Massive Gas Giant Found Circling TOI-2180

A team of professional astronomers and citizen scientists has discovered a giant exoplanet orbiting the G5-type star TOI-2180.

TOI-2180 is a bright, slightly evolved G5 star about 380 light-years away in the constellation of Draco.

Otherwise known as TIC 298663873, HD 238894 or SAO 31031, it hosts at least one planet.

Named TOI-2180b, the alien world has the same diameter as Jupiter, but is 2.8 times more massive.

“TOI-2180b is such an exciting planet to have found,” said Dr. Paul Dalba, an astronomer with the University of California, Riverside.

“It hits the trifecta of: (i) having a several-hundred-day orbit, (ii) being relatively close to Earth, and (iii) us being able to see it transit in front of its star.”

“It is very rare for astronomers to discover a planet that checks all three of these boxes.”

TOI-2180b is special because it takes 260.8 days to complete a journey around its star, a relatively long time compared to many known gas giants outside our Solar System.

The planet also contains 105 times the mass of Earth in elements heavier than helium and hydrogen.

“Nothing quite like it exists in our own Solar System,” Dr. Dalba and his colleagues said.

The single-transit event of TOI-2180b was spotted by citizen scientists in the data from NASA’s Transiting Exoplanet Survey Satellite (TESS).

Using the Automated Planet Finder telescope at Lick Observatory, the astronomers observed the planet’s gravitational tug on the star, which allowed them to calculate the mass of TOI-2180b and estimate a range of possibilities for its orbit.

Hoping to observe a second transit event, they organized a campaign using 14 different telescopes across three continents in the northern hemisphere.

Over the course of 11 days in August 2021, the effort resulted in 20,000 images of the TOI-2180 star, though none of them detected the planet with confidence.

“Single-transit discoveries like TOI-2180b highlight the exciting potential of the TESS mission to find planets with long orbital periods and low irradiation fluxes despite the selection biases associated with the transit method,” the researchers said.

The team’s paper was published in the Astronomical Journal.

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Paul A. Dalba et al. 2022. The TESS-Keck Survey. VIII. Confirmation of a Transiting Giant Planet on an Eccentric 261 Day Orbit with the Automated Planet Finder Telescope. AJ 163, 61; doi: 10.3847/1538-3881/ac415b