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REVIEW 3 major objections 6 minor 49 references

No Planet around the K Giant Star 42 Draconis

T0 review · 3 major / 6 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read The claimed 3.9-Jupiter-mass planet in a 479-day orbit around 42 Draconis is not there; the radial-velocity signal faded and infrared photometry varies at the same period, so the variations are intrinsic stellar oscillations.

desk verdict The RV amplitude drop is a convincing refutation of 42 Dra b, but the DIRBE 'conclusive' claim overreaches and needs a baseline check. read the letter →

arxiv 2505.05260 v1 pith:MXGIQFIB submitted 2025-05-08 astro-ph.EP astro-ph.SR

Artie P. Hatzes , Volker Perdelwitz , Marie Karjalainen , Jana Köhler , Michael Hartmann This is my paper · ORCID
classification astro-ph.EPastro-ph.SR
keywords 42DraconisKgiantstarsradialvelocityplanetsexoplanetfalsepositivesstellaroscillationsCOBE/DIRBEphotometrylong-periodvariabilityplanetretraction
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 sets out to show that the giant planet previously reported around the K giant star 42 Draconis does not exist. Fifteen years of radial-velocity measurements show the 479-day signal attributed to a 3.9-Jupiter-mass companion did not stay coherent: its amplitude fell by a factor of about four, and a second period near 530 days appears in the later data, so the two close periods can beat and mimic a decaying signal. Independent near-infrared photometry from the COBE/DIRBE archive varies at the proposed orbital period and at a second period of about 170 days, which the authors take as direct evidence that the variations are intrinsic to the star rather than orbital reflex motion. If correct, the result adds 42 Dra to a list of long-period K-giant 'planet' signals that vanish under longer monitoring, with consequences for planet-occurrence statistics around intermediate-mass stars.

What carries the argument

The central mechanism is beating between two closely spaced radial-velocity periods, 487.8 and 534.8 days, whose superposition makes a single roughly 479-day signal appear to shrink and fade over time. The analysis runs on Lomb-Scargle periodograms with pre-whitening, on the bisector velocity span of the cross-correlation function, and on the COBE/DIRBE 1.25-micron photometric time series, which supplies the independent evidence that the star itself varies at the proposed orbital period and at 169.8 days. The Hill stability criterion seals the interpretation: two hypothetical planets at 1.24 and 1.32 AU would be too close for a stable system, so the two periods cannot be a compact planetary pair.

What would settle it

Check the first and last epochs of 42 Dra in the COBE/DIRBE 1.25-micron photometry: if they span less than 479 days, the peak at the orbital frequency in Figure 6 is not a confirmed periodic signal. Alternatively, obtain fresh high-cadence radial velocities over two or more 479-day cycles; if the signal's amplitude and phase remain coherent, the planet hypothesis would be revived.

Watch

Extended reading notes

Core claim

The paper's central claim is that 42 Draconis hosts no giant planet. The original detection, a 479.1-day radial-velocity signal with a semi-amplitude near 110 m/s and a minimum mass of 3.9 Jupiter masses, survived the standard checks: no H-alpha correlation, no line-shape variations at the orbital period, and no Hipparcos photometric signal at that period. Continued monitoring broke it: the 2008-2011 data alone show no peak at 479 days, and a fit that fixes all orbital parameters except amplitude gives K = 27.4 m/s compared with 96.6 m/s for the earlier subset. A frequency analysis of the full 2004-2018 dataset finds periods of 487.8 and 534.8 days whose beating can reproduce the amplitude decline, and 1.25-micron COBE/DIRBE photometry shows significant power at the orbital period and at 169.8 days. These two periods cannot be two planets: at their masses and separations the system would be dynamically unstable by the Hill criterion. The authors conclude that the variations are stellar oscillations, most likely oscillatory convection modes.

Load-bearing premise

The refutation's strongest independent evidence assumes the COBE/DIRBE 1.25-micron time series is long enough and well enough sampled to constrain a 479-day period, but the paper never states that baseline, and a shorter baseline could make the orbital-frequency peak a low-frequency trend.

