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Aligned Stellar Obliquities for Two Hot Jupiter-hosting M Dwarfs Revealed by MAROON-X: Implications for Hot Jupiter Formation

T0 review · 3 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read Two more hot Jupiters around M dwarfs orbit aligned with their stars

desk verdict The two new RM detections are credible and genuinely useful; the KL population toy model is the softest part and should be framed as speculation. read the letter →

arxiv 2508.13145 v1 pith:XH5JIMVE submitted 2025-08-18 astro-ph.EP

Drew Weisserman , Erik Gillis , Ryan Cloutier , Nina Brown , Jacob L. Bean , Andreas Seifahrt , Tanya Das , Madison Brady
show 9 more authors
Bertram Bitsch Emily Deibert Thomas M. Evans-Soma Noah Fenlon Laura Kreidberg Michael Line Ralph Pudritz Evgenya L. Shkolnik Luis Welbanks
This is my paper · ORCID
classification astro-ph.EP
keywords hotJupitersMdwarfstarsstellarobliquitiesRossiter-McLaughlineffectKozai-Lidovmigrationtidaldampingradialvelocitybinary
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

Hot Jupiters, giant planets on orbits of a few days, are two to three times rarer around small, cool M dwarf stars than around Sun-like stars, suggesting a different formation pathway. The paper reports the second and third detections of the Rossiter-McLaughlin effect for hot Jupiters around M dwarfs, using transits of TOI-3714 b and TOI-5293 A b observed at high spectral resolution. Both measurements are consistent with aligned stellar obliquities: $\lambda = 21^{+14}_{-11}{}^\circ$ and $-12^{+19}_{-14}{}^\circ$. If these systems are representative, the emerging sample of aligned hot-Jupiter hosts supports the view that M dwarfs, with their deep convective envelopes, tidally realign misaligned orbits produced by high-eccentricity migration.

What carries the argument

The central mechanism is the Rossiter-McLaughlin effect, the anomalous radial-velocity distortion produced when a transiting planet occults different Doppler-shifted parts of the rotating stellar disk; its shape encodes the sky-projected obliquity $\lambda$. The forward model is computed with a spherical-harmonic-based stellar surface code, fitting $v\sin i_\star$, $\lambda$, velocity offsets, jitter, and linear trends while adopting orbital and transit priors from each planet's discovery paper. Deprojected obliquities $\psi$ follow from the geometric relation $\cos\psi = \cos i_\star \cos i_p + \sin i_\star \sin i_p \cos\lambda$, with stellar inclinations inferred probabilistically from $v\sin i_\star$, rotation periods, and radii. The dynamical context comes from Kozai-Lidov oscillations, a secular angular-momentum exchange between a planet and a wide binary companion that cycles eccentricity and mutual inclination, and from tidal obliquity damping timescales computed from convective-zone masses.

What would settle it

Re-observe a transit of TOI-3714 b or TOI-5293 A b with simultaneous high-precision space photometry and high-resolution spectroscopy: a spot-crossing anomaly in the light curve overlapping the Rossiter-McLaughlin sequence would break the clean-signal assumption and could shift $\lambda$; alternatively, one additional old hot-Jupiter-hosting M dwarf with a short tidal-damping timescale and $\psi \gtrsim 40^\circ$ would contradict the claim that M dwarfs efficiently align their hot Jupiters.

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Extended reading notes

Core claim

Measured through the Rossiter-McLaughlin effect, the projected spin-orbit angles of TOI-3714 b and TOI-5293 A b are $\lambda = 21^{+14}_{-11}{}^\circ$ and $-12^{+19}_{-14}{}^\circ$, with deprojected obliquities $\psi = 26^{+11}_{-10}{}^\circ$ and $24^{+11}_{-10}{}^\circ$; both are consistent with well-aligned orbits. The signals are detected at $7.6\sigma$ and $7.8\sigma$, establishing these as just the second and third hot Jupiters around M dwarfs with a Rossiter-McLaughlin detection. The paper argues that because a wide binary companion in each system can drive Kozai-Lidov oscillations on timescales far shorter than the system ages, while the stellar tidal damping timescales are about $10^3$\,--\,$10^4$ years, the alignment is naturally explained if these planets once had high eccentricities and inclinations that were later damped. A population-level toy model adds that Kozai-Lidov migration is more efficient around early M dwarfs than around A or FGK stars and can by itself account for the observed hot Jupiter occurrence rate around M dwarfs.

