Project

The SPECULOOS project searches for potentially habitable exoplanets around the smallest and coldest stars of the solar neighborhood.

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ince 1995, hundreds of planets have been detected outside our solar system (exoplanets), revealing that most of the stars of our Galaxy have their own planetary systems. In parallel to these detections, many projects have succeeded in characterizing in detail some giant exoplanets, in particular to analyze their atmospheres. Applying the techniques developed by these pioneering studies to telluric exoplanets is the next step to study other Earths and search for traces of life beyond our solar system.

To pave this way, the SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) project aims to detect terrestrial planets eclipsing some of the smallest and coldest stars of the solar neighborhood. This strategy is motivated by the possibility of studying such planets in detail with the James Webb Space Telescope (JWST) or with the futur giant telescopes of the future like the E-ELT. The exoplanets detected by SPECULOOS should thus offer us the opportunity to analyze the atmosphere of extrasolar worlds similar to our Earth, in order to look for traces of biological activity.

SPECULOOS aims to detect exoplanets using the so-called transit method based on the apparent dimming of a star when one of its planets ’transits’ it, i.e. passes in front of it. When an exoplanet transits, we can accurately measure the ratio between its size and that of its star. As stellar sizes are now easy to estimate, transits give thus access to the measurement of the size of the planet. In addition, its mass can sometimes be measured by a complementary method. The combination of the measurements of the mass and the radius leads directly to an estimate of its average density, which allows to constrain its internal composition. Moreover, the particular geometrical configuration of the orbit gives the possibility to study directly the planetary atmosphere without having to spatially resolve the planet and its host star. Indeed, photometric and spectroscopic measurements obtained when the planet is hidden by its star allow to characterize its emission and to study its composition and atmospheric structure. Similar measurements performed when the planet transits in front of its star, allow to detect some atoms and molecules in its atmospheric limb.

The ability of this method to detect and study in detail small rocky planets, like the Earth, is all the stronger as the host star is small, cold, and close. Indeed, for the same planet, the smaller the star, the larger the fraction of its surface hidden by the planet or by its atmospheric limb during the transit. The smaller and colder the star, and therefore the less luminous it is, the greater the contribution of the planet to the light measured by our instruments. And the closer the system is, the more precise our measurements are. Taking these elements into account, we can estimate that the detection and study of an atmosphere of an exoplanet similar to the Earth (size, mass, atmosphere, temperature) in transit is only possible if its star is an ultra-cool dwarf. This category includes stellar objects whose surface temperature is less than half that of the Sun. These objects have respectively a mass and a radius of less than 10% and 15% of those of our star. If their mass is less than ~7% of that of the Sun, they are not stars, but brown dwarfs, i.e. objects that form like stars but do not transform hydrogen into helium in their core (nuclear fusion).

The goal of SPECULOOS is to observe all ultra-cool dwarfs that are sufficiently close and bright to allow the atmospheric study of a potentially habitable Earth-sized exoplanet. About 90% of these objects are very low mass stars, and the rest are brown dwarfs. To be clear, the goal of SPECULOOS is to detect transiting exoplanets around as many of these stars as possible, not to study their atmospheres. For that, we need an extremely powerful space telescope like the James Webb Space Telescope.

SPECULOOS is a project led by the University of Liege (project leader: Michaël Gillon) and carried out in partnership with the University of Cambridge, the University of Birmingham, the Massachusetts Institute of Technology, the University of Bern, and the University of Zurich. It is based on a network of robotic telescopes distributed on two main observatories, SPECULOOS-South in Chile (4 telescopes) and SPECULOOS-North in Tenerife (1 telescope, soon 2), complemented by the SAINT-Ex telescopes (1 telescope in Mexico). 

The project was born in 2011 as a prototype on the TRAPPIST-South telescope in Chile. This prototype discovered the extraordinary planetary system TRAPPIST-1 (= SPECULOOS-1) composed of seven planets similar to the Earth in orbit around an ultra-cold dwarf star located at 40 light years. After this prototype phase, the project itself started its operations in 2019. In 2022, it discovered a new potentially habitable planet around a very-low-mass star named SPECULOOS-2 (Delrez et al. 2022). In 2024, it then found an Earth-sized planet on a very short orbit around SPECULOOS-3, an ultracool dwarf 55 light years away (Gillon et al. 2024). Many other discoveries should follow.

SPECULOOS is funded by the European Research Council, the FNRS, the University of Liege, the Walloon Region, the SNSF, as well as by the Balzan Prize, the MERAC Prize, Francqui, Simons, and Heising-Simons Foundations.