Editorial extensions

If this is right

  • The named exoplanet 42 Dra b (Orbitar) would have to be retracted from planet lists.
  • Long-period radial-velocity signals around K giants should not be regarded as confirmed planets unless monitoring spans more than a decade and checks for amplitude and phase changes.
  • Photometric variation at the proposed orbital period is a decisive test: when the star itself brightens and dims on the 'planet' period, the companion interpretation is ruled out.
  • Planet-occurrence statistics for intermediate-mass stars may be inflated by this class of false positives, potentially biasing formation theories.
  • If oscillatory convection modes are confirmed, they become a known source of long-period RV variability that future surveys will need to account for.

Reading between the lines

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

  • The paper calls the DIRBE evidence conclusive but never states the time baseline of the photometry; if that baseline is shorter than 479 days, the peak at the orbital frequency could be a low-frequency trend, so the independent photometric refutation would need re-examination.
  • The same DIRBE-based search could be applied to other K giants with claimed long-period planets; a star whose infrared photometry varies at the RV period would be a strong candidate for retraction.
  • The 530-day and 294-day periods found here could be a signature of the suspected oscillation modes, testable by looking for similar period ratios in other luminous K giants.
  • If Gaia astrometry resolves the motion of 42 Dra over the same epochs, a real companion would leave a measurable acceleration that the oscillation interpretation would not, giving a dynamical test independent of photometry.
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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 / 6 minor

Summary. The manuscript reports new radial velocity (RV) measurements of the K giant 42 Dra spanning 2004–2018 and re-examines the previously claimed 479-day, 3.9 Jupiter-mass companion. The authors find that the 479-day signal is not stable: the RV amplitude declines from ~96 m/s in the early subset to ~27 m/s in 2008–2011 data, and a periodogram of the later subset shows no peak at the orbital frequency. A frequency analysis of the full RV set yields periods of roughly 488 and 530 days, which the authors interpret as beating that modulates the observed amplitude. They also analyze COBE/DIRBE 1.25 micron photometry and report a peak at the planet's orbital period plus a 170-day period. The paper concludes that the planet hypothesis is conclusively ruled out and that the variations are intrinsic stellar oscillations, possibly oscillatory convection modes, with implications for the statistics of planets around K giant stars.

Significance. If the core conclusion holds, the paper removes a widely cited exoplanet from the literature and strengthens the emerging caution that long-period RV signals in K giants can be mimicked by intrinsic stellar variability. The authors explicitly test the detectability of the 479-day signal in the later subset by injecting a synthetic orbit with the same sampling, which is a constructive and falsifiable check. The comparison of RV amplitudes between two well-sampled epochs is a strong, simple argument against a Keplerian companion. However, the paper currently overstates the certainty of the photometric evidence, which is important because the abstract and Section 4.4 assign the word 'conclusively' specifically to the DIRBE photometry. The manuscript is therefore significant but needs revision to bring the claims in line with the actual support.

