Astronomers Found a Giant Planet Circling a Star Hotter Than 8,000 Degrees Every Two Days on a Stretched, Egg-Shaped Orbit, and Its Orbit Is Tilting Out of View. After About 2033 It Will Stop Passing in Front of Its Star and Vanish From Earth's Sight for Hundreds of Years.

A team led by astronomers at the University of Tokyo has found a rare kind of planet: a gas giant heavier than Jupiter that whips around a star far hotter than the Sun once every two Earth days, on an orbit that is distinctly oval rather than round. The catch is that the orbit is slowly shifting, and by around 2033 the planet will no longer cross the face of its star as seen from Earth. Astronomers will lose the ability to observe its transits for hundreds of years.

The planet is TOI-1355 b, and its star, TOI-1355, is an early A-type star with a surface temperature of about 8,400 degrees Celsius (15,100 Fahrenheit), compared with roughly 5,500 C (9,900 F) for the Sun. The work, by Project Researcher Noriharu Watanabe and Professor Norio Narita of the university's Graduate School of Arts and Sciences and an international team, was published in the journal Publications of the Astronomical Society of Japan under the title "Discovery of an eccentric hot super-jupiter leaving the transiting geometry of the early-A-type star TOI-1355."

"Planet surveys around stars as hot as or cooler than the sun are flourishing, and thousands of planets have been discovered around such stars. But planet surveys around hotter stars are still not advanced," Watanabe said. "In this project, we hunted for planets around hot stars to examine the diversity of exoplanets more broadly."

NASA's Transiting Exoplanet Survey Satellite, TESS, caught the star dimming periodically, and the team confirmed the dips were a planet passing in front of it. The clue that something was strange came from the secondary eclipse, the smaller dip when the planet passes behind the star. "When we investigated the change in brightness of TOI-1355 from prior data, we found that the secondary eclipse occurred earlier than the timing assumed for a circular orbit," Watanabe said. "This was unusual for hot Jupiters around hot stars, and this is why we began to research this planet in detail." On a circular orbit the two eclipses are evenly spaced; an early secondary eclipse means the planet is racing through one side of its orbit and loitering on the other, the signature of eccentricity.

Extracting the planet's mass from the TESS light curve alone was the hardest part. "We had to derive more complex model equations due to its elliptical orbit," Watanabe said. The team measured both the mass and the eccentricity from the changes in brightness of the star and planet, without relying on the radial-velocity spectroscopy that is difficult on fast-spinning hot stars.

The discovery matters because it supports a specific story about how hot Jupiters come to be. Giant planets cannot form two days from their star; they must migrate inward. One route is quiet, through a disk of gas. The other is violent: gravitational tussles with other giant planets throw one of them onto a highly stretched orbit, and the star's tides then gradually shrink and circularize it over millions of years. A hot Jupiter caught while still eccentric is a planet caught mid-process. "It is considered that the planetary orbit had become highly elliptical initially due to gravitational interaction with other celestial bodies, such as other giant planets, and shrunk gradually later due to the tidal effect of its host star," Watanabe said.

The clock is running because the orbit's orientation is slowly changing relative to our line of sight. Nothing dramatic will happen to the planet itself. But the geometry that lines it up between the star and Earth is temporary, and after roughly 2033 the transits stop. The team is preparing a second paper measuring how the orbit is tilted relative to the star's spin, and is drafting a proposal to point the James Webb Space Telescope at the system while there is still something to see.