TOI-1355 b is visible in transit now, but probably not for much longer. A team led by Noriharu Watanabe of the University of Tokyo reports that the giant planet’s path across its star has moved steadily towards the stellar limb. If that measured drift continues at the same rate, the grazing transits should cease around the middle of 2033.
The planet will not disappear, change orbit suddenly or leave its system. Only the viewing geometry from Earth will change. Its orbital plane is precessing, so the narrow line of sight that lets the planet cross the face of its star is sliding out of alignment.
The forecast comes from one newly published analysis, not a guaranteed timetable. The peer-reviewed paper in the Publications of the Astronomical Society of Japan extrapolates a change measured from 2019 to 2024. The University of Tokyo’s 18 September research release says the transit geometry may then remain unavailable for hundreds of years, although neither source gives a precise return date.
The shadow has already reached the stellar edge
NASA’s Transiting Exoplanet Survey Satellite observed TOI-1355 in 2019, 2020, 2022 and 2024. Ground observatories added follow-up photometry, including MuSCAT3 on the two-metre Faulkes Telescope North in Hawaii, two small European observatories and a 28-centimetre telescope at a private observatory in Germany. The result is a time series of transit shapes spanning five years.
Astronomers describe a transit chord with an impact parameter, measured in units of the star’s radius. A value of zero would carry the planet across the centre of the stellar disc. A value near one takes its centre across the limb.
TOI-1355 b’s fitted impact parameter rose from about 0.853 in 2019 to 0.862 in 2020, 0.904 in 2022 and 0.928 in 2024. The team’s preferred cloud-free model gives a rate of 0.01633 per year, with an uncertainty of roughly 0.0008 per year. The progression, rather than a difference between only two epochs, is the evidence for a moving orbital plane.
Because the planet has a finite size, its silhouette can still overlap the star after its centre has moved beyond the limb. The paper calculates that the transit became grazing at the start of 2024 and should lose all overlap in mid-2033 if the measured rate remains constant.
A simple extrapolation places the moment when the planet’s centre misses the stellar disc around late 2028. Its outer edge could continue clipping the star for roughly another five years. Between now and mid-2033 the planet will complete about 1,100 of its rapid orbits, although observing seasons, daylight, weather and telescope schedules make only a fraction of those transits useful.
A fast-spinning star turns the orbital plane
TOI-1355, also catalogued as HD 210058, is an early A-type star about twice the Sun’s mass and roughly 247 parsecs, or 806 light-years, away. Spectroscopy gives it a projected rotation speed of 80.8 kilometres per second. Fast rotation makes a star slightly oblate and gives its gravitational field a quadrupole component rather than the symmetry of a perfect sphere.
If a planet’s orbital plane is tilted relative to that star’s equator, the uneven gravitational field can torque the orbit. The plane slowly swivels around the stellar spin axis. This is nodal precession, and from Earth it appears as a changing transit chord. It is different from apsidal precession, in which the long axis of an eccentric orbit rotates within the orbital plane.
The orbit should not yet be called polar. The new analysis constrains several possible ranges for the true spin-orbit angle, but does not pin down one solution. A spectroscopic attempt in 2023 lacked enough signal to trace the planet’s shadow across the rapidly rotating star. The authors say better Doppler tomography during future transits is needed to measure the projected angle and test whether TOI-1355 b resembles the near-polar hot Jupiters already known to precess.
One useful comparison is TOI-1518 b. A 2024 PASJ study found its impact parameter moving in the opposite direction, towards the centre of its star. Precession can therefore bring a transit deeper into view as well as carry one away. The direction seen at any moment depends on which part of the much longer precession cycle astronomers happen to catch.
TOI-1355 b joins only four hot Jupiters previously reported with detected nodal precession: Kepler-13Ab, WASP-33b, KELT-9b and TOI-1518b. All four circle rapidly rotating hot stars on near-polar paths. That pattern makes TOI-1355 b’s still-unmeasured three-dimensional alignment especially useful.
The eccentric orbit may retain its migration history
TOI-1355 b has about 5.84 times Jupiter’s mass and 1.42 times its radius, which is why the paper calls it a hot super-Jupiter. Its orbit has a semimajor axis of only 0.040 astronomical units, about six million kilometres, and a period of 2.170 days. Yet its eccentricity is 0.2203, making the path distinctly elliptical.
That is uncommon among very close giant planets around stars hotter than 7,000 kelvin. Tides tend to erase eccentricity over time, leaving many short-period systems nearly circular. The combination of a hot host, a short period and a surviving eccentricity makes TOI-1355 b useful for testing high-eccentricity migration.