major comments (3)
  1. [§4.4] The DIRBE photometric analysis is presented as conclusively ruling out the planet, but the manuscript never states the time baseline or sampling of the DIRBE 1.25 micron time series. The Price et al. (2010) data are weekly averages from the cryogenic COBE mission, whose useful DIRBE data span only about ten months, shorter than the 479-day period. In a periodogram of a time series shorter than the candidate period, a broad low-frequency peak can arise from a monotonic or low-order trend; the windowed bootstrap as described tests white noise at the chosen frequency but does not test the null of a low-frequency trend. The phased data in Figure 7 cover only a fraction of a cycle, making a sine fit nearly indistinguishable from a linear drift. The authors should report the exact baseline, detrending or trend tests, and then downgrade the photometric claim from 'conclusive' to supporting evidence.
  2. [§4.2] The frequency analysis of the full RV data set does not account for the instrumental zero-point offset that the authors explicitly state exists between the 2004–2011 and 2014–2018 measurements. If the offset is not modeled, a two-segment time series with different means can inject power at low frequencies and can bias the recovered periods and amplitudes of the 487.8-day and 534.8-day signals. The manuscript should either include a fitted zero-point offset in the pre-whitening procedure or demonstrate that the recovered periods are unchanged when the offset is marginalized.
  3. [§4.1] The central amplitude-drop argument needs more detail and internal consistency. The text refers to 'JD = 245660' when the intended boundary is presumably JD 2454660, and the caption of Figure 3 gives a different range. The statement that two data points were removed 'in order to provide a larger time gap between the two subset data' requires specification of which points were removed and a sensitivity test, because the K-amplitude comparison is load-bearing for the refutation. The authors should also show that fitting K alone, with all other orbital parameters fixed, is not biased by phase coverage or by the choice of the adopted orbital solution.
minor comments (6)
  1. [§4.4] In the bootstrap description, the text says 'the RV data are randomly shuffled' but the analysis is applied to the DIRBE photometry; this should be corrected.
  2. [§4.2] The pre-whitening description should state the frequency grid, the window-function treatment, and the number of significant peaks checked beside the three listed in Table 5; the 534.8-day period is close to the 487.8-day period and a beat interpretation should be checked against aliases.
  3. [Abstract / §5] The abstract states the second period as 530 days while Table 5 lists 534.8 ± 2.2 days; the text should use a consistent value or clearly explain the rounding.
  4. [§5] The Hill radius formula is written as rH ≈ a (m / 3M)^(1/3) without parentheses; it should be rH ≈ a (m / (3M))^(1/3).
  5. [General] There are several typographical issues, including 'firmly established' in the Conclusions, 'these that we are seeing stellar oscillations', and the inconsistent JD boundary in §4.1; a careful language pass is needed.
  6. [Figure 3] The caption should explicitly indicate the data subsets used in the upper and lower panels and state how the two removed points were selected.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the planet refutation is tested against new RV data and an external photometric time series.

full rationale

The paper refutes a previously published 479-d planet claim for 42 Dra using two independent lines of evidence: (1) new RV data spanning 2004-2018 in which the 479-d amplitude drops from K = 96.6 m/s to K = 27.4 m/s, and which, analyzed alone, show no significant peak at the published orbital frequency even though a simulation of the published orbit sampled at the same epochs would have been detected (Fig. 2, lower panel); and (2) an external photometric time series (COBE/DIRBE 1.25 micron) whose strongest periodogram peak coincides with the orbital frequency (Fig. 6) with a windowed bootstrap FAP of about 2e-3 at that frequency. No load-bearing step of this refutation reduces by construction to its own inputs. The 487.8-d and 534.8-d periods in Table 5 are fitted to the RV data by standard pre-whitening, and the statement that their beating 'may account for' the observed amplitude variations is a hedged post hoc interpretation of that fit, not a first-principles prediction of that amplitude drop. The simulated-detection test uses the published D09 orbit as a benchmark null and therefore assumes the planet hypothesis only to test it, which is the correct non-circular falsification procedure. Self-citations (Hatzes & Cochran 1993; Hatzes 2013; Hatzes et al. 2015, 2018; Hatzes 2019) are contextual or methodological: the windowing-bootstrap FAP is described inline so no ansatz is hidden behind the citation; the Aldebaran and gamma Dra cases are external observational analogies whose oscillation interpretations the paper explicitly leaves unconfirmed; and D09 is the original detection under test, so citing it is necessary rather than circular. No uniqueness theorem is imported from the authors' prior work, and no known result is renamed in new coordinates. A statistical-robustness concern noted in review (that the DIRBE cryogenic baseline may be shorter than 479 d, so the low-frequency peak could be an unresolved trend rather than a resolved periodicity) is a correctness risk about the strength of the photometric evidence, not a circularity, because the DIRBE data are external inputs rather than outputs of the paper's own fits; under the review rules, such concerns do not raise the circularity score, and the independent RV-amplitude evidence carries the core refutation regardless.

