gives only a typical facular field strength, not a distribution of east-west differences along a transit chord. The 300 G | SSD case is the only one used in the WASP-39 comparison, and it is called 'somewhat extreme but still realistic'—that qualification is doing a lot of work. None of the MURaM-derived brightness contrasts carry error bars, and the four magnetisation levels don't sample the parameter space of filling factors or limb geometry. Since the asymmetry scales nearly linearly with the field difference, the uncertainty in that contrast maps directly onto the headline amplitude.

Two smaller comments. The claim that impact parameter has no effect on ingress/egress asymmetry is asserted without demonstration; it is probably true for the contact-depth difference in a symmetric model, but it deserves a derivation. And the WASP-39 comparison uses one G9 MURaM model plus one 3D climate model; the claim that stellar and planetary wavelength dependencies are distinguishable is made for that example, not for all stars.

Who this is for: anyone working on JWST ingress-egress asymmetry measurements. It deserves a serious referee, because even if the amplitude is unproven, the mechanism should be on the table and the disentangling proposal is testable. Send it out, but ask the authors to calibrate the field-contrast assumption with solar or stellar magnetogram statistics and to add sensitivity studies across the plausible parameter range.

Referee Report

3 major / 4 minor

Summary. This paper proposes that small-scale stellar magnetic fields, by brightening the stellar limb relative to the non-magnetic case, can introduce an asymmetry between the ingress and egress of a planetary transit light curve. Using MURaM magnetoconvection simulations and MPS-ATLAS spectral synthesis, the authors model a star with different magnetization levels on the eastern and western hemispheres and find ingress-egress depth differences up to about 600 ppm for a 10,000 ppm transit at 0.6 µm. They compare this stellar signal with the morning-evening terminator asymmetry expected for WASP-39b and argue that the two have distinct wavelength dependencies, which could be used to disentangle them.

Significance. The paper identifies a genuinely new contribution to transit asymmetry that is not captured by standard 1D stellar atmosphere models and that is distinct from the usual unocculted-spot contamination. The modeling is based on established 3D MHD simulations and ray-by-ray spectral synthesis, and the forward-model nature of the calculation means there is no fitting to the target result. The proposed wavelength-dependence discriminant is a useful, falsifiable prediction. However, the quantitative amplitude of the effect is highly sensitive to the assumed magnetization contrast between the two stellar limbs, which is not observationally calibrated in this work; this limits the immediate certainty of the 'significant interference' claim.

major comments (3)

[Section 2, Figure 3] The claim that near-limb east-west field differences 'can readily reach several hundred Gauss' lacks any supporting magnetogram statistics or reference. Since the computed asymmetry is nearly linear in the magnetization difference (as seen in Figure 2b), the headline 600 ppm value scales directly with this assumed contrast. If the typical contrast is only a few tens of gauss, the signal would be reduced to roughly the 50 ppm level, which is below the planetary signals of interest. Please provide quantitative constraints from solar or stellar magnetograms, or re-frame the result as a parametric scaling with the unknown field contrast.
[Section 2] The two-hemisphere model assigns a uniform magnetization to each half of the stellar disk, but the transit asymmetry is set by the field distribution along the ingress/egress chord near the limb. The paper does not demonstrate that a hemispheric average correctly represents the near-limb region, where the magnetic brightening is strongest. A localized active region of finite size near one limb would produce a different asymmetry amplitude than a uniformly magnetized hemisphere. Please test the sensitivity of the amplitude to the spatial scale and location of the magnetized region, or provide a justification for why the hemispheric approximation is adequate.
[Sections 2 and 3] The amplitudes reported (e.g., 600 ppm) are not accompanied by any uncertainty estimates. The manuscript does not state how many MURaM snapshots or time averages are used for each magnetization level, nor how the results vary across realizations. Given that the 9x9 Mm simulation boxes are small and the magnetic field configuration evolves, a discussion of snapshot-to-snapshot variability is needed to assess the robustness of the predicted amplitude.

minor comments (4)

