2239619

Target RA (J2000): 48.582321 and deg Target Dec (J2000): -34.03414 and deg Gaia GMag: 11.412 Programme ID: CH_PR240023 PI of observing programme: Scarsdale Title of programme: Period Measurement of The Brightest-Host Transiting Habitable Zone Terrestrial Exoplanet Abstract: We propose to use CHEOPS photometry to measure the period of the Transiting Exoplanet Survey Satellite (TESS) planet candidate TOI4353.01 (TIC176797879) initially a duo-transit object that was confirmed with a transit detection in previous CHEOPS observations (priv. comm.). This object is a terrestrial-size exoplanet with three transits each separated by 718 days (two in TESS one in CHEOPS). This even separation and minimal phase coverage means that the period still needs to be determined. Based on the transit duration and a priori likelihood of transit the true period is very unlikely to be 718 days and instead is more likely to be an alias of that spacing in the 34-60 day range. This planet is therefore a likely-terrestrial planet candidate in its star’s optimistic habitable zone (OHZ). It has potential for future mass measurement and even transmission spectroscopy thanks to its bright (for an M star) host. In fact no other OHZ terrestrial has a host star as bright as TOI4353.01. Because of the rarity of terrestrial OHZ exoplanets and the difficulty of discovering more with TESS due to its short sectors followup of candidates like this one is of critical importance. [truncated! Please see actual data for full text]