TOI-2180 b is a giant

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

orbiting the G-type star TOI-2180, also known as HD 238894. It was discovered with the help of the

Transiting Exoplanet Survey Satellite Transiting Exoplanet Survey Satellite (TESS, Explorer 95 or MIDEX-7) is a space telescope for NASA's Explorer program, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the ''Kepler' ...

and is currently the exoplanet with the longest orbital period TESS was able to uncover (as of September 2022). 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 Citizen science (CS) (similar to community science, crowd science, crowd-sourced science, civic science, participatory monitoring, or volunteer monitoring) is scientific research conducted with participation from the public (who are sometimes re ...

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 (CTOI) and it was soon promoted into a regular TOI. The TESS-Keck Survey collaboration performed

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

follow-up observations for nearly 2 years with the

Automated Planet Finder The Automated Planet Finder (APF) Telescope a.k.a. Rocky Planet Finder, is a fully robotic 2.4-meter optical telescope at Lick Observatory, situated on the summit of Mount Hamilton, east of San Jose, California, USA.Steven S. Vogt et al., APF - ...

and

Keck I The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and when com ...

. 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 In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...

. TOI-2180 b has a high eccentricity of the orbit at 0.37. The radial velocity monitoring also showed acceleration due to an outer planet or low-mass star in the system. 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 (74.9 °C, 166.7 °F). This is closer to Jupiters 165 K than most discovered giant exoplanets. TOI-2180 b belongs to a small sample of temperature 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 upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals are generally defined as ...

in its envelope and interior.

Future observations

The exoplanet is a poor target for

transmission spectroscopy Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating f ...

because of its high surface gravity and the large radius of the host star. The large radius of the star causes a relative shallow transit depth of about 0.5%. The system is still an excellent target to find rings and exomoons around TOI-2180 b. It is also a good target to study the migration of exoplanets.

Host star

The host star is a 9.16 magnitude bright and slightly

evolved star Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...

with a spectral type of G5. It has a mass of 1.1 and a radius of 1.6 . The radius is increased due to the evolved nature of the star. The star is 116 parsec (379

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s) distant from earth and has an age of about 8.1 billion years.

Habitable Zone

Currently the habitable zone around TOI-2180 is between 1.5 and 2.2 astronomical units.Using a luminosity of 2.544 by Dalba et al. and using step 2 by Morris Because TOI-2180 is slightly evolved, it had a habitable zone closer to the star in the past. At an age of about 3 billion years the habitable zone was located between 1.1 and 1.6 astronomical units.Using MIST evolutionary track with initial mass=1.115 Msun (best match from isochromes), initial metallicity e/H0.269 (Delba et al for TOI-2180) and using synthetic photometry output UBV etc. Then the approach by Morris was used, assuming the star was an F-type back then.

Notes

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References

Transiting exoplanets Exoplanets detected by radial velocity Exoplanets discovered by TESS Exoplanets discovered in 2022 2180