Assumptions & free parameters 5 free parameters · 5 assumptions · 1 invented entities

The central claim rests on standard periodogram and orbital-fitting machinery plus several domain assumptions. The fitted RV periods and amplitudes are model outputs, not independent inputs. The most important unstated premise is that the COBE/DIRBE photometric time series is long enough to establish a 479-day period; this is not documented and is questionable given the mission duration. The paper also assumes the 2014-2018 RV zero-point offset does not affect the 530-day period found in the combined data.

free parameters (5)
  • RV amplitude of early subset = K = 96.6 +/- 6.3 m/s
    Fitted to RV data up to JD 2454400 with orbital parameters fixed; used to show the signal was once strong.
  • RV amplitude of late subset = K = 27.4 +/- 6.1 m/s
    Fitted to RV data from JD 2454660 onward; the factor-of-four drop is the core evidence against a Keplerian companion.
  • Full-data frequency analysis periods = P1 = 487.8 d, P2 = 534.8 d, P3 = 294.1 d
    Three periodicities extracted by pre-whitening the full 2004-2018 RV data set; P1 and P2 are used to construct the beating explanation.
  • Full-data frequency analysis amplitudes = K1 = 79.3 m/s, K2 = 49.2 m/s, K3 = 20.7 m/s
    Fitted amplitudes of the three periods in the RV data; these are model outputs, not physical constants.
  • Single-Keplerian fit to 2004-2011 data = P = 473.9 d, K = 65.8 m/s, e = 0.25, omega = 181.2 deg
    Revised orbital solution over the 2004-2011 data set; the already reduced K relative to D09's 110.5 m/s supports amplitude instability.
assumptions (5)
  • domain assumption The COBE/DIRBE photometric time series is long enough to constrain a 479-day period.
    The paper does not state the time baseline of the DIRBE data in Section 4.4; the standard COBE/DIRBE cryogenic mission is about ten months, shorter than the period, so the 479-day peak may be a low-frequency trend.
  • domain assumption A giant planet on a 479-day orbit cannot produce 1.25 micron brightness variations at the same period.
    Used in Section 4.4 to interpret the DIRBE periodogram peak as refuting the planet. This is reasonable for a non-transiting giant planet around a K giant, but it is an external assumption not tested in this paper.
  • domain assumption The zero-point offset of the 2014-2018 RV data does not materially affect the combined frequency analysis.
    Section 3 states the later data have a different zero point, but Section 4.2 combines the full data set without describing an offset fit. The 530-day period may be influenced by this offset.
  • domain assumption The 487.8-day and 534.8-day periods are both real and beat together to explain the amplitude variations.
    This is the proposed mechanism in Section 4.2 and Section 5. The two periods are derived from the same data set and the beat period is comparable to the total data span, so the reality of the 530-day period is not independently confirmed.
  • domain assumption A single Keplerian orbit has constant RV amplitude, so a factor-of-four decrease rules out one companion.
    This is the physical basis for the amplitude comparison in Section 4.1. It is standard orbital mechanics, but the conclusion depends on the amplitude fits to the two data subsets being on a consistent velocity scale.
invented entities (1)
  • Oscillatory convection modes in 42 Dra
    purpose: Proposed explanation for the multi-periodic long-period RV and photometric variations, replacing the planet hypothesis.
    The paper suggests these modes in the Abstract and Section 5 but provides no direct detection, model, or independent observable for them in 42 Dra. The suggestion is based on analogy with gamma Dra and CEMP stars, so it is a hypothesis rather than a measured entity.

