天文学家仅剩不到七年观测巨行星凌星，轨道进动将使其错位数百年

TOI-1355 b's transit chord is moving off its rapidly rotating star. If the measured drift holds, the final grazing transit will occur around mid-2033.

By Deep Field · Edited by Lachlan Brown

Published September 21, 2026 · How we edit

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.

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.