A giant planet racing around its star every two days is on a countdown. Researchers say TOI-1355 b, a so-called hot Jupiter, will stop passing in front of its star as seen from Earth around 2033, leaving astronomers only a few years to study it with the method that revealed it.

The planet itself will not disappear or be destroyed. Its orbit is slowly changing orientation, so its path will no longer cross the face of its star from our point of view. After that, astronomers will be unable to observe its transits for hundreds of years, according to the University of Tokyo.

The findings were published Sept. 18 in the Publications of the Astronomical Society of Japan. The work was 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; the paper lists 23 authors. Its title describes the planet as an eccentric hot super-Jupiter leaving the transiting geometry of an early A-type star, a class of star hotter and more massive than the sun.

An Unusual Giant Around a Very Hot Star

TOI-1355 b is a hot Jupiter, a gas giant that orbits extremely close to its star. It completes one orbit in about 2.17 days and has roughly 5.84 times the mass and 1.42 times the radius of Jupiter, according to a summary of the study published by Universe Today. The system lies about 805 light-years from Earth.

Its star is unusual too. TOI-1355 has a surface temperature of about 8,400 degrees Celsius (about 15,200 degrees Fahrenheit), compared with about 5,500 degrees Celsius for the sun, according to the University of Tokyo.

Watanabe said planet surveys around stars as hot as or cooler than the sun have found thousands of planets, while surveys around hotter stars "are still not advanced." The team set out to search hot stars to examine the diversity of exoplanets more broadly.

An Oval Orbit That Tilts Over Time

What caught the team's attention was timing. The planet's secondary eclipse, when it passes behind its star, came earlier than expected for a circular orbit. That pointed to an oval, or eccentric, orbit, with an eccentricity of about 0.22. Earth's orbit, by comparison, has an eccentricity of about 0.0167, meaning it is nearly circular.

The team used data from NASA's Transiting Exoplanet Survey Satellite, known as TESS, which recorded periodic dimming of the star, along with ground observations including the Okayama Observatory of Kyoto University. Measuring the planet's mass from TESS brightness data was the biggest challenge, Watanabe said, because the eccentric orbit required more complex model equations.

The orbit is also slowly tilting relative to our line of sight. When a planet transits, it blocks a small amount of starlight, which lets astronomers measure its size and study its atmosphere. As TOI-1355 b's orbit shifts, its path will move off the face of the star as seen from Earth, and by around 2033 its transits will no longer be visible.

A Clue to How Hot Jupiters Form

Hot Jupiters are thought to form farther from their stars and migrate inward. A highly eccentric orbit suggests a dramatic past, possibly involving gravitational interactions with other planets, followed by gradual shrinking of the orbit because of tides raised by the star.

Watanabe said the orbit likely became highly elliptical "due to the gravitational interaction with other celestial bodies," such as other giant planets, before shrinking gradually because of the host star's tidal effect. According to the university, the discovery supports a theory that hot Jupiters around hot stars can have very dramatic histories.

That makes TOI-1355 b a useful test case. Over time, tides are expected to make eccentric orbits more circular. Catching a planet partway through that process helps astronomers check their models of how giant planets migrate and settle.

Limits of the Countdown

The 2033 date is a projection based on the orbital changes measured so far, not an exact deadline. It depends on how precisely the rate of tilting has been measured, and future observations could refine it.

What will be lost is the ability to watch the planet's transits, which are the main tool for measuring its size and probing its atmosphere, and the method TESS used to find it. The University of Tokyo notes there would be no major changes for the planet itself.

The team is already planning next steps. Watanabe said the researchers are preparing another paper on how the planet's orbit is inclined relative to the star's rotation. "We are also planning a proposal for an observation," he said, referring to NASA's James Webb Space Telescope.

TOI-1355 b is far too faint to see without professional telescopes, but its changing orbit is a reminder that planetary systems are not static, and that for some distant worlds the observing window is measured in years.

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What Readers Want to Know

What is TOI-1355 b?

It is a hot Jupiter, a gas giant about 5.84 times Jupiter's mass that orbits its star every 2.17 days, located about 805 light-years from Earth.

Why will it vanish from view around 2033?

Its orbit is slowly changing orientation, so it will stop passing in front of its star as seen from Earth. The planet itself will not disappear.

How long will its transits stay hidden?

The University of Tokyo says astronomers will be unable to observe its transits for hundreds of years.

What makes this planet unusual?

It has an eccentric, oval orbit and circles a star much hotter than the sun, a combination that is unusual among known hot Jupiters.

How was it studied?

Researchers used data from NASA's TESS space telescope and ground observatories, including the Okayama Observatory of Kyoto University.

What happens next?

The team is preparing another paper on the orbit's tilt and plans to propose observations with NASA's James Webb Space Telescope.