TOKYO — Astronomers have only a few years left to watch a giant planet cross the face of its star. TOI-1355 b, a hot super-Jupiter that races around its scorching host star once every 2.17 days, is slowly drifting out of alignment as seen from Earth — and researchers calculate that its transits will cease around the middle of 2033, closing a rare window on a strange and instructive world.
The planet itself is going nowhere. It will keep orbiting its star long after 2033. What changes is geometry: the plane of its orbit is precessing, gradually sliding the narrow corridor through which the planet appears to cross the stellar disc out of Earth's line of sight. The University of Tokyo, whose researchers led the study, says astronomers will then be unable to observe its transits for hundreds of years.
Illustration: A giant gas planet on an elongated elliptical orbit around a hot blue-white star.
What Happened
The findings were published on September 18, 2026, in the journal Publications of the Astronomical Society of Japan, in a paper led by Project Researcher Noriharu Watanabe and Professor Norio Narita of the University of Tokyo's Graduate School of Arts and Sciences, with an international team of 23 authors. The study, titled "Discovery of an Eccentric Hot Super-Jupiter Leaving the Transiting Geometry of the Early-A-type star TOI-1355," drew on observations from NASA's Transiting Exoplanet Survey Satellite (TESS) in 2019, 2020, 2022 and 2024, plus follow-up photometry from ground observatories.
The headline result is a countdown. By tracking how the planet's transit chord — the path its silhouette takes across the star — has moved over five years, the team measured a steady drift. The transit impact parameter, which is zero when a planet crosses the centre of its star and near one when it grazes the edge, rose from about 0.853 in 2019 to 0.862 in 2020, 0.904 in 2022 and 0.928 in 2024. Extrapolating that drift at a constant rate, the researchers calculate that the planet's centre will miss the stellar disc around late 2028, while its outer edge will keep clipping the star until roughly mid-2033, when all overlap ends.
That forecast is a projection, not a guaranteed timetable. It assumes the measured drift continues at the same rate; real precession traces a cycle, so a longer baseline of observations could move the predicted final transit earlier or later. The paper does not give a precise date for when transits might return, and the University of Tokyo's "hundreds of years" description should be read as a broad estimate rather than a fixed calendar.
The discovery itself is unusual in two ways. TOI-1355 b is a hot super-Jupiter with about 5.84 times the mass of Jupiter and 1.42 times its radius, orbiting just 0.040 astronomical units — about six million kilometres — from its star. And despite that tight, two-day orbit, its path is distinctly elliptical, with an eccentricity of 0.2203. Most hot Jupiters in such close orbits have been circularised by tides; TOI-1355 b has kept its oval shape, which makes it a valuable test case for how giant planets migrate toward their stars.
Background
TOI-1355, also catalogued as HD 210058, is an early A-type star roughly twice the mass of the Sun and about 806 light-years from Earth, with a surface temperature of around 8,400 degrees Celsius compared with the Sun's roughly 5,500 degrees. Planet surveys around stars as hot as this are still far less advanced than surveys around Sun-like stars, Watanabe said, which is why his team deliberately hunted for planets around hot stars.
The first clue came from TESS, which detected periodic dimming of the star as the planet passed in front of it. But the secondary eclipse — the moment the planet passes behind its star — arrived earlier than expected for a circular orbit. That timing mismatch told the team the orbit was elliptical, not circular, and prompted the detailed study.
Measuring the planet's mass was the hardest part. Rather than a conventional radial-velocity orbit, 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. The resulting photometric mass is consistent between the team's cloudy and cloud-free models, though it still carries an uncertainty of about 14 per cent.
The star's own spin is what drives the vanishing act. TOI-1355 rotates fast, with a projected rotation speed of 80.8 kilometres per second, which makes it slightly oblate and gives its gravitational field an uneven, quadrupole component. Because the planet's orbital plane is tilted relative to the star's equator, that uneven field torques the orbit, slowly swivelling the plane around the star's spin axis. Astronomers call this nodal precession — different from apsidal precession, in which the long axis of an eccentric orbit rotates within its plane.
Illustration: Astronomers at an observatory telescope watching a distant star dim as a planet transits it.
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. TOI-1355 b's true three-dimensional alignment is not yet pinned down — a spectroscopic attempt in 2023 lacked enough signal to trace the planet's shadow across its fast-spinning star — so the authors say better Doppler tomography during future transits is needed to test whether it resembles those near-polar cases.
The eccentric orbit also preserves a clue about the planet's past. In the high-eccentricity migration picture, gravitational interactions with another planet or star can tilt and stretch a giant planet's orbit, after which tides gradually shrink and circularise it. The authors estimate a tidal circularisation time of about 33 million years, shorter than the star's approximate age of 370 million years — a mismatch that could mean the eccentricity was excited relatively recently, or simply that the assumed tidal dissipation is too strong. No perturbing companion has been identified in the system, so scattering remains a scenario rather than an observed event.