Load-bearing premise

The load-bearing assumption is that no occulted starspot or plage crossed the stellar disk during either transit; a hidden active region would distort the Rossiter-McLaughlin shape and bias the inferred obliquity, and the paper cannot rule this out without simultaneous photometry.

Editorial extensions

If this is right

  • Together with TOI-4201 b, all three hot Jupiters around M dwarfs with a Rossiter-McLaughlin detection now have obliquities consistent with zero, giving the first empirical alignment sample for this population.
  • The measured tidal damping timescales, roughly $10^3$ to $10^4$ years, are orders of magnitude shorter than the system ages, so aligned orbits are the expected endpoint even if the planets arrived via high-eccentricity migration.
  • The toy model yields Kozai-Lidov hot Jupiter formation efficiencies of $0.16^{+0.12}_{-0.06}$, $0.65^{+0.14}_{-0.12}$, and $1.01^{+0.42}_{-0.36}$ for A, FGK, and early M stars, implying the channel is fully capable of producing the M-dwarf hot Jupiter population.
  • The elevated multiplicity fraction among hot-Jupiter-hosting M dwarfs, $0.47\pm0.16$ versus $0.28\pm0.08$ for field M dwarfs, is consistent with a binary-driven formation route.
  • If tidal damping is as efficient as these numbers suggest, future Rossiter-McLaughlin measurements of other hot-Jupiter-hosting M dwarfs should continue to find aligned systems rather than a scattered obliquity distribution.

Reading between the lines

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

  • This reading implies that a genuinely misaligned hot-Jupiter-hosting M dwarf, if found, would most plausibly be young or have an unusually long damping timescale; age and stellar mass should therefore correlate with obliquity in a larger sample.
  • A direct test of the main systematic worry is available: simultaneous space-based photometry during a future transit would reveal occulted starspots, whose absence is currently assumed rather than demonstrated.
  • The toy model's neglect of in-situ formation and planet-planet scattering around M dwarfs could be tested by searching for cold Jupiter companions and inner planet populations in these systems; a firm detection of either would shift some of the inferred Kozai-Lidov efficiency to other channels.
  • The mild $2\sigma$ offsets in $\psi$ might sharpen into real misalignments if stellar rotation periods or radii are revised, so independent rotation measurements would discriminate between true alignment and a slight viewing-geometry effect.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 5 minor

Summary. This paper presents MAROON-X radial-velocity observations of transits of the hot Jupiters TOI-3714 b and TOI-5293 A b, with the goal of measuring the Rossiter-McLaughlin effect and hence the stellar obliquities of their M-dwarf hosts. The authors report highly significant RM detections (7.6σ and 7.8σ), projected obliquities λ = 21^{+14}_{-11} deg and -12^{+19}_{-14} deg, and deprojected obliquities ψ = 26^{+11}_{-10} deg and 24^{+11}_{-10} deg, concluding that both systems are well-aligned. They also refine the stellar parameters using decontaminated Ks-band photometry, constrain the wide binary orbits with Gaia DR3 astrometry, evaluate the plausibility of Kozai-Lidov migration in each system, and present a toy model comparing the efficiency of KL migration around A, FGK, and early M stars. The paper argues that KL migration is more efficient around early M dwarfs and that the emerging population of aligned HJMD hosts supports efficient tidal obliquity damping in M dwarfs.

