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Decoding Lights from Exotic Worlds

Final Report Summary - EXOLIGHTS (Decoding Lights from Exotic Worlds)

Current statistical estimates indicate that, on average, every star in our Galaxy hosts at least one planetary companion, i.e. our Milky Way is crowded with over one thousand billion planets. The number of known planets has increased by two orders of magnitude (~4000) in the last two decades and it is expected to increase further in the next decade thanks to a long list of planned exoplanet missions and surveys from the ground. The most revolutionary aspect of this young field is the discovery that the Solar System does not appear to be the paradigm in our Galaxy but rather one of the many possible configurations we are seeing out there. For instance, the most common planets in our galaxy are objects heavier than the Earth and lighter than Neptune, so called super-Earths, of which we know little to nothing.

The characterisation of exoplanetary atmospheres represents the next major advance in this new field, as the atmospheric chemical composition is a key diagnostic of the formation and evolution processes responsible for the diversity among the planets in our Galaxy. However, before the ExoLights program started, work in exoplanet atmospheric spectroscopy had been undertaken piecemeal with one or perhaps two spectra over a narrow wavelength range being studied at any one time. This approach is inadequate to provide answers to the key questions of exoplanetary science spelled out above.

To bring our understanding of these exotic worlds to a new level, the ExoLights team has incorporated a wide range of interdisciplinary and intersectorial fields, ranging from quantum physics modelling of complex molecules, to space-mission engineering and atmospheric modelling, to the use of deep learning techniques to calibrate and correlate large data sets to unprecedented precision. ExoLights has created the infrastructure needed to enter the era of big-data for the characterisation of exoplanets.

It is through the integrated and internally coherent approach of sophisticated AI data de-trending and Bayesian analysis techniques that we could move the field from its patch-work understanding to a self-consistent footing. In recent years the study of exoplanetary atmospheres has therefore shifted from the investigation of individual planets to the characterisation of populations, with the first catalogue of 30 exoplanet atmospheres being studied at any one time published only very recently by the ExoLights team (Tsiaras+, 2018). Other high-impact results achieved by the ExoLioghts team include the first detection of an atmosphere around a super-Earth and the first water detection in the atmosphere of a super-Earth in the habitable-zone of its parent star (Tsiaras+,2017,2019).

These studies have indicated that a statistically significant number of planets (approximately two orders of magnitude larger than the sample expected to be observed with future general purpose facilities) needs to be observed systematically in order to fully test models and understand which are the relevant physical parameters. This requires observations of a large sample of objects, generally on long timescales, which can only be done with a dedicated space instrument, rather than with multi- purpose telescopes not optimised for the specific application.

During the past years, the ExoLights team has been leading the effort to plan such missions in Europe.
ARIEL, proposed to the European Space Agency in 2015 (Tinetti+,2015, is currently the next medium-size M4 450 M€ ESA space mission, to be launched in 2028. ARIEL will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 exoplanet atmospheres, simultaneously in visible and infrared wavelengths (Tinetti+,2018). 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.
Twinkle is a new (semi-)commercial satellite which aims to exploit the off-the-shelf capabilities developed by the Earth observation community to launch a cost-effective, quick (3 years to launch) precursor mission to Ariel (Tessenyi+, 2014). A Phase-A study was completed by a consortium of research institutes and industries (Savini+, 2016) funded through the ExoLights program. Twinkle is just entering its mission definition phase led by Airbus and funded by private investments; the aim is for launch 2022. In its seven year-mission, Twinkle will observe spectroscopically exoplanets, stars and Solar-system bodies in the visible and infrared light.