A Search for Long-Period Transiting Exoplanets with TESS and CHEOPS

Detecting exoplanets via the transit method is inherently biased towards short-period planets. Due to the nature of its observing strategy, the Transiting Exoplanet Survey Satellite (TESS) is particularly susceptible to this detection bias; only 12% of planets confirmed by TESS have orbital periods longer than 20 days. It’s therefore crucial that we expand the sample of longer-period planets from TESS. One way to do this is using duotransits - planet candidates with two observed transits separated by about two years. These duotransits do not have a unique period, instead they have a discrete set of possible period aliases. We use these aliases to perform targeted follow-up of the duotransit with the CHaracterising ExOPlanets Satellite (CHEOPS), to recover the period and ultimately confirm the planet. To select the optimal targets for this CHEOPS program we have developed a specialised pipeline that searches for duotransits in the TESS data. I will present this duotransit pipeline and its first results. We have discovered 3 new duotransits, all of which are non-TOI planet candidates with P > 20 days and RP < 5 REarth, that have since been successfully observed by CHEOPS. This work is revealing a sample of small, longer-period transiting exoplanets, amenable to radial velocity follow-up and future atmospheric characterisation with the recently launched James Webb Space Telescope (JWST)