An international team of scientists, led by Laetitia Delrez, astrophysicist at the University of Liège (Belgium), has just announced the discovery of two ‘super-Earth’ type planets orbiting LP 890-9. Also known as TOI-4306 or SPECULOOS-2, this small, cool star located about 100 light-years from our Earth is the second coolest star around which planets have been detected, after the famous TRAPPIST-1. This important discovery is published in the journal Astronomy & Astrophysics.
A first planet, LP 890-9b (or TOI-4306b), the innermost in the system, was initially identified by NASA’s Transiting Exoplanet Survey Satellite (TESS), a space mission dedicated to the search for exoplanets orbiting nearby stars. This planet, which is about 30% larger than the Earth, completes an orbit around the star in just 2.7 days. The ULiège researchers used their ground-based SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) telescopes to confirm and characterise this planet, and also to probe the system in depth for other planets that might have been ‘missed’ by TESS.
“TESS searches for exoplanets using the transit method, by monitoring the brightness of thousands of stars simultaneously, looking for slight dimmings that could be caused by planets passing in front of their stars,” explains Laetitia Delrez, FNRS Postdoctoral Researcher in the Astrobiology and STAR (Faculty of Sciences) research units at ULiège, and lead author of the article. “However, a follow-up with ground-based telescopes is often necessary to confirm the planetary nature of the detected candidates and to refine the measurements of their sizes and orbital properties.” This follow-up is particularly important in the case of very cold stars, such as LP 890-9, which emit most of their light in the near-infrared and for which TESS has a rather limited sensitivity.
In contrast, the telescopes of the SPECULOOS consortium, led by ULiège and installed at the European Southern Observatory (ESO) in Paranal, Chile (SPECULOOS South) and at the Teide Observatory in Tenerife (SPECULOOS North), are optimised to observe this type of star with high precision, thanks to cameras that are very sensitive in the near infrared. “The goal of SPECULOOS is to search for potentially habitable terrestrial planets transiting the smallest and coolest stars in the solar neighbourhood, such as the TRAPPIST-1 planetary system, which we discovered in 2016 thanks to a pilot project with our TRAPPIST-South telescope,” recalls Michaël Gillon, FNRS Senior Research Associate, co-director of the Astrobiology research unit at ULiège and principal investigator of the SPECULOOS project. “This strategy is motivated by the fact that such planets are particularly well suited to detailed studies of their atmospheres and the search for possible chemical traces of life with large observatories, such as the JWST.”
The observations of LP 890-9 obtained by SPECULOOS have proved fruitful, as they have not only helped to confirm the first planet, but have also made it possible to detect a second, previously unknown one. This second planet, LP 890-9c (renamed SPECULOOS-2c by the ULiège researchers), is similar in size to the first one (about 40% larger than the Earth) but has a longer orbital period of about 8.5 days. This orbital period, later confirmed with the MuSCAT3 instrument in Hawaii, places the planet in the so-called ‘habitable zone’ around its star. “Although this planet orbits very close to its star, at a distance about 10 times shorter than that of Mercury around our Sun, the amount of stellar irradiation it receives is still low, and could allow the presence of liquid water on the planet’s surface, provided it has a sufficient atmosphere,” explains Francisco J. Pozuelos, researcher at the Institute of Astrophysics of Andalusia, a former postdoctoral researcher in the Astrobiology and STAR research units at ULiège and one of the main co-authors of the paper. “This is because the star LP 890-9 is about 6.5 times smaller than the Sun and has a surface temperature half that of our star. This explains why LP 890-9c, despite being much closer to its star than the Earth is to the Sun, could still have conditions that are suitable for life.
The research team will then study the atmosphere of this planet, for example with the JWST, for which LP 890-9c appears to be the second most favourable target among the potentially habitable terrestrial planets currently known, surpassed only by the TRAPPIST-1 planets. “This comparison does not, however, consider the fact that LP 890-9c is located close to the inner boundary of the habitable zone and could therefore have an atmosphere that is particularly rich in water vapour, which would then boost its atmospheric signals,” explains Laetitia Delrez. “Moreover, models often differ as to the exact position of this inner boundary of the habitable zone depending on the characteristics of the star. The discovery of LP 890-9c therefore offers a unique opportunity to better understand and constrain the habitability conditions around the smallest and coolest stars in our solar neighbourhood”, concludes the researcher.