TOI-4507 b: Cold, Super-Puffy Polar Planet

• TOI-4507 b is a transiting sub-Saturn orbiting a young F star on a 105-day, nearly polar orbit with a remarkably low density (<0.3 g/cm³) indicative of a massive hydrogen envelope.
• The discovery combined TESS photometry with targeted ground-based transit observations and detailed spectroscopic measurements via the Rossiter–McLaughlin effect.
• Its unique long-period, polar orbit and significant scale height make it a benchmark target for probing atmospheric composition through JWST transmission spectroscopy and studying migration histories.

Searching arXiv for the discovery paper and closely related contextual papers on super-puffs and polar orbits. arXiv_search(query="TOI-4507 b A Cold and Super-Puffy Planet on a Polar Orbit", max_results=5, sort_by="relevance") arXiv_search(query="(Espinoza-Retamal et al., 30 Sep 2025)", max_results=5, sort_by="relevance") TOI-4507 b is a transiting sub-Saturn orbiting a young F star on a 105-day, nearly polar orbit. It was reported as a planet with a density $<0.3\ {\rm g/cm^3}$ around a $700$ Myr old host, making it both one of the longest-period planets for which the stellar obliquity has been measured and among the longest-period and youngest “super-puff” planets yet discovered (Espinoza-Retamal et al., 30 Sep 2025). The system is notable because the photometric transit signal is secure, the radial-velocity Keplerian signal is not detected, and the planetary interpretation is instead strengthened by a spectroscopic transit via the Rossiter–McLaughlin effect, which yields a three-dimensional stellar obliquity of $\psi = 82.0^{+2.6}_{-2.4}\deg$ (Espinoza-Retamal et al., 30 Sep 2025).

1. Discovery and observational identification

TOI-4507 b was identified in TESS 2-minute photometry, where the transits were first flagged by the SPOC pipeline across Sectors 2, 3, 5–13, 27–30, 32–39, 61–69 and 87–89 (Espinoza-Retamal et al., 30 Sep 2025). The signal consisted of a periodic dip of 6,100 ppm, corresponding to a planet-to-star radius ratio $R_p/R_\star \simeq 0.072$, recurring every $\sim 104$ days with very high signal-to-noise because of TESS’s near-continuous coverage (Espinoza-Retamal et al., 30 Sep 2025).

The discovery photometry was supplemented by ground-based follow-up from Antarctica. Two additional transits were observed with the ASTEP 400 cm telescope at Concordia: a full event on 2025 May 24 and an egress on the previous night, in Sloan-$R$ and once in $B$ (Espinoza-Retamal et al., 30 Sep 2025). These data were reduced with aperture-photometry pipelines on site and incorporated into the system-level fit.

This detection pathway is significant because it combines long-baseline space photometry with targeted ground-based transit recovery at a period where complete events are observationally expensive. A plausible implication is that TOI-4507 b occupies an observational regime that is intrinsically sparse in current transit surveys: long-period, low-density planets around relatively young stars.

2. Photometric modeling and transit ephemeris

The TESS Presearch Data Conditioning Simple Aperture Photometry light curves were stitched together with lightkurve, with corrections for instrumental systematics (Espinoza-Retamal et al., 30 Sep 2025). For a uniform determination of the transit ephemeris and depths, the combined photometry was modeled with juliet, which employs batman for the transit shapes and dynesty for nested sampling (Espinoza-Retamal et al., 30 Sep 2025). The adopted setup used uninformative priors on the period $P$ and mid-transit time $t_0$, constrained the stellar density $\rho_\star$ by spectroscopy and isochrones, and included a Matern-3/2 Gaussian process to absorb residual correlated noise in the TESS light curve (Espinoza-Retamal et al., 30 Sep 2025).

The resulting linear ephemeris is

$700$0

All observed transits align to better than five minutes of this linear solution, although one ASTEP epoch deviated by $700$1 min, hinting at possible TTVs (Espinoza-Retamal et al., 30 Sep 2025). The report does not claim a TTV detection; the statement is explicitly limited to a hint.

The photometric analysis establishes the timing and depth structure of the system with high precision. In context, the transit-only solution already indicates an unusually inflated planet for its incident flux, but the paper treats mass and architecture as requiring spectroscopic confirmation.

