A team of researchers including those from the University of Tokyo spotted a rare kind of planet that’s unusually hot and eccentric. TOI-1355 b orbits its star TOI-1355, which is also much hotter than our sun, in a strange way and incredibly quickly at around two Earth days. This unusual orbit could provide a rare opportunity to study this class of planet and how they move and evolve. But the evolving orbit of the planet means it won’t be possible to observe it after around 2033.

Project Researcher Noriharu Watanabe and Professor Norio Narita from the Graduate School of Arts and Sciences at the University of Tokyo and their international team explore extra-solar planets and are especially interested in those very unlike anything we find in our own solar system. They’ve recently discovered TOI-1355 b, a so-called hot Jupiter, which, as the name of the class suggests, is a Jupiter-like planet that orbits very close to its host star. In this case, the star is much hotter than our own, at 8,400 degrees Celsius, as opposed to 5,500 degrees Celsius at the surface for our sun. And TOI-1355 b orbits in a peculiar elliptical way that’s enticed the team.

“Planet surveys around stars as hot 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,” said Watanabe. “In this project, we hunted for planets around hot stars to examine the diversity of exoplanets more broadly. When we investigated the change in brightness of TOI-1355 from prior data, we found that the secondary eclipse, a phenomenon in which a planet passes behind a star, occurred faster than the timing assumed for a circular orbit. This was unusual for hot Jupiters around hot stars, and this is why we began to research this planet in detail.”

NASA’s Transiting Exoplanet Survey Satellite (TESS) space telescope recently found periodic dimming of the star TOI-1355 and Narita’s team determined this was due to a planetary transit. They measured the planetary mass and orbital eccentricity — the amount of deviation from a circular orbit — from the change in the stellar and planetary brightness observed by TESS. The discovery of a hot Jupiter with a highly eccentric orbit is important as it supports a theory that hot Jupiters around hot stars can have very dramatic histories involving dynamic migration, perhaps due to planets interacting through gravity and altering each other's orbits. This kind of interaction is predicted to cause a high eccentricity that will later become more circularized with time.

“It is considered that the planetary orbit had become highly elliptical, initially due to the gravitational interaction with other celestial bodies, such as other giant planets, and shrunk gradually later due to the tidal effect of its host star,” said Watanabe. “The biggest challenge so far has been to measure the planetary mass from the brightness change in the TESS data. We had to derive more complex model equations due to its elliptical orbit. We are preparing another paper about our investigation into how much the planetary orbit is inclined relative to the stellar rotation. We are also planning a proposal for an observation using NASA’s James Webb Space Telescope.”