Significance. If the measurements are correct, this paper triples the number of hot Jupiters around M dwarfs with a detected RM effect, from one to three, and provides two of the best-sampled RM signals for such systems. The RM detections are statistically strong, the modeling follows standard practice with starry, and the authors are careful to adopt literature priors from the discovery papers. The stellar parameter refinement via Ks-band decontamination and the Gaia astrometric binary fits are useful contributions in their own right. The population-level KL toy model is explicitly labeled as a toy, and the paper includes a candid caveats section. However, the alignment conclusion rests on an assumed spot-free stellar disk, one transit requires a poorly understood linear detrend, and the KL-efficiency claim depends on several unconstrained model assumptions. Given the small sample, the population interpretation should be treated as preliminary rather than as a definitive measurement.

major comments (3)
  1. [§4.2, §5.1] The forward model assumes a uniform, unspotted stellar disk, but the paper states in §4.2 that it cannot reliably assess the occurrence of an occulted active region with MAROON-X data and that simultaneous photometry would be needed. For these slowly rotating stars (vsini ≈ 1.1 km/s) with RM semi-amplitudes of only 29-31 m/s, an occulted spot or plage with a disk-averaged contrast of roughly 0.5% can produce a perturbation of a few m/s, comparable to ~10% of the RM amplitude and enough to shift the inferred λ by about 10-15 degrees, which is comparable to the quoted uncertainties. Because the central claim that both systems are aligned depends on these λ values, I ask the authors to provide a quantitative assessment of this systematic, for example using TESS photometry, the SERVAL activity metrics (dLW, CRX, Hα), or injection-recovery tests with a spotted-star model, or to weaken the alignment claim accordingly.
  2. [§4.1, Table 2] For the first transit of TOI-5293 A b, the model includes a linear detrending slope of -12.6 ± 2.0 m/s/hr whose physical origin is unknown, despite the strong correlation with BERV (ρ = 0.995). Because this transit contributes to the reported λ and vsini, and because a linear slope can partially absorb the curvature of the RM ingress and egress over the 1.94-hour transit, I request a robustness check such as fitting without the slope, fitting with a BERV-correlated model, or comparing the λ and vsini posteriors from the two transits separately. The paper should also justify why a single linear term is preferred over higher-order or wavelength-dependent systematics.
  3. [§5.4.2, Eq. (5)] The population-level conclusion that KL migration is more efficient around early M dwarfs and can fully account for the HJMD occurrence rate rests on f_pps = f_in-situ = 0, a fixed binary suppression factor S_bin = 0.3, and f_misaligned = 0.368 from an isotropic mutual-inclination distribution. Under these assumptions, f_HJ/f_HJ,pred is a ratio of observed occurrence rates to an assumed KL contribution, not a physically measured efficiency. The caveats in §5.4.3 are welcome, but the abstract and §6 state the KL-efficiency result more strongly than the model supports. I recommend adding a sensitivity analysis that varies S_bin and relaxes f_pps = 0 (or uses an a-dependent multiplicity treatment), or explicitly demoting these statements to a speculative interpretation in the abstract and conclusions.
minor comments (5)
  1. [Title] The title contains a typo: 'F ormation' should be 'Formation'.
  2. [§5.4.3] The section heading 'Kozi-Lidov migration' should be 'Kozai-Lidov migration'.
  3. [Table 2] The formatting of the TOI-5293 A rows for v0 and m, with upper and lower values for different transits, is ambiguous; please label the rows or add a footnote so the reader can tell which value corresponds to which transit.
  4. [Figure 1] The shaded in-transit windows are not labeled with the transit times; adding tick labels or annotation would make the figure easier to interpret.
  5. [§5.2] The deprojected obliquity ψ depends on the literature rotation period Prot through i⋆, but the text does not propagate the possible systematic error in Prot; a brief discussion of how ψ changes for the 1σ range of Prot would be helpful.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the RM/obliquity measurements are direct fits to new MAROON-X data, and the population toy model is a literature-input ratio rather than a self-derived prediction.