3. Radial velocities, non-detection of the Keplerian signal, and planetary confirmation

Planet validation and mass constraints were pursued with HARPS, FEROS, and CORALIE (Espinoza-Retamal et al., 30 Sep 2025). HARPS on the ESO 3.6 m telescope provided 31 out-of-transit spectra with 1200–1500 s exposures, median S/N $700$2 per pixel at 550 nm, and formal errors $700$3 between October 2019 and March 2025, together with 20 in-transit observations covering $700$4 of a transit on 2024 Oct 27 for the Rossiter–McLaughlin anomaly (Espinoza-Retamal et al., 30 Sep 2025). FEROS on the MPG/ESO 2.2 m yielded 11 spectra with 600–1800 s exposures, S/N $700$5, and RV errors $700$6, while CORALIE on the Euler 1.2 m contributed 10 spectra with 900–1800 s exposures and RV errors $700$7 (Espinoza-Retamal et al., 30 Sep 2025). All RVs were reduced with their respective pipelines and modeled with independent jitter terms.

The joint RV-plus-photometry analysis was performed with ironman, combining radvel Keplerian orbits, Hirano’s analytic RM model, and batman transits under dynesty sampling (Espinoza-Retamal et al., 30 Sep 2025). To accommodate a possible non-detection, the semi-amplitude $700$8 was allowed to float over positive and negative values. The paper gives the canonical Keplerian semi-amplitude as

$700$9

In the fit, the Keplerian signal was not detected:

$\psi = 82.0^{+2.6}_{-2.4}\deg$0

implying a 95% upper limit of $\psi = 82.0^{+2.6}_{-2.4}\deg$1 and hence $\psi = 82.0^{+2.6}_{-2.4}\deg$2 (Espinoza-Retamal et al., 30 Sep 2025).

The absence of a detected orbital RV semi-amplitude does not undermine the planetary interpretation in this case, because the RM sequence yielded a definitive spectroscopic transit signature around the star (Espinoza-Retamal et al., 30 Sep 2025). This is a system where confirmation depends critically on time-resolved in-transit spectroscopy rather than on a conventional mass detection.

4. Spin–orbit architecture and polar geometry

The defining dynamical property of TOI-4507 b is its nearly polar orbit. In the Rossiter–McLaughlin fit, the sampled parameters were the sky-projected obliquity $\psi = 82.0^{+2.6}_{-2.4}\deg$3, the stellar spin period $\psi = 82.0^{+2.6}_{-2.4}\deg$4, the stellar radius $\psi = 82.0^{+2.6}_{-2.4}\deg$5, and $\psi = 82.0^{+2.6}_{-2.4}\deg$6 (Espinoza-Retamal et al., 30 Sep 2025). These were combined into the three-dimensional stellar obliquity $\psi = 82.0^{+2.6}_{-2.4}\deg$7 through

$\psi = 82.0^{+2.6}_{-2.4}\deg$8

where $\psi = 82.0^{+2.6}_{-2.4}\deg$9 is the orbital inclination (Espinoza-Retamal et al., 30 Sep 2025).

The inferred values are

$R_p/R_\star \simeq 0.072$0

These measurements establish TOI-4507 b on an almost perfectly polar orbit (Espinoza-Retamal et al., 30 Sep 2025). The distinction between $R_p/R_\star \simeq 0.072$1 and $R_p/R_\star \simeq 0.072$2 is important: the former is sky-projected, whereas the latter is the three-dimensional stellar obliquity. The system’s exceptional status comes from the 3D inference, not merely from a projected misalignment.

The paper further states that TOI-4507 b, at 105 d, boasts the longest period among planets with measured 3D obliquities (Espinoza-Retamal et al., 30 Sep 2025). This places it in a sparsely populated region of spin–orbit parameter space, particularly among cold and low-density planets. A plausible implication is that the system provides leverage on migration and excitation channels that are difficult to isolate in short-period populations, where tidal effects are much stronger.

