UNIVERSITY PARK, Pa. — A newly discovered gas giant planet with an elliptical, comet-like orbit is closing in on its host star, according to new research by a Penn State graduate student and a team of collaborators. The researchers believe the planet, discovered using data from NASA’s Transiting Exoplanet Satellite Survey (TESS), will eventually become a “hot Jupiter,” a Jupiter-sized planet located extremely close to its host star that completes an orbit every few days. The researchers describe the new planet, which may also experience dramatic temperature changes and extreme weather events, in a paper appearing Oct. 7 in the Astrophysical Journal Letters.

“Astronomers have several theories about how hot Jupiters have come to be located so close to their host stars,” said Jiayin Dong, a graduate student at Penn State and lead author of the new study. “One theory suggests that the exoplanet began with a much larger orbit that ended up shrinking over time. This newly discovered planet appears to be doing just that, which gives us the rare opportunity to study the planet in action as its orbit shrinks.”

The planet orbits TOI-3362, an unexceptional star 50% larger than the sun. The research team was systematically searching for gas giant planets located close to their stars from observations of nearly 3 millions stars made by TESS in its first year when Dong realized this gas giant, called TOI-3362 b, has an 18-day orbit that is extremely eccentric.

“The time that the planet spends passing in front of its host star is short compared to what we expected based on the planet’s orbital period,” said Dong, “This short duration implies that the planet has an elongated orbit, unlike all the planets we have in the solar system.”

To confirm this theory, Dong turned to data from the MINERVA-Australis exoplanet observatory in Australia and the CHIRON instrument at the Astronomical Observatory Cerro Tololo in Chile, using radial velocity measurements that map out the slight motion of the star due to the gravitational pull of the orbiting planets. The researchers confirmed that TOI-3362 b has a highly elliptical orbit, with its distance to the host star at the farthest point nine times the distance at its closest point.

According to the researchers, such a highly elliptical orbit is uncommon, especially for a planet without a long orbital period. In the solar system, only small bodies such as Halley's Comet can have eccentricities as high as TOI-3362 b. In fact, TOI-3362 b currently holds the record of the highest orbital eccentricity among giant planets with an orbital period less than 30 days. Only two other transiting planets — planets that we observe pass in front of their host star — are known to have similar or higher eccentricity, but they are on orbits much farther away from their stars.

The research team is especially interested in TOI-3362 b, because its extreme orbit suggests it is “migrating” closer to its star and might provide an example of how and where large planets form. Because the formation of a giant planet requires a substantial amount of material from the gas and dust surrounding a new-born star, most theories predict these planets are born far away from the star where the materials are plenty, and with circular orbits, similar to Jupiter and Saturn. To explain why many of the giant planets discovered are so close to their host star, these planets’ orbits must have shrunk.

“One way this orbit shrinkage can happen is that an external star or planet gave the giant planet a kick in the past, so its orbit got elongated and the tidal interaction between the star and the planet started to drag the planets closer and closer to the star,” said Rebekah Dawson, Shaffer Career Development Associate Professor in Science, associate professor of astronomy and astrophysics at Penn State, and an author of the study. “The prediction is that TOI-3362 b will eventually become a hot Jupiter, one of the first types of known transiting exoplanets, with a 3.5-day circular orbit."

Because the dramatic change in the distance of TOI-3362 b to its host star happens in a brief 18-day cycle, the researchers believe the planet may experience extreme weather events. When TOI-3362 b is closest to its host star, its surface temperature may reach three times the temperature compared to when it’s the farthest away from its host star. This temperature contrast is twice as much compared to the temperature ratio between Death Valley and the coldest part of Antarctica. Metals such as aluminum may be in vapor form in TOI-3362’s atmosphere at its hottest point, but are likely to condense into liquid form, like raindrops, when the planet moves to the cooler part of the orbit in nine days.

“It is also thought that exoplanets with such an extreme orbit like TOI-3362 b have incredibly powerful winds at its equator with speeds up to several thousand miles per hour,” said Dong. “Missions such as NASA’s Hubble and the upcoming James Webb Space Telescope may be able to probe these predicted weather phenomena.”

In addition to Dong and Dawson, the research team includes Jonathan Jackson, a graduate student at Penn State, as well as researchers from the Massachusetts Institute of Technology, the University of Southern Queensland, Michigan State University, Harvard University, the University of Birmingham, Indiana University, the University of Arizona, the University of Cote d’Azur, the European Space Agency, Swarthmore College, Hazelwood University, the University of Liege, Cadi Ayyad University, the University of Texas at Austin, the University of California at Riverside, the University of Louisville, Nanjing University, Princeton University, the NASA Ames Research Center, and Caltech/IPAC-NASA Exoplanet Science Institute.

This work was supported by the NN-EXPLORE program, which is the joint NASA-NSF Exoplanet Observational Research program. This work has made use of data from the European Space Agency (ESA) mission Gaia.