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Since 1995, hundreds of exoplanets have been discovered, revealing that most of the stars in our galaxy host their own planetary system. Some of them are home to planets located in their star’s habitable zone, motivating the space search for other life forms. Recently, an international team discovered two super-Earths in the habitable zone of the red dwarf LP 890-9. One of them may be the second most habitable exoplanet discovered so far.
In the James Webb Space Telescope era, temperate terrestrial exoplanets transiting ultra-cold red dwarfs offer unique opportunities to characterize their atmospheres as well as search for gas biosignatures. The goal is to understand how often and under what conditions life can arise.
It is for this purpose that the SPECULOOS project (“Search for habitable Planets EClipsing ULtra-cOOl Stars”) was created. Although SPECULOOS officially began science operations in 2019, it was initiated in 2011 as a prototype survey targeting fifty of the brightest red dwarfs on the south side with the TRAPPIST South telescope. This research prototype led to the discovery of the TRAPPIST-11 system, consisting of seven terrestrial planets transiting a nearby ultracool M8V dwarf.
The discovery of this reference system prompted a wave of theoretical and observational follow-up studies, so that today the TRAPPIST-1 planets are the best-studied terrestrial planets outside our solar system. In fact, the authors of this discovery declared in 2017 in a press release: “ The Trappist-1 system is the largest treasure trove of Earth-sized planets ever discovered around a single star “.
Recently, an international team of scientists, led by Laetitia Delrez, astrophysicist at the University of Liège, has just announced the discovery of two “super-Earth” planets orbiting a dark red dwarf star, TOI-4306. It is the second coldest star around which planets have been discovered, after TRAPPIST-1, which is about a hundred light years from our Earth. These rocky planets are slightly larger than Earth and appear to be habitable. In fact, one of them may be the second most habitable exoplanet discovered to date. This work is published in the journal Astronomy and astrophysics.
Discovery in synergy
This detection of two temperate super-Earths passing through the nearby dwarf star LP 890-9 was fortuitous. In fact, the innermost planet (TOI-4306.01) has been discovered for the first time by TESS. This announcement triggered intensive photometric monitoring by the SPECULOOS Southern Observatory, which led to the discovery of another long-period transiting planet, previously undetected by TESS.
Specifically, the first planet is about 30% larger than Earth and completes a full orbit around the star in just 2.7 days, far too fast to sustain life there. The ULiège researchers used their ground-based SPECULOOS telescopes to confirm and characterize this planet, and also probe the system in depth in search of other planets.
Laetitia Delrez, FNRS researcher in the Astrobiology and STAR (Faculty of Natural Sciences) research units at ULiège, and lead author of the study, explains in a press release: ” TESS searches for exoplanets using the transit method, monitoring the brightness of thousands of stars simultaneously and looking for small drops in light output that may be caused by planets passing in front of their stars “.
However, the use of terrestrial telescopes is essential to confirm the telluric nature of the discoveries and to allow precise measurements of size and orbital properties. This tracking is especially important in the case of very cool stars, such as LP 890-9, which emit their light mainly in the near-infrared and for which TESS has only limited sensitivity.
This is why the SPECULOOS telescopes, directed by ULiège and installed at the European Southern Observatory (ESO) in Paranal in Chile (SPECULOOS South) and at the Teide Observatory in Tenerife (SPECULOOS North), aim to detect terrestrial planets that eclipsing some of the smallest and coolest stars in the solar system. They are equipped with very sensitive cameras in the near infrared.
Michaël Gillon, FNRS senior researcher, co-director of the Astrobiology Research Unit at ULiège, declares: ” The purpose of SPECULOOS is to search for potentially habitable terrestrial planets that pass around the smallest and coolest stars in the solar system, such as the TRAPPIST-1 planetary system, which we discovered in 2016 through a pilot project with our TRAPPIST telescope. “.
A planet close to Earth’s characteristics
The observations via SPECULOOS thus made it possible to confirm the first planet, but also to detect another one as previously mentioned. This second planet, LP 890-9c (renamed SPECULOOS-2c by the ULiège researchers), is similar in size to the first – about 40% larger than 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.
Francisco J. Pozuelos, researcher at the Institute of Astrophysics in Andalusia, points out: “ Although this planet is very close to its star, at a distance about 10 times smaller than that of Mercury around our Sun, the amount of stellar radiation it receives remains low and could allow the presence of liquid water on the surface of the planet, provided that has sufficient atmosphere “.
This potential presence of liquid water would be mainly due to the fact that the star LP 890-9 is about 6.5 times smaller than the Sun and has a surface temperature half as high. De factoeven though the planet is closer, it still has “conditions suitable for life”.
Subsequently, the scientists want to study this system, in particular SPECULOOS-2c, using the James Webb Space Telescope to characterize its atmosphere, as was recently the case for the exoplanet WASP-39b. As Laetitia Delrez points out, unlike the planets in the TRAPPIST-1 system, it must be taken into account that ” LP 890-9c is located near the inner boundary of the habitable zone and may therefore have an atmosphere that is particularly rich in water vapor, which would then enhance its atmospheric signals “.
The authors conclude: The discovery of LP 890-9c provides a unique opportunity to better understand and constrain the conditions for habitability around the smallest and coldest stars in our solar neighborhood “.