ASTEP: Antarctic Search for Transiting ExoPlanets

Exoplanets are now discovered on a daily basis thanks to many astronomical surveys. These surveys require additional follow-up observations for validation, for joint observations at different wavelengths and for a precise and regular determination of the transit ephemerides. This need has become even more important with JWST. In this domain, the potential of robotic medium size (30-80 cm) telescopes has been demonstrated by telescopes such as TRAPPIST in Chile and ASTEP in Antarctica. Since 2020, the ASTEP observations contributed to the publication in peer-review journals of more than 35 articles. In particular, ASTEP has proven its ability to detect small planets (transit depth of 0.06%), very long period systems (a planet candidate with a 500 days period, a triple planetary system with an outer planet of an 83 days orbital period), a circumbinary planet, and generally systems which are extremely difficult to observe from other latitudes (e.g. a 12 hour long planetary transit).

Thanks to the support from the STFC, ESA, the University of Birmingham, OCA and CNRS/INSU, ASTEP has seen several major improvements: in 2022, two new highly sensitive cameras have been installed, combining a higher efficiency and simultaneous observations in two colors. In 2023, the ASTEP mount was replaced with a new direct-drive mount capable of functioning at -80°C, developed at the Lagrange laboratory. Observations have proved to be much more reliable, with a pointing stability increased by a factor 2 and a pointing speed increased by an order of magnitude. ASTEP has been operated continuously since 2017.

We propose to pursue the operations of ASTEP in 2027 and beyond. ASTEP will continue the highly successful follow-up of TESS transiting planet candidates, focusing particularly on transiting planets on long orbital periods: While these planets are challenging to discover and validate, they represent choice targets for atmospheric characterization with the future JWST and ARIEL telescopes. ASTEP is also part of the PLATO follow-up program: Its capability to observe simultaneously in two colors will be an important asset to differentiate bona fide long-period planets from binary interlopers. ASTEP will also monitor known transiting exoplanets in order to obtain precise ephemerides for observations with larger facilities.

In this next phase of ASTEP operations, we will also seek to develop a mature astronomy program at Concordia with two lines of progress: First, combining with other facilities in particular in the infrared with the Cryoscope project (at Concordia) and ExTrA (Chile). Second, building an astronomical tower to limit turbulence, frost and generally lead to greatly improved observation conditions. These developments will be possible thanks to additional sources of funding.