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SPECULOOS: searching for habitable planets amenable for biosignatures detection around the nearest ultra-cool stars

Final Report Summary - SPECULOOS (SPECULOOS: searching for habitable planets amenable for biosignatures detection around the nearest ultra-cool stars)

The SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) project aims to explore for transits the ~1400 “ultracool dwarf” stars known within 40 parsecs (Gillon 2018). With masses ranging from 7 to 10% of the Sun, these Jupiter-sized stars are located at the extreme bottom of the Main Sequence. Their luminosities are less than 0.1% of the Sun, which makes their habitable zones correspond to orbits of a few days only, maximizing the transit probability and frequency of a putative temperate planet. Their Jupiter-like sizes translate into transit depths of ~1% for an Earth-sized planet, within reach of ground-based telescopes. The combination of their small sizes, masses, and temperatures maximizes the potential for the atmospheric characterization of a transiting temperate Earth-sized planet with upcoming giant telescopes like JWST. The ERC Starting Grant 336480 enabled the initiation of the project by funding two of the four 1m robotic telescopes that compose its core facility, the SPECULOOS Southern Observatory (SSO), located at Cerro Paranal in the Chilean Atacama desert. These two telescopes, Io and Europa, were built and tested in Germany between 2014 and 2016, then installed in Chile in 2017. Their installation was followed by an intense commissioning phase that was concluded at the end of 2018. While SSO started officially its scientific operations in Jan 2019, it made an important first discovery in 2018 during its commissioning phase: the detection of young binary brown dwarf bringing a key constraint on the evolution models of brown dwarfs (Triaud et al. 2020). The photometric performances of SSO are nominal (Murray et al. 2020), and the project has reached the sensitivity required to detect temperate Earth-sized planets.
In fact, SPECULOOS started back in 2011 as a prototype mini-survey on the 60cm TRAPPIST robotic telescope with a target list composed of the 50 brightest southern ultracool dwarf stars. The goal of this prototype was to assess the feasibility of SPECULOOS, but it did much better than expected. Indeed, it detected around one of its targets, TRAPPIST-1, an amazing planetary system composed of seven Earth-sized planets in temperate orbits of 1.5 to 19 days. At least three of these planets orbit within the habitable zone of the star, and all of them are particularly well-suited for a detailed atmospheric study with JWST. Thanks to the resonant and transiting configuration of the system, the masses and radii of the planets could be very precisely measured. The detection of TRAPPIST-1 out of a target list of only 50 objects suggest that compact systems of rocky planets could be very frequent around ultracool dwarf stars, in agreement with recent theoretical predictions. If this is the case, then SPECULOOS should find many other similar systems, to eventually produce a catalogue of several dozens temperate rocky planets well-suited for detailed atmospheric characterization with next-generation major astronomical facilities.