Exoplanet transits through the exoclock project in support of the Ariel space mission

Any follow-up observation of the transit events, either with photometry, low or high-resolution spectroscopy, or even with polarimetry, relies on a reasonably accurate knowledge of the timing of the transit. Most observations need a certain amount of out-of-transit observations before and after the transit event to establish models for the systematics in the data. If an individual planetary system is not observed, our knowledge of the transit timing degrades with time. This is because the timing uncertainty increases linearly with the number of transit epochs that passed since the last observations (Mallonn et al. 2019, A&A, 622, A81; Dragomir et al. 2020, AJ, 159 5 , 219; Zellem et al. 2020, Publ. Astron. Soc. Pac., 132 1011 , 054401).

Due to the large number of exoplanets discovered per year nowadays, there is a non-negligible number of systems for which the timing uncertainty reached values of one hour or more. This uncertainty is too high for follow-up observations with space-based or large ground-based telescopes, where observing time is very expensive and a good coverage of out-of-transit observations cannot be guaranteed within a limited observing interval (Alonso et al. 2014, A&A, 567, A112; Benneke et al. 2017, ApJ, 834 2, 187).

Ariel Ephemeris WG and the ExoClock project

The ESA Ariel space mission will study what exoplanets are made of, how they formed and how they evolved by surveying a diverse sample of about 1000 known extrasolar planets, probing their atmosphere through spectroscopy in visible and infrared simultaneously (Barnes & Haswell 2022, Exp Astron, 53, 589–606; Tinetti et al. 2018, Exp Astron, 46, 135-209). It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. For this technique to be as efficient as possible and to organise large-scale surveys we need to have a good knowledge of each exoplanet’s expected transit time well before Ariel’s launch in 2029. In fact, when planning observations for a single planet or for a small number of planets, ephemeris updates can be done on a per-case basis. However, in the new era of characterising large numbers of planets, such an effort needs to be organised in a much more efficient way through an open, interactive, collaborative platform, in order to make the best use of all the currently available resources, such as the ExoClock project (Kokori et al. 2022a, Exp Astron, 53, 547–588). ExoClock has been developed by the Ariel Ephemerides working group in a manner to make the best use of all available resources: observations reported in the literature, from space instruments and, mainly, from ground-based telescopes (which include both professional and amateur observatories). In this effort, the ExoClock team has been actively collaborating with both professional and amateur astronomers coming from various countries around the world to achieve an effective pro-am collaboration. Participants contribute with observations of exoplanets by using a wide range of telescopes, from backyard ones to large facilities owned by organisations and universities. Apart from the science goal, the team’s efforts include public engagement with science, by creating educational and user-friendly tools to facilitate participation of broader communities such as citizen scientists and school students (Kokori 2024, EPSC2024-480).

Contributions from the Europlanet Telescope Network by the Sabadell Team

The final scientific product of ExoClock is a verified catalogue of homogenous ephemerides for Ariel candidate targets that is continuously updated and incremented. This is compiled with yearly basis publications (see Kokori et al. 2022b, ApJS, 258 2, 40; Kokori et al. 2023, ApJS, 265 1, 4., with the latest update being the Data Release 4 (Kokori et al. 2024, in prep). These public catalogues will be beneficial to both the Ariel mission and, most importantly, to the exoplanet community as a whole, providing a much required homogeneous and self-coherent catalogue spacing many host-stars parameters.

The upcoming DR4 includes 24 light-curves obtained from 3 telescopes that are part of the Europlanet Telescope network (Heward et al. 2020). The ETN provides access to professional and trained amateur astronomers involved in planetary science or exoplanet research to small and medium-sized telescopes from professional observatories in the network around the globe. We highlight these contributions: the light-curves were obtained by the Sabadell team, which is composed mostly of amateur astronomers within the Sabadell Astronomical Society, but also university students and professionals. By receiving funding for several nights of telescope time under the Europlanet 2024 RI NA2 Call, further collaborations with the IAC80 telescope at the Teide Observatory (Tenerife), the 1.23m telescope at the Calar Alto Observatory (Almería) and the Joan Oró telescope (TJO; Colomé et al. 2010) at the Montsec Observatory (Lleida) were made possible, even beyond the NA2 Call. The work up until DR4 includes at least 11 low SN transits were the use of larger apertures was necessary (e.g. TOI-4479b, TOI-1272b, TOI-969b, K2-284b, LHS1478b), highlighting the importance of the addition of larger apertures to the network as well as the ongoing collaboration with such facilities in supporting the Ariel mission.