full rationale

The paper's central claim is the detection of the Rossiter-McLaughlin effect in two new MAROON-X transit time series and the resulting sky-projected obliquities (Section 5.1). These values are obtained by fitting a starry-based RM model to new spectroscopic data, with orbital and transit parameters adopted as priors from the independent discovery papers (C22, C23). No target quantity is used as an input to define the model that produces it; lambda and vsini are free parameters constrained by the new RVs. The deprojected obliquities in Section 5.2 are straightforward trigonometric combinations of the fitted lambda, inferred stellar inclination from v_eq and vsini, and known orbital inclination; this is a derived quantity, not a circular prediction. The KL timescale and tidal damping timescale calculations use literature formulas and literature/numerical inputs and are not used to manufacture the obliquity values. The toy model in Section 5.4.2 computes f_HJ,pred from literature occurrence rates, multiplicity fractions, a literature suppression factor, and an isotropic misalignment fraction; the 'efficiency' f_HJ/f_HJ,pred is a ratio of independent observed quantities to that product. The early-M efficiency being near unity is a numerical outcome of the chosen inputs, not an identity forced by construction. The paper explicitly flags the model's simplifying assumptions (f_pps = f_in-situ = 0, S_bin = 0.3, isotropic misalignment) in Section 5.4.3, which is the behavior of a model comparison rather than a circular derivation. The only genuine limitations are physical assumptions, such as neglecting occulted starspots/plages (Section 4.2), and these are correctness risks rather than circularity: the paper admits it cannot reliably assess an occulted active region without simultaneous photometry, but this does not make the measured obliquities equal to the model inputs. Self-citations appear only for instrument description, data-reduction software, and standard methodology (Seifahrt et al. for MAROON-X, Bean et al. for the reduction pipeline), none of which carries the load of the central scientific claim. No step in the derivation chain reduces by construction, definition, or self-citation to its own inputs.

Assumptions & free parameters 2 free parameters · 6 assumptions · 0 invented entities

The central obliquity measurements rest on the classical RM model and on the adopted orbital priors from the discovery papers; the KL and population-level interpretations rest on several external assumptions, most notably the isotropic binary mutual inclination distribution and the neglect of other migration channels.

free parameters (2)
  • Linear detrending slope m = -12.6±2.0 m/s/hr for TOI-5293 A b Transit 1; consistent with zero for other transits
    Fitted to instrumental/atmospheric drift of unknown origin in the RV time series; most significant for Transit 1 of TOI-5293 A b.
  • RV jitter log sigma = 0.15^{+0.05}_{-0.07} (TOI-3714), 0.47^{+0.02}_{-0.05} (TOI-5293 A)
    Fitted noise term in the RV model; does not affect the obliquity itself but is part of the likelihood.
assumptions (6)
  • domain assumption Classical RM model assumptions: uniform surface brightness, rigid-body rotation, quadratic limb darkening with TESS-band parameters from discovery papers.
    Used in starry forward models; ignores surface inhomogeneities (see starspot assumption below).
  • domain assumption Empirical mass-M_Ks and radius-M_Ks relations for M dwarfs (Mann et al. 2015, 2019) are model-independent.
    Used to refine stellar parameters in Section 2; assumes these calibrations are accurate for the target metallicity and age.
  • domain assumption Binary companion masses derived from Ks-band magnitudes and empirical relations are correct.
    Used as inputs to the lofti_gaia astrometric orbit fits in Section 5.3.
  • domain assumption Mutual inclination distribution of binaries with S-type HJs is isotropic, giving f_misaligned = 0.368.
    Adopted in the toy model from Christian et al. 2025 (Section 5.4.2); if the distribution is not isotropic, the KL efficiency estimates change.
  • ad hoc to paper Planet-planet scattering and in-situ formation contributions are negligible (f_pps = f_in-situ = 0).
    Set for convenience in the toy model (Eq. 5); the paper acknowledges this in Section 5.4.3.
  • domain assumption No occulted starspots or plages contaminated the transit RV time series.
    Assumed in Section 4.2; the paper admits this cannot be assessed without simultaneous photometry.