5. Planetary and stellar properties

From the transit depth and a stellar radius of $R_p/R_\star \simeq 0.072$3, the planet radius is

$R_p/R_\star \simeq 0.072$4

Combined with the $R_p/R_\star \simeq 0.072$5 mass upper limit, the mean density satisfies

$R_p/R_\star \simeq 0.072$6

which qualifies TOI-4507 b as a textbook “super-puff” (Espinoza-Retamal et al., 30 Sep 2025).

The host star is described as a young, mildly metal-poor, main-sequence F star with age $R_p/R_\star \simeq 0.072$7 Gyr and metallicity $R_p/R_\star \simeq 0.072$8 (Espinoza-Retamal et al., 30 Sep 2025). Its atmospheric and bulk properties are

$R_p/R_\star \simeq 0.072$9

These were determined through an iterative combination of ZASPE spectroscopic fitting and PARSEC isochrone matching to broadband photometry and Gaia DR3 parallax (Espinoza-Retamal et al., 30 Sep 2025).

A clear 1.7-day rotational modulation with amplitude $\sim 104$0 ppm in the TESS light curve yields $\sim 104$1 d, consistent with the spectroscopic $\sim 104$2, and implies that the stellar spin axis is tipped by $\sim 104$3 away from the line of sight (Espinoza-Retamal et al., 30 Sep 2025). This stellar orientation is a key ingredient in promoting the projected RM information to a three-dimensional obliquity estimate.

6. Thermal regime, origin of the low density, and competing interpretations

The orbital semimajor axis is $\sim 104$4 au (Espinoza-Retamal et al., 30 Sep 2025). Under assumptions of zero albedo and full redistribution, the equilibrium temperature is given as

$\sim 104$5

which the paper characterizes as modest enough that tidal inflation, effective only at short periods, cannot explain the planet’s puffiness (Espinoza-Retamal et al., 30 Sep 2025).

Instead, the low density is interpreted as likely arising from an exceptionally massive H/He envelope, perhaps accreted in a dust-poor, cold region of the parent disk and subsequently migrated inward, as envisioned by Lee & Chiang (2016) (Espinoza-Retamal et al., 30 Sep 2025). This is presented as an interpretation rather than a demonstrated evolutionary history. A plausible implication is that TOI-4507 b is informative not only for atmospheric structure but also for disk thermochemistry and migration pathways.

The paper explicitly considers an alternative explanation: an oblique ring system. That possibility is disfavored because Poynting–Robertson drag would dissipate rings at $\sim 104$6 K over the star’s lifetime (Espinoza-Retamal et al., 30 Sep 2025). This addresses a recurrent ambiguity in the interpretation of very low-density transiters, where large apparent radii can in principle be mimicked by circumplanetary structures. In this case, the preferred explanation remains an intrinsically extended, hydrogen-rich envelope.

7. Comparative significance and follow-up value

TOI-4507 b is placed within a small cohort of “cold Neptunes” on polar orbits (Espinoza-Retamal et al., 30 Sep 2025). Within that cohort, the paper emphasizes three linked properties: the 105-day orbital period, the nearly polar three-dimensional geometry, and the super-puff density regime. Together these make the system unusual among currently characterized transiting planets.

The planet’s atmosphere is expected to be highly favorable for transmission spectroscopy because of its extreme scale height, low density, and $\sim 104$7 (Espinoza-Retamal et al., 30 Sep 2025). The paper therefore identifies it as an outstanding target for JWST transmission spectroscopy, with the prospect of probing a cool, hydrogen-dominated atmosphere in a very inflated configuration (Espinoza-Retamal et al., 30 Sep 2025). This suggests that TOI-4507 b is relevant to two distinct observational programs: dynamical studies of spin–orbit misalignment at long period, and atmospheric studies of low-gravity, cold H/He-dominated planets.

The broader significance of the system lies in the combination of properties rather than in any single metric. Long period reduces the plausibility of tidal inflation; youth constrains the evolutionary timescale; the RM detection secures the transiting body’s association with the F star; and the low density places the planet in the super-puff regime (Espinoza-Retamal et al., 30 Sep 2025). In aggregate, these features make TOI-4507 b a benchmark object for studying the intersection of envelope retention, migration history, and spin–orbit architecture in cold, low-density exoplanets.