[Section 2, Figure 1] The references to 'Figure 1, right panel' and 'Figure 1, left panel' appear to be swapped relative to the caption, which identifies the left panel as quiet (SSD) and the right panel as active (300 G). Please correct the cross-references.
[Figure 4 caption] The caption states 'The orange and blue lines show planetary asymmetry', but the stellar curve is also described as blue, making the color assignment ambiguous. Please specify distinct line styles and colors for each curve.
[Section 2] The phrase 'To pinpoint the asymmetry of these transit light curves' might be more clearly rendered as 'To isolate the asymmetry' or 'To display the asymmetry'.
[Section 3] The reference to Frazier (1971) for the typical field strength in solar faculae is dated; consider citing more recent measurements if available, though this does not affect the conclusions.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: forward model with independently grounded inputs; only a minor self-citation for the limb-brightening input.

full rationale

The paper's chain is a forward model: (i) MURaM/MPS-ATLAS simulations yield limb intensity profiles for quiet (SSD) and magnetized (100/200/300 G) stellar atmospheres; (ii) these profiles are assigned to eastern and western hemispheres; (iii) transit light curves are computed by integrating the stellar disk. The resulting ingress–egress asymmetry is the sensitivity of the model to the assumed magnetization contrast, not a fitted quantity. No equation is defined in terms of the target asymmetry; the 600 ppm value is conditional ('can reach up to ... depending on the magnetization'), and Section 2 explicitly notes the asymmetry increases almost linearly with the imposed field difference. The claim that near-limb fields can differ by several hundred Gauss is asserted rather than calibrated, and Section 2 concedes hemispheric contrasts 'may be rare'; this is a realism/calibration concern, not circularity, because the derivation does not presuppose the conclusion. The main self-citation, Kostogryz et al. (2024), supplies the limb-darkening input; that prior work is an independent MHD simulation study with published validation against solar observations, so relying on it is standard use of prior evidence rather than a self-referential reduction. The planetary comparison uses published climate models, not fitted outputs. No pattern of self-definitional reasoning, fitted-input-called-prediction, imported uniqueness, ansatz-smuggling via citation, or renaming of known results is present. Score 1 reflects the minor load-bearing self-citation for the input limb-brightening effect, which is nevertheless externally grounded.

Assumptions & free parameters 3 free parameters · 5 assumptions · 0 invented entities

The paper introduces no new physical entities. Its central claim depends on representative magnetic field values and on the fidelity of MURaM and MPS-ATLAS simulations; these are modeling assumptions rather than fitted parameters, but they are not independently constrained within this paper.

free parameters (3)

magnetization difference between limbs = 0-300 G (SSD vs 300 G)
The amplitude of the asymmetry scales with the difference in imposed vertical magnetic field between the eastern and western half-disks; the chosen values (100/200/300 G vs SSD) are representative and determine the 600 ppm headline.
transit depth normalization = 10,000 ppm (rp/R*=0.1)
The reported asymmetry is quoted for a 10,000 ppm transit; the absolute ppm value scales with the planet-to-star radius ratio.
G9 stellar model parameters = Teff=5250 K, logg=4.4, M/H=0.0
These parameters are adopted for the WASP-39 comparison and are not varied or fitted; the resulting asymmetry amplitude depends on the specific stellar model.

assumptions (5)

domain assumption MURaM 3D radiative MHD simulations with box-in-a-star setup reproduce the magnetic effect on stellar limb darkening.
Invoked in Section 2; the central result relies on these simulated brightness profiles being accurate, especially near the limb.
domain assumption MPS-ATLAS ray-by-ray spectral synthesis provides accurate limb spectra for the simulated atmospheres.
Used to convert MURaM snapshots to wavelength-dependent intensities in Section 2.
domain assumption Small-scale dynamo (SSD) simulations represent the quiet stellar surface.
The quiet (SSD) case serves as the reference for computing the magnetic brightening (Section 2).
domain assumption Each stellar hemisphere can be represented by a single MURaM snapshot with a homogeneous mean vertical field.
This idealization is the basis for combining half-disks into asymmetric light curves (Section 2).
domain assumption The WASP-39b planetary asymmetry models of Carone et al. 2023 (with CH4 omitted) adequately represent the true planetary signal for the comparison.
Used in the WASP-39 example to argue that stellar and planetary signals can be disentangled (Section 2).

