TOI-2180 b
TOI-2180 b is a giant exoplanet orbiting the G-type star TOI-2180, also known as HD 238894. It was discovered with the help of the Transiting Exoplanet Survey Satellite and is currently the exoplanet with the longest orbital period TESS was able to uncover. TOI-2180 b orbits its host star every 260.16 days.
Discovery
TOI-2180 b was first identified as an exoplanet candidate due to a single transit with TESS at 12./13. December 2019 by a group of citizen scientists called the Visual Survey Group, which includes Thomas Lee Jacobs, a former U.S. naval officer. The group was using the light curve processing tool lcTools. In May 2020 the Planet Hunters: TESS collaboration announced this object as a Community TESS Object of Interest and it was soon promoted into a regular TOI.The TESS-Keck Survey collaboration performed radial velocity follow-up observations for nearly 2 years with the Automated Planet Finder and Keck I. The follow-up observations uncovered that the single transit was caused by a long-period planet.
Orbital properties
TOI-2180 b has a long orbital period of 260.16 days, which also leads to a long transit duration of 24 hours. The distance to the host star is 82.8% the sun-earth distance. The planet does not orbit inside the habitable zone, despite this close resemblance in semi-major axis. TOI-2180 b has a high eccentricity of the orbit at 0.37.The second transit was not detected from the ground and the third transit was not observed. The fourth transit was observed at 31. January/01. February 2022, refining the orbital period. The next transit will occur on 2022 October 18 at 21:28 UTC.
Physical properties
The planet has the same size as Jupiter, but is 2.8 times heavier than Jupiter. TOI-2180 b stand out because of its cold estimated temperature of about 348 Kelvin. This is closer to Jupiters 165 K than most discovered giant exoplanets. TOI-2180 b belongs to a small sample of temperate Jupiters with a temperature <400 K that transit, such as Kepler-167 e, WD 1856+534 b, Kepler-1704 b, KOI-3680 b, Kepler-1514 b and Kepler-539 b. TOI-2180 b has by far the brightest host star with a visual magnitude of 9.16, which is about 3 magnitudes brighter than the next brightest system in this sample.The planet is likely enriched in metals compared to its host star. The discovery team inferred that TOI-2180 b is enriched in metals by a factor of about 5 compared to its host star. This means it has about 100 of heavy elements in its envelope and interior.