Discovery of an Eccentric Hot Super-Jupiter Leaving the Transiting Geometry of the Early-A-type star TOI-1355

Abstract: Hot Jupiters orbiting hot stars ($T_\mathrm{eff} > 7000$ K) are suggested to have experienced high-eccentricity migration, often evidenced by the tendency for misaligned orbits, despite their circular orbits. In this paper, we present the discovery of TOI-1355 b: an eccentric ($e\sim0.22$) hot Jupiter with a mass of $m_{\mathrm{p}}\sim5.8M_J$ and a radius of $R_{\mathrm{p}}\sim 1.4R_J$ orbiting an A-type star with a period of about $2.17$ days, identified from the TESS transit survey and subsequent follow-up observations. We measured the stellar parameters using the data from the high-resolution spectrograph Seimei/GAOES-RV and obtained the planetary parameters from the photometric data acquired by TESS and ground-based telescopes. This is one of the rare eccentric hot Jupiters around hot stars. This system could be undergoing high-eccentricity migration. We detected nodal precession by measuring the change in its impact parameter. This implies that its transit will no longer be observable from the middle of 2033. Nevertheless, TOI-1355 b is anticipated to be a compelling target for future atmospheric observations, given the hint of atmospheric variability detected in this study.

• The paper reports TOI-1355 b, a 5.84-Jupiter-mass planet with a 2.17-day orbit and eccentricity of 0.2203, making it one of only two known eccentric hot Jupiters around hot stars.
• The paper combines five years of TESS data, ground-based transits, phase-curve modeling, spectroscopy, and speckle imaging to detect nodal precession as the impact parameter increased from 0.853 to 0.928.
• The paper predicts that transits will cease by mid-2033 and finds evidence for atmospheric variability, creating a time-sensitive opportunity to test high-eccentricity migration and characterize the planet with JWST.

Overview and motivation

Watanabe et al. report the discovery of TOI-1355 b, a massive, eccentric hot super-Jupiter transiting the early A-type star TOI-1355 (TIC 372264750, HD 210058), identified in TESS photometry and characterized with ground-based follow-up. The planet has $m_p = 5.84^{+0.83}_{-0.78}\,M_J$, $R_p = 1.424^{+0.039}_{-0.041}\,R_J$, $P_{\rm orb} \simeq 2.1703$ days, and an eccentricity of $e = 0.2203^{+0.0013}_{-0.0011}$ — among the highest measured for hot Jupiters around hot stars (2608.17387).

The system is scientifically valuable because hot stars ($T_{\rm eff} > 7000$ K) lack deep convective envelopes and are expected to retain primordial spin–orbit misalignments rather than realign tidally. Only about twenty hot Jupiters around hot stars were previously known; nearly all have circular orbits, with only one eccentric example (TOI-159 b, $e = 0.24$) reported prior to this work. A hot Jupiter that retains significant eccentricity before tidal circularization provides direct evidence bearing on the high-eccentricity migration pathway (planet–planet scattering or Kozai–Lidov cycles followed by tidal dissipation), as opposed to disk migration which should produce aligned, circular orbits.

Observations

The photometric dataset spans five years of TESS coverage (Sectors 16–18 in 2019, Sector 24 in 2020, Sectors 57–58 in 2022, and Sectors 77–78 in 2024), supplemented by ground-based transits from MuSCAT3 on the 2 m Faulkes Telescope North, RCO 40 cm, OACT 91 cm, and a private SCT observatory in Herges-Hallenberg. A key data challenge is contamination from TIC 372264743, an RS CVn variable with an $\sim$11.56-day period within the TESS aperture; the authors removed this signal by iteratively subtracting dominant frequency components from periodograms of the out-of-transit light curves.

Spectroscopy was obtained with Seimei/GAOES-RV ($R \sim 65{,}000$, 516–593 nm) during two transits in 2023. Doppler tomography was attempted but the SNRs (~80 and ~60 per pixel) proved insufficient to detect the planetary signature; the spectra were instead used for stellar characterization. NESSI speckle imaging at WIYN ruled out close companions down to $\Delta{\rm mag} \sim 6$ at 1″, supporting the planetary interpretation.

Stellar and planetary parameters

Stellar parameters were derived in two stages: PySME spectral fitting yielded $T_{\rm eff} = 8780 \pm 350$ K, $R_p = 1.424^{+0.039}_{-0.041}\,R_J$0, $R_p = 1.424^{+0.039}_{-0.041}\,R_J$1 km s$R_p = 1.424^{+0.039}_{-0.041}\,R_J$2, classifying the star as A3V. These values served as Gaussian priors for ARIADNE SED fitting with Kurucz models combined with Gaia parallax, giving $R_p = 1.424^{+0.039}_{-0.041}\,R_J$3, $R_p = 1.424^{+0.039}_{-0.041}\,R_J$4, distance $R_p = 1.424^{+0.039}_{-0.041}\,R_J$5 pc, and a young isochrone age of $R_p = 1.424^{+0.039}_{-0.041}\,R_J$6 Gyr.