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Cite this review

Pith. "Pith review of The effect of stellar magnetic activity on measurements of morning and evening asymmetry of planetary terminator." pith.science (2026). https://pith.science/paper/CEWZ6GST

@misc{pith2026250716451,
  author       = {Pith},
  title        = {Pith review of: The effect of stellar magnetic activity on measurements of morning and evening asymmetry of planetary terminator},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/CEWZ6GST}},
  note         = {Machine review of arXiv:2507.16451}
}

read the original abstract

Differences in the ingress and egress shapes of transit light curves can indicate morning-evening temperature contrasts on transiting planets. Here, we pinpoint an alternative mechanism that can introduce asymmetries in transit light curves, potentially affecting the accurate determination of morning-evening differences. Small-scale magnetic field concentrations on the surfaces of the host star affect the visibility of stellar limb regions, making them brighter relative to the non-magnetic case. A difference in magnetization between the star's western and eastern limbs can thus create an asymmetry in limb brightness and, consequently, an asymmetry between transit ingress and egress. We model the limb darkening and stellar limb asymmetry in solar-like stars using the 3D radiative MHD code MURaM to simulate magnetized stellar atmospheres and the MPS-ATLAS code to synthesize spectra using ray-by-ray approach. Our results show that ingress-egress depth differences can reach up to 600 ppm for a 10000 ppm transit at 600 nm, depending on the magnetization of the stellar limbs--significantly interfering with planetary signals. Observations of the Sun show that such concentrations are often not accompanied by spots and do not manifest in photometric variability, indicating that even photometrically quiet stars can produce such asymmetries. However, planetary and stellar asymmetries exhibit distinct wavelength dependencies, which we propose to leverage for disentangling them.

Figures

Figures reproduced from arXiv: 2507.16451 by the authors.

**Figure 2.** Asymmetry of the transit light curve caused by asym [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Magnetically-induced asymmetry of the transit li [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

Figures from the paper (1 more)

**Figure 4.** Figure 4: Planet vs. stellar asymmetry for the illustrative [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]

Discussion (0). Continue with ORCID to comment.

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Pith tools

Reviewed August 6, 2026 · model on record in the stance chip above.

[1] [1]

2021, An Introduction to Stellar Magnetic Activity, doi: 10.1088/2514-3433/ac2956

Basri, G. 2021, An Introduction to Stellar Magnetic Activity, doi: 10.1088/2514-3433/ac2956

doi:10.1088/2514-3433/ac2956 2021

[2] [2]

S., Cameron, R

Bhatia, T. S., Cameron, R. H., Solanki, S. K., et al. 2022, A&A, 663, A166, doi: 10.1051/0004-6361/202243607

doi:10.1051/0004-6361/202243607 2022

[3] [3]

Buehler, D., Lagg, A., & Solanki, S. K. 2013, A&A, 555, A33, doi: 10.1051/0004-6361/201321152

doi:10.1051/0004-6361/201321152 2013

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2023, arXiv e-prints, arXiv:2301.08492, doi: 10.48550/arXiv.2301.08492

Helling, C. 2023, arXiv e-prints, arXiv:2301.08492, doi: 10.48550/arXiv.2301.08492

work page doi:10.48550/arxiv.2301.08492 2023

[5] [5]

Danilovic, S., Sch¨ ussler, M., & Solanki, S. K. 2010, A&A, 513, A1, doi: 10.1051/0004-6361/200913379 de Wit, J., Gillon, M., Demory, B. O., & Seager, S. 2012, A&A, 548, A128, doi: 10.1051/0004-6361/201219060

doi:10.1051/0004-6361/200913379 2010

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E., Kirk, J., et al

Espinoza, N., Steinrueck, M. E., Kirk, J., et al. 2024, Nature, 632, 1017, doi: 10.1038/s41586-024-07768-4

doi:10.1038/s41586-024-07768-4 2024

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Faedi, F., Barros, S. C. C., Anderson, D. R., et al. 2011, A&A, 531, A40, doi: 10.1051/0004-6361/201116671

doi:10.1051/0004-6361/201116671 2011

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work page doi:10.1007/bf00155772 1971