Why It Matters
The vanishing transit makes visible a selection effect built into the most productive planet-hunting method we have. The transit method works only when an orbit happens to cross its star from our line of sight; a repeated dip constrains the planet's size, and starlight filtering through its atmosphere during transit can reveal composition. When TOI-1355 b's geometry closes, transmission spectroscopy — which depends on that stellar backlight — will go with it, even though other methods may still register the planet or its effect on the star.
That is why the deadline matters scientifically, not just dramatically. The authors propose high-resolution ground spectroscopy and observations with Hubble or Webb before 2033, and the University of Tokyo team says it is preparing a proposal for James Webb Space Telescope observations. The window may also narrow in quality before it closes in geometry: grazing transits cover less of the star, so the signal becomes shallower and more sensitive to limb darkening and the exact transit path, making prompt observations valuable even while transits remain available.
The drift itself is also a measurement. From the precession rate, the team estimated the star's Love number — a quantity describing how readily its internal mass distribution responds to distortion — at a logarithmic value of minus 1.83, with an uncertainty of 0.40, which 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. Losing the geometry is unfortunate for future observers, yet the rate at which it is being lost carries information a fixed transit could not provide.
Finally, TOI-1355 b sharpens a broader question: how typical is our own solar system? Planets around hot stars remain comparatively little surveyed, and oddball worlds like this one help astronomers understand how planetary systems form, migrate and evolve — and how many kinds of system current techniques may still be missing simply because their geometry does not favour detection from Earth.
Illustration: A planet's tilted orbit precessing so its path drifts off the face of its star.
What Happens Next
Between now and mid-2033, TOI-1355 b will complete roughly 1,100 of its rapid orbits, though observing seasons, daylight, weather and telescope schedules mean only a fraction of those transits will be useful. Each new well-measured transit epoch will test whether the linear forecast still holds and could shift the predicted final transit.
Watch for three things. First, Doppler tomography during upcoming transits, which could measure the sky-projected spin-orbit angle and settle whether the orbit is near-polar like its precessing cousins. Second, repeated light curves that refine the stellar quadrupole and Love number, turning the drift rate into a sharper probe of the star's interior. Third, spectroscopy during the diminishing transits — and any Webb observations the team secures — that could test the model's hints of a very hot dayside, with an equilibrium temperature near 2,740 kelvin and a dayside brightness temperature above 3,000 kelvin in the cloud-free case. Those temperature indications are model-dependent, not detections of specific molecules or changing weather, and should be read that way.
The team is also preparing another paper on how much the planet's orbit is inclined relative to the star's rotation. If that work confirms a strongly tilted orbit, TOI-1355 b will become an even cleaner laboratory for the physics of precession around hot, fast-spinning stars — a laboratory with a closing door. After mid-2033, astronomers who want to study this world will have to wait, on the University of Tokyo's estimate, for hundreds of years before its transits return to view.
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FAQ
What is TOI-1355 b?
TOI-1355 b is a hot super-Jupiter — a gas giant with about 5.84 times Jupiter's mass and 1.42 times its radius — that orbits the early A-type star TOI-1355 (also HD 210058), about 806 light-years away, once every 2.17 days.
Will the planet disappear in 2033?
No. The planet will keep orbiting its star. What will end around mid-2033 is its transits as seen from Earth: its precessing orbital plane will no longer carry it across the face of its star from our line of sight, so the transit method will stop working for this system for hundreds of years, according to the University of Tokyo.
Why is its orbit unusual?
Its orbit is distinctly elliptical, with an eccentricity of 0.2203, despite a tight two-day orbit around a star much hotter than the Sun. Tides usually circularise such close orbits, so the surviving eccentricity supports theories in which gravitational interactions stretched the orbit before tides shrank it.
How do astronomers know the transits will end?
By measuring the transit chord from TESS observations in 2019, 2020, 2022 and 2024. The impact parameter rose steadily from about 0.853 to 0.928, showing the orbital plane is precessing. Extrapolating that drift gives a final grazing transit around mid-2033, though the date is a projection that future observations could refine.
What can astronomers still learn before 2033?
Doppler tomography could measure the orbit's tilt, repeated light curves could refine the star's interior properties, and spectroscopy — including possible Webb observations — could probe the planet's hot atmosphere while the transit backlight is still available.
Sources
- University of Tokyo, "Time's running out to catch giant planet's strange orbit," press release, September 18, 2026.
- Watanabe, N., et al., "Discovery of an Eccentric Hot Super-Jupiter Leaving the Transiting Geometry of the Early-A-type star TOI-1355," Publications of the Astronomical Society of Japan, September 18, 2026, DOI: 10.1093/pasj/psag103.
- Space Daily / Universe, "Astronomers have less than seven years to watch a giant planet transit its star," September 21, 2026 (impact-parameter series, precession analysis and 2033 forecast detail).
- NASA Transiting Exoplanet Survey Satellite (TESS) observations of TOI-1355, 2019–2024, as reported in the study.
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