The joint light-curve analysis modeled full-orbit phase curves including transit, secondary eclipse, thermal emission (with day/night brightness temperatures and a phase offset $R_p = 1.424^{+0.039}_{-0.041}\,R_J$7), reflection, Doppler boosting, and ellipsoidal variation, fitted jointly across all epochs via MCMC under two scenarios: a cloud-free case ($R_p = 1.424^{+0.039}_{-0.041}\,R_J$8) and a cloudy case ($R_p = 1.424^{+0.039}_{-0.041}\,R_J$9). The cloud-free case was preferred by BIC ($P_{\rm orb} \simeq 2.1703$0BIC = 43 relative to the cloudy case), and the authors note that reflective clouds should be vaporized at the inferred day-side temperatures exceeding 3000 K — physically consistent with the cloud-free preference. Key results:

Parameter	Value
$P_{\rm orb} \simeq 2.1703$1	$P_{\rm orb} \simeq 2.1703$2 d
$P_{\rm orb} \simeq 2.1703$3	$P_{\rm orb} \simeq 2.1703$4
$P_{\rm orb} \simeq 2.1703$5	$P_{\rm orb} \simeq 2.1703$6
$P_{\rm orb} \simeq 2.1703$7	$P_{\rm orb} \simeq 2.1703$8
$P_{\rm orb} \simeq 2.1703$9	$e = 0.2203^{+0.0013}_{-0.0011}$0 AU
$e = 0.2203^{+0.0013}_{-0.0011}$1 (2019 → 2024)	$e = 0.2203^{+0.0013}_{-0.0011}$2
$e = 0.2203^{+0.0013}_{-0.0011}$3	$e = 0.2203^{+0.0013}_{-0.0011}$4 yr$e = 0.2203^{+0.0013}_{-0.0011}$5

TRICERATOPS validation using the speckle contrast curve yields a false positive probability of $e = 0.2203^{+0.0013}_{-0.0011}$6, formally validating the planet. No transit timing variations are detected relative to the refined linear ephemeris.

Nodal precession and the end of transits

Because the planet transits near the stellar limb ($e = 0.2203^{+0.0013}_{-0.0011}$7), the changing impact parameter directly traces nodal precession driven by the stellar quadrupole moment of the rapidly rotating host. The authors derive an analytic relation between $e = 0.2203^{+0.0013}_{-0.0011}$8 and the spin–orbit angles for an eccentric orbit, accounting for relativistic, tidal, and rotational contributions to apsidal motion. Assuming the stellar spin axis is effectively fixed (justified since $e = 0.2203^{+0.0013}_{-0.0011}$9), they constrain the projected obliquity to $T_{\rm eff} > 7000$0 or $T_{\rm eff} > 7000$1, and the true obliquity to $T_{\rm eff} > 7000$2 or $T_{\rm eff} > 7000$3. The obliquity constraints remain loose; notably, unlike the four previously known precessing systems (Kepler-13Ab, WASP-33b, KELT-9b, TOI-1518b), a near-polar orbit cannot be confirmed without a Doppler tomographic measurement of $T_{\rm eff} > 7000$4. The stellar Love number inferred from the apsidal motion budget, $T_{\rm eff} > 7000$5, agrees with the solar value within $T_{\rm eff} > 7000$6.

Extrapolating the measured $T_{\rm eff} > 7000$7, grazing transits began in early 2024 and the transit geometry ceases entirely by mid-2033. This gives the community a finite observational window of roughly nine years for transit-based atmospheric characterization — a concrete scheduling constraint for JWST and high-resolution spectroscopic programs.

Migration state and atmosphere

With $T_{\rm eff} > 7000$8 AU and adopting $T_{\rm eff} > 7000$9, the circularization timescale is $e = 0.24$0 yr, shorter than the stellar age of $e = 0.24$1 yr. This tension implies either that the planet acquired its eccentricity recently (e.g., through ongoing scattering) or that $e = 0.24$2 is substantially larger than assumed — the paper leaves this ambiguity open, as $e = 0.24$3 remains poorly constrained observationally.

The day-side brightness temperature exceeds the equilibrium temperature ($e = 0.24$4 K versus $e = 0.24$5 K in most epochs), suggesting either TiO/VO-driven thermal inversion or inefficient heat redistribution. More strikingly, the phase offset varies between epochs: $e = 0.24$6 in 2020 versus $e = 0.24$7 in 2022, inconsistent beyond $e = 0.24$8, hinting at atmospheric variability analogous to that reported for WASP-121 b. The night-side temperature in 2020 ($e = 0.24$9 K) is anomalously low compared to other epochs, though consistent with equilibrium within its large uncertainty. An amplitude spectrum of the residual light curves shows no $\sim$0 Scuti pulsations down to $\sim$130 ppm, confirming that stellar variability does not compromise the transit modeling despite the star lying in the instability strip.

Limitations and open questions

Several caveats bear on the interpretation. First, the planetary mass comes from photometric phase-curve modeling (Doppler boosting and ellipsoidal variation) rather than radial velocities, so it inherits model dependence on the beaming factor and gravity-darkening coefficient. Second, the obliquity constraints on $\sim$2 are bimodal and weak; whether TOI-1355 b joins the near-polar population of precessing hot Jupiters requires Doppler tomography at higher SNR than achieved here. Third, the circularization argument depends on an assumed $\sim$3; independent constraints on tidal quality factors would resolve whether the eccentricity is freshly excited. Fourth, the physical origin of the epoch-to-epoch variation in phase offset and night-side temperature is unidentified — candidate explanations include time-variable clouds, circulation changes, or tidal heating, none of which can be discriminated with current data. Finally, the small wavy residuals of $\sim$4100 ppm in the phase curves suggest unmodeled components in the atmospheric emission model.

Conclusion

TOI-1355 b is one of only two known eccentric hot Jupiters orbiting a hot star, with a precisely measured $\sim$5 and a detected nodal precession rate of $\sim$6 yr$\sim$7. The system offers a rare opportunity to test high-eccentricity migration around an A-type star whose misalignment should be preserved, and its transits will permanently end by mid-2033, making continued TESS monitoring and JWST atmospheric observations time-sensitive. The open questions this work leaves are the true obliquity $\sim$8, the planet's tidal quality factor, and the mechanism behind the apparent atmospheric variability in the phase curve.