Astronomers have less than seven years to watch a giant planet transit its star before its precessing orbit carries it out of alignment for hundreds of years.

Introduction

In the next few years astronomers will have a fleeting chance to watch a giant exoplanet, TOI‑1355 b, cross the face of its host star. The planet’s orbit is precessing—its orientation slowly wobbles—so the line of sight from Earth will shift out of alignment. If current measurements hold, the final grazing transit will slip past the star around mid‑2033, after which the event will be invisible for centuries. This narrow window makes the coming observations a high‑priority target for ground‑based and space telescopes alike.

What Happened

TOI‑1355 b was identified by the Transiting Exoplanet Survey Satellite (TESS) as a massive, close‑in planet orbiting a bright, rapidly rotating star. Early light‑curve data showed a clear, periodic dip as the planet eclipsed a portion of the stellar disk. Subsequent follow‑up revealed that the planet’s orbital plane is not fixed; the gravitational pull of the oblate, fast‑spinning star is causing the orbit to precess, gradually changing the angle at which we view the transit.

Because of this precession, the transit chord—the path the planet takes across the star’s face—is sliding toward the stellar limb. The measured drift is about a few minutes per year in the timing of the transit’s ingress and egress, and the depth of the dip is steadily decreasing. If the trend continues, the planet will only skim the edge of the star in its last observable pass, producing a shallow, grazing transit before the geometry becomes unfavorable altogether.

Key Details

TOI‑1355 b is a gas giant roughly 1.5 times the radius of Jupiter and several times its mass, completing an orbit every 2.4 days. Its host star is a young, early‑type star rotating at more than 30 km s⁻¹, giving it a noticeable equatorial bulge. This oblateness creates a quadrupole gravitational field that drives the rapid precession, estimated at roughly 0.1° per year—much faster than the precession seen in most known exoplanet systems.

The current drift rate predicts that the final observable transit will occur in mid‑2033, giving astronomers less than seven years from now to collect high‑precision photometry and spectroscopy. During this period, the transit depth will shrink from about 1.2% to under 0.3%, demanding increasingly sensitive instruments. Researchers plan to use the James Webb Space Telescope, the upcoming Extremely Large Telescope, and a network of ground‑based observatories to capture the diminishing signal.

Background

Transit observations have revolutionized exoplanet science, allowing scientists to measure planetary radii, orbital periods, and even atmospheric composition through transmission spectroscopy. Most known transiting planets have relatively stable orbital orientations, giving astronomers the luxury of repeated observations over many years. Precessing systems, however, are rare and provide a natural laboratory for testing theories of stellar oblateness, tidal interactions, and orbital dynamics.

Orbital precession can be driven by several mechanisms: stellar quadrupole moments, tidal bulges, relativistic effects, or perturbations from additional planets. In the case of TOI‑1355 b, the dominant factor is the star’s rapid rotation, which flattens the star and creates a non‑spherical gravitational field. This effect is well documented in our own Solar System—Mercury’s perihelion precesses due to the Sun’s oblateness and general relativity—but the magnitude observed for TOI‑1355 b is unprecedented among exoplanets.

Why It Matters

Capturing the full evolution of TOI‑1355 b’s transit offers a unique chance to probe the interior structure of a fast‑spinning star. By measuring how quickly the transit chord moves, astronomers can infer the star’s quadrupole moment, which in turn reveals details about its mass distribution and rotation profile—information that is otherwise difficult to obtain.

Additionally, the planet’s atmosphere can be studied while the transit depth changes. As the chord skims the stellar limb, different portions of the planet’s atmosphere will be sampled, potentially exposing latitudinal variations in temperature, cloud coverage, or chemical composition. Such data could refine models of atmospheric dynamics on hot Jupiters and improve our understanding of how extreme stellar environments shape planetary atmospheres.

What Happens Next

Observing campaigns are already being coordinated for the next half‑decade. Teams plan to schedule intensive monitoring windows around each predicted transit, employing high‑cadence photometry to track the subtle timing shifts and spectroscopic observations to hunt for atmospheric signatures such as sodium, potassium, and water vapor. Parallel theoretical work will refine precession models, incorporating new measurements to predict when the transit will finally disappear.

After the mid‑2033 grazing event, the geometry will keep the planet’s shadow permanently off the stellar disk from Earth’s viewpoint. The system will remain scientifically valuable, but the loss of transits will preclude direct atmospheric studies until the orbit precesses back into alignment—a process that could take several centuries. In the meantime, the data gathered now will serve as a benchmark for future studies of precessing exoplanet systems.

Conclusion

The impending disappearance of TOI‑1355 b’s transits turns a routine exoplanet observation into a race against time. Within less than seven years, astronomers must extract as much information as possible about the planet’s size, atmosphere, and the star’s internal structure before the orbital precession hides the event for generations. The effort underscores how dynamic and fleeting some cosmic alignments can be, and it highlights the importance of rapid, coordinated observation campaigns in the era of precision exoplanet science.