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Cite this review

Pith. "Pith review of Aligned Stellar Obliquities for Two Hot Jupiter-hosting M Dwarfs Revealed by MAROON-X: Implications for Hot Jupiter Formation." pith.science (2026). https://pith.science/paper/XH5JIMVE

@misc{pith2026250813145,
  author       = {Pith},
  title        = {Pith review of: Aligned Stellar Obliquities for Two Hot Jupiter-hosting M Dwarfs Revealed by MAROON-X: Implications for Hot Jupiter Formation},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/XH5JIMVE}},
  note         = {Machine review of arXiv:2508.13145}
}
abstract

Hot Jupiters (HJs) are $2-3\times$ less common around early M dwarfs than around AFGK stars, suggesting that HJs may form and/or migrate via distinct pathways around different types of stars. One source of insight into HJ formation mechanisms is to trace their dynamical histories through measurements of host stellar obliquities via the Rossiter-McLaughlin (RM) effect. Here we present measurements of the RM effect for the HJs TOI-3714 b and TOI-5293 A b using the Gemini-North/MAROON-X spectrograph. Our measurements represent just the second and third hot Jupiters around M dwarfs (HJMD) with a detection of the RM effect. We find that both systems are well-aligned with sky-projected obliquities of $\lambda = 21^{+14}_{-11}$$\mathrm{^{\circ}}$ and $-12^{+19}_{-14}$$\mathrm{^{\circ}}$ and deprojected obliquities of $\psi = 26^{+11}_{-10}$$\mathrm{^{\circ}}$ and $24^{+11}_{-10}$$\mathrm{^{\circ}}$ for TOI-3714 and TOI-5293 A, respectively. Both stars are in wide binary systems. We refine the stellar parameters by decontaminating their unresolved $K_s$-band photometry and constrain the binary orbits using Gaia DR3 astrometry. We find that the minimum mutual inclination of the planet and binary companion in the TOI-5293 system is sufficiently large to drive Kozai-Lidov (KL) migration while the result for TOI-3714 is inconclusive. We present a population-level analysis of HJs around AFGK versus early M dwarfs and argue that KL migration is more efficient around the latter, which is expected to produce misaligned stellar obliquities in HJMD systems in the absence of efficient tidal damping. The emerging population of well-aligned HJMD hosts supports the expectation that M dwarfs, with their deep convective envelopes, do efficiently dampen misaligned obliquities.

Figures

Figures reproduced from arXiv: 2508.13145 by the authors.

Figure 1
Figure 1. Our raw MAROON-X data of TOI-3714 b (top panel) and TOI-5293 A b (two lower panels). The blue arm, red arm, and combined RVs are depicted by the translucent blue, red, and solid markers, respectively. The solid lines de￾pict random draws from each planet’s known Keplerian orbit posterior (TOI-3714 b; C22, TOI-5293 A b; C23). The verti￾cal shaded regions highlight each planet’s in-transit window. 4.2. Full RV model W… view at source ↗
Figure 2
Figure 2. The RM effect signatures for TOI-3714 b (left panel) and TOI-5293 A b (right panel). Each set of markers depicts the combined MAROON-X RV observations (i.e. blue plus red channels) for each transit sequence observed. The black curves represent random draws from our RM model posteriors after removing the median detrending model for each transit and the planet-induced Keplerian orbital signals. 5293 A, respectively, a… view at source ↗
Figure 3
Figure 3. The distribution of projected stellar obliquities |λ| for giant exoplanets (Rp ≥ 5 R⊕) as a function of host star effective temperature. All stellar obliquity measurements come from measurements of the Rossiter-McLaughlin effect and are retrieved from the NASA Exoplanet Archive. Hot Jupiters around M dwarfs (HJMDs) occupy the shaded red region of parameter space with Teff < 3900 K, while host stars above the Kraft b… view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: Results of orbital fits to Gaia DR3 astrometric data for the binary systems TOI-3714 (upper row) and TOI-5293 (lower row). Left column: a random selection of sky-projected binary orbit solutions from our astrometric fit posteriors. Right column: the distribution of min…
Figure 5
Figure 5. Figure 5: Left: A comparison of the various frequency terms featured in Eq. 5 obtained from the literature for early M dwarfs, FGK stars, and A stars. The terms fHJ, fCJ, fmult, and fmisaligned denote, respectively, the observed occurrence rates of hot Jupiters, cold Jupiters, t…

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