In that family of models, interactions with another planet or star can tilt and stretch a giant planet’s orbit. Repeated close passages then dissipate energy through tides, shrinking and circularising the orbit. A major review of hot-Jupiter origins sets out this route alongside disc migration and formation close to the star, while cautioning that no single channel explains the full population.
Space Daily has previously examined TIC 241249530 b, a much more eccentric giant caught on a possible migration path. TOI-1355 b is less extreme, but it has two unusual advantages: it orbits a hot, rapidly rotating star and its changing transit geometry can be measured over a human observing career.
No perturber has been identified in the TOI-1355 system, so planet scattering or the pull of a distant companion remains a formation scenario rather than an observed event. The authors also estimate a tidal circularisation time of about 33 million years, shorter than the star’s approximate 370-million-year age, but that calculation assumes a poorly known planetary dissipation factor. The mismatch could mean the eccentricity was excited relatively recently, or simply that the assumed dissipation is too strong.
The vanishing transit is itself a measurement
The transit method works only when an orbit happens to cross a star from our line of sight. A repeated dip constrains the planet-to-star radius ratio and, once the stellar radius is known, the planet’s radius. When some starlight passes through an atmosphere, the changing spectrum can identify absorbers and constrain temperature structure. Combine the transit with a mass measurement and astronomers can also estimate bulk density.
TOI-1355 b makes the method’s geometric selection effect visible. The planet is not becoming intrinsically fainter. The precessing orbital plane is simply carrying that thin transit corridor away from Earth’s line of sight.
Space Daily previously described Kepler-453 b, whose circumbinary precession makes its transits visible only about nine per cent of the time. The torque there comes from two stars rather than one rapidly rotating, oblate star, but both systems show that a planet can transit during only a limited observing epoch.
The study did not derive TOI-1355 b’s mass from a conventional radial-velocity orbit. Instead, the researchers modelled subtle changes across the TESS phase curve, including Doppler boosting as the star moves and ellipsoidal variation as the planet’s gravity distorts it. Their photometric mass is consistent between the cloudy and cloud-free models, while still carrying an uncertainty of about 14 per cent.
The moving shadow probes the star as well. From the precession rate, the team estimated the star’s Love number, a quantity related to how readily its internal mass distribution responds to distortion. Their logarithmic value of minus 1.83, with an uncertainty of 0.40, agrees with the Sun’s reference value within one standard deviation. The uncertainty is too broad for a fine map of the stellar interior, but repeated transit measurements could narrow it.
This is the less obvious value of a disappearing transit. Losing the geometry is unfortunate for future observers, yet the rate at which it is being lost carries information that a fixed transit could not provide.
The atmospheric opportunity closes with the geometry
The paper’s phase-curve models place the planet’s equilibrium temperature near 2,740 kelvin and its dayside brightness temperature above 3,000 kelvin in the cloud-free case. The authors report a brightness-temperature excess that could reflect a thermal inversion or inefficient heat redistribution, along with changes in the phase offset between observing years. Those are model-dependent indications, not detections of titanium oxide, vanadium oxide or changing weather.
Transit spectroscopy is especially useful because it compares the star before, during and after the planet crosses. Space Daily recently described how Webb separated the morning and evening conditions on WASP-121 b during one transit. TOI-1355 b is a different world around a different star, but the same dependence on an edge-on view applies.
The authors propose high-resolution ground spectroscopy and observations with Hubble or Webb before 2033. After the last transit, other methods may still register the planet or its effect on the star, but transmission spectroscopy will lose the backlight that makes atmospheric characterisation possible.
The window may narrow in quality before it closes in geometry. Grazing transits cover less of the star, and the signal becomes shallower and more sensitive to limb darkening and the exact path of the planet. That makes prompt observations useful even if telescope time closer to 2033 remains available.
What astronomers can settle before 2033
The mid-2033 date assumes the measured impact parameter continues to rise at a constant rate. Real precession traces a cycle, so a longer baseline should reveal curvature eventually. Each new epoch can test whether the linear forecast remains adequate and can move the predicted final transit earlier or later.
Doppler tomography could measure the sky-projected spin-orbit angle and tighten constraints on the orbit’s true three-dimensional tilt. Repeated light curves can refine the stellar quadrupole and Love number. Spectroscopy during the diminishing transits can test the atmospheric hints without treating them as established chemistry.
The participation of small telescopes in the discovery paper is useful context, though the roughly half-percent dip is a photometric measurement rather than something an observer can watch by eye. Coordinated observations at different longitudes can cover events that would otherwise be lost to daylight or weather.
The University of Tokyo’s “hundreds of years” description should remain broad. The paper does not publish a precise reappearance date, and the present data cover only five years of the changing chord. The next well-measured transits, rather than 2033 itself, are where the forecast will be tested.
About this article
This article is for general information and reflection. It is not professional advice. For your specific situation, consult a qualified professional.