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Cite this review

Pith. "Pith review of No Planet around the K Giant Star 42 Draconis." pith.science (2026). https://pith.science/paper/MXGIQFIB

@misc{pith2026250505260,
  author       = {Pith},
  title        = {Pith review of: No Planet around the K Giant Star 42 Draconis},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/MXGIQFIB}},
  note         = {Machine review of arXiv:2505.05260}
}
read the original abstract

Published radial velocity (RV) measurements of the K giant star 42 Dra reveal variations consistent with a 3.9 M_Jup mass companion in a 479-d orbit. This exoplanet can be confirmed if these variations are long-lived and coherent. Continued monitoring may also reveal other companions. We have acquired additional RV measurements of 42 Dra spanning fifteen years. Periodogram analyses were used to investigate the stability of the planet RV signal. We also investigated variations in the spectral line shapes using the bisector velocity span as well as infrared photometry from the COBE mission. The new RV measurements do not follow the published planet orbit. An orbital solution using the 2004 - 2011 data yields a period and eccentricity consistent with the published values, but the RV amplitude has decreased by a factor of four from the earlier measurements. Including some additional RV measurements taken between 2014 and 2018 reveal the presence of a second period at 530 d. The beating of this period with the one at 479-d may account for the observed amplitude variations. The planet hypothesis is conclusively ruled out by COBE/DIRBE 1.25 micron photometry that shows variations with the planet orbital period as well as an additional 170 d period. The amplitude variations in the RV as well the COBE/DIRBE photometry firmly establish that there is no giant planet around 42 Dra. The presence of multi-periodic variations suggests that these may be stellar oscillations, most likely oscillatory convection modes. These oscillations may account for some of the long period RV variations attributed to planets around K giant stars. This may skew the statistics of planet occurrence around intermediate mass stars. Long-term monitoring with excellent sampling is required to exclude amplitude variations in the long-periods found in radial velocity of K giant stars.

Figures

Figures reproduced from arXiv: 2505.05260 by the authors.

Figure 1
Figure 1. The RV measurements for 42 Dra. The vertical dashed line marks the boundary between the old and new RV measure￾ments. The dashed curve is the orbital solution from D09 and the solid curve a new solution based on all the 2004 − 2011 data. At the Thuringer Landessternwarte Tautenburg (TLS) we ¨ have been monitoring a sample of K giant stars with the Doppler method to search for exoplanet companions. This pro￾gram has … view at source ↗
Figure 2
Figure 2. (Top) The Lomb-Scargle periodogram of the RV mea￾surements up to JD = 2454337. (Middle) The periodogram of the RV measurements taken after JD ≈ 2454660. (Bottom) The periodogram of a simulated orbit over the same time range as the middle panel. The 479-d should have been present in the periodogram of the latter RV measurements. 53200 53400 53600 53800 54000 54200 54400 −200 −100 0 100 200 K = 97 m s−1 54600 54800 55… view at source ↗
Figure 3
Figure 3. (Top) The orbital solution of the data up to JD ≈ 2454400. The K-amplitude is 96 m s−1 . (Bottom) An orbital solution to the RV data taken from JD ≈ 2454400 − 2455600. All orbital parameters except the RV amplitude were kept fixed for both data subsets. The K-amplitude for the latter data is 27 m s−1 . We continued to monitor 42 Dra with RV measurements for an additional nine years, three of these with good cadence.… view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: The complete RV measurements of 42 Dra from 2004 − 2018 (points). The curve represents a two-component fit with two periods, P1 = 487.3 d and P2 = 530 d. The RV data show a clear variation in the amplitude of the 479-d signal. We took our revised orbital solution and u…
Figure 5
Figure 5. Figure 5: shows the low frequency end of L-S periodogram of the Hipparcos photometry. There is indeed a peak at ν = 0.00145 ± 0.00018 d−1 (P = 690 ± 90 d) that is consistent with the one seen in the BVS, but the signal is not very significant (FAP ∼ 0.05). The Hipparcos photomet…
Figure 7
Figure 7. Figure 7: The DIRBE photometry phased to the 479-d “orbital” period. The red squares are phase-binned values. The curve represents a sine fit to the data. Data measurements are repeated to the right of the vertical dashed line. Clearly, these are p-mode oscillations. Kjeldsen & …

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