mission control Searching for red worlds The SPECULOOS project aims to detect terrestrial well suited for detailed atmospheric characterization, explains Principal Investigator Michaël Gillon.

tudying alien worlds circling stars (La Silla , Chile) and other than the Sun is no longer science TRAPPIST-North (Oukaïmeden Sfiction. Within the last 15 years, the first Observatory, Morocco), also participate in observational constraints have been gathered SPECULOOS, focusing on its ~100 brightest on the atmospheric properties of some giant targets. In fact, SPECULOOS started back exoplanets in orbit around bright nearby in 2011 as a prototype mini-survey on stars1. Extending these pioneering studies TRAPPIST-South with a target list composed to smaller and more temperate exoplanets of the 50 brightest southern ultracool dwarf holds the promise of revolutionizing our Fig. 1 | The SPECULOOS Southern Observatory stars. The goal of this prototype was to assess understanding of rocky planets by enabling at Paranal. Credit: M. Gillon. the feasibility of SPECULOOS, but it did us to assess their diversity at the Galactic much better than expected. Indeed, it detected scale, not only in terms of orbits, but also in around one of its targets, TRAPPIST-1, an terms of atmospheric compositions, surface 12 ×​ 12 arcmin and a pixel scale of 0.35 amazing planetary system composed of seven conditions, and, eventually, habitability. A arcsec on the CCD. The observations are Earth-sized planets in temperate orbits of promising shortcut to this revolution consists carried out using a single ‘I +​ z’ filter that 1.5 to 19 days4,5, at least three of which orbit of the detection of temperate rocky planets has a transmittance of more than 90% from within the habitable zone of the star. transiting host stars that are small, cool, 750 nm to beyond 1 μ​m, with the red end of The detection of TRAPPIST-1 out of and nearby enough to make their detailed the effective bandpass being defined by the a target list of only 50 objects, and the atmospheric characterization — including spectral responses of the CCD. The typical apparent low densities of most of its planets, the detection of potential biosignatures — exposure times are between 30 and 60 s. The suggest that compact systems of water- possible by eclipse spectroscopy with the mean monitoring duration per target is ~19 rich rocky planets could be very common James Webb Space Telescope (JWST) and nights, and is fine-tuned as a function of each around ultracool dwarf stars, in agreement other upcoming facilities2. target’s spectral type so as to reach a 70% with recent theoretical predictions6. If this The SPECULOOS (Search for habitable probability of observing the transit of a planet is the case, then SPECULOOS should find Planets EClipsing ULtra-cOOl Stars) project that receives the same irradiation from its many other TRAPPIST-1-like systems, to aims to seize this opportunity by exploring host star as the Earth does from the Sun. For eventually produce a catalogue of several the nearest 'ultracool dwarf'3 stars for a given night, each SPECULOOS telescope dozen temperate rocky planets well suited transits. With masses ranging from 7–10% observes the same target continuously, so for detailed atmospheric characterization. of the Sun, these Jupiter-sized stars are as to maximize its capacity to detect a low- SPECULOOS is a project led by the located at the extreme bottom of the main amplitude transit. University of Liège () and carried sequence. Their luminosities are less than SPECULOOS will eventually be based out in collaboration with MIT, the 0.1% of the Sun, which makes their habitable on two nodes, one in each hemisphere. Universities of Cambridge, Jeddah, Bern, zones correspond to orbits of only a few The southern one, the SPECULOOS Birmingham, California (San Diego), Cadi days, maximizing the transit probability Southern Observatory (SSO), is currently Ayyad and the Astrophysics Institute of and frequency of a putative temperate being commissioned at the ESO Paranal the Canaries. It is primarily funded by the planet. Their Jupiter-like sizes translate Observatory in the Chilean Atacama Desert European Research Council and several into transit depths of ~1% for an (Fig. 1). It consists of four 1-m telescopes private sponsors and the Simons and the Earth-sized planet — within reach of that will explore ~500 southern ultracool Heising-Simons foundations. ❐ ground-based telescopes. dwarf stars for transits. This exploration The basic concept of SPECULOOS is to should take ~7 years. The northern node of Michaël Gillon search for transits in the ~1,000 ultracool SPECULOOS, the SPECULOOS Northern FNRS Research Associate at the University of Liège, dwarf stars that are bright enough in the Observatory (SNO), is also planned to Liège, Belgium. near-infrared to make the atmospheric consist of four 1-m telescopes, and it will e-mail: [email protected] characterization of an Earth-sized planet with be located at Observatory in the JWST possible. For this purpose it uses (). Its first telescope will be Published online: 5 April 2018 ground-based 1-m-class optical telescopes, installed at the end of 2018, and the full https://doi.org/10.1038/s41550-018-0443-y equipped with robotic equatorial mounts observatory is planned to be operational for References and deeply-depleted 2k ×​ 2k CCD cameras early 2020. SAINT-EX, a new robotic 1-m 1. Crossfeld, I. J. M. Publ. Astron. Soc. Pacifc 127, 941 (2015). that individually monitor each of the project’s telescope being installed at San Pedro Mártir 2. Kaltenegger, L. & Traub, W. A. Astrophys. J. 698, 519–527 (2009). 1,000 targets. The telescopes have a Ritchey– Observatory (Mexico) will also partially 3. Kirkpatrick, J. D. Ann. Rev. Astron. Astrophys 43, 195–245 (2005). 4. Gillon, M. et al. Nature 533, 221–224 (2016). Chrétien optical design with a focal length of contribute to SPECULOOS. Finally, two 5. Gillon, M. et al. Nature 542, 456–460 (2017). 8 m that translates into a field of view of 60-cm robotic telescopes, TRAPPIST-South 6. Alibert, Y. & Benz, W. Astron. Astrophys. 598, L5 (2017).

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