Science

Discovery of two potentially habitable super-Earths just 100 light-years away

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Hundreds of exoplanets have been discovered since 1995, suggesting that most of the stars in our galaxy have their own planetary systems. Some of them are home to planets located in their star’s habitable zone, which motivates the cosmic 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 era of the James Webb Space Telescope, temperate exoplanets transiting ultracold red dwarfs offer unique opportunities for characterizing their atmospheres as well as searching for gaseous biosignatures. The goal is to understand how often and under what conditions life can appear.

It is for this that the SPECULOOS project (“Search for habitable planets that outshine ULTRA-COOL STARS”) was created. Although SPECULOOS officially began its science activities in 2019, it was launched in 2011 as a prototype survey of the fifty brightest south-facing red dwarfs by the TRAPPIST South Telescope. This research prototype led to the discovery of the TRAPPIST-11 system, consisting of seven terrestrial planets transiting the nearby ultracool dwarf M8V.

The discovery of this frame of reference sparked a wave of subsequent theoretical and observational studies, so that the TRAPPIST-1 planets are today the most studied terrestrial planets outside our solar system. Indeed, the authors of this discovery stated in a 2017 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 Delreze, an astrophysicist at the University of Liege, just announced the discovery of two “super-Earth”-like planets in orbit around the dark red dwarf star TOI-4306. It is the second coldest star around which planets have been discovered, after TRAPPIST-1, located 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 & Astrophysics.

Discovery in synergy

This discovery of two temperate super-Earths passing through the neighboring dwarf star LP 890-9 was accidental. Indeed, the innermost planet (TOI-4306.01) was first discovered by TESS. This announcement prompted intense photometric monitoring by the SPECULOOS Southern Observatory, which led to the discovery of a second transit planet with a longer period, previously undiscovered by TESS.

Specifically, the first planet is about 30% larger than Earth and completes an orbit around the star in just 2.7 days, which is too fast to support life on it. The ULiège researchers used their ground-based SPECULOOS telescopes to confirm and characterize this planet, and to explore the system in depth in search of other planets.

Laetitia Delres, FNRS Researcher in the Research Unit of Astrobiology and STAR (Faculty of Sciences) at ULiège and lead author of the study, explains in a press release: “TESS is looking for exoplanets using the transit method, tracking the luminosity of thousands of stars at the same time, looking for small dips in the light output which may be caused by the passage of planets in front of their stars.”

However, the use of ground-based telescopes is essential to confirm the telluric nature of the discoveries and to provide accurate 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 light primarily in the near infrared and for which TESS has only limited sensitivity.

This is why the Houlliège-operated SPECULOOS telescopes at the European Southern Observatory (ESO) on Paranal in Chile (SPECULOOS south) and at the Teide Observatory in Tenerife (SPECULOOS north) aim to detect terrestrial planets eclipsing some of the smallest and coolest stars in the vicinity of the sun. They are equipped with very sensitive cameras in the near infrared range.

Michael Guyon, Senior Scientist at FNRS, Co-Director of the Astrobiology Research Unit at ULiège, states: “The goal of SPECULOOS is to search for potentially habitable terrestrial planets orbiting the smallest and coldest stars in the vicinity of the solar system, such as the TRAPPIST-1 planetary system. , which we discovered in 2016 through a pilot project with our TRAPPIST-South telescope.”

Planet close in characteristics to Earth

Thus, the SPECULOOS observations made it possible to confirm the first planet, as well as to detect the second one, as mentioned earlier. This second planet, LP 890-9c (renamed SPECULOOS-2c by 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 by the MuSCAT3 instrument in Hawaii, places the planet in the so-called “habitable” zone around its star.

Comparison of the LP 890-9 system and the inner solar system. The LP 890-9 system is much more compact: two of its planets would easily fit in the orbit of Mercury, the innermost planet in our solar system. © Adeline Deward (RISE-Illustration)

Francisco J. Pozuelos, a researcher at the Institute of Astrophysics of Andalusia, notes: “Although this planet is very close to its star, at a distance of about 10 times less than the distance from Mercury around our Sun, the amount of stellar radiation it receives remains weak , and can tolerate liquid water on the planet’s surface as long as it has a sufficient atmosphere.”

This potential presence of liquid water may be due mainly to the fact that the star LP 890-9 is about 6.5 times smaller than the Sun and that its surface temperature is half that. De facto, even if the planet is closer, it still has “conditions conducive to life.”

Subsequently, the researchers want to study this system, in particular SPECULOOS-2c, using the James Webb Space Telescope to characterize its atmosphere, as happened recently with exoplanet WASP-39b. As Letitia Delres points out, unlike the TRAPPIST-1 planets, it must be taken into account that “LP 890-9c is located near the inner boundary of the habitable zone and therefore may have a particularly vapor-rich atmosphere. water, which will then amplify its atmospheric signals.”

The authors conclude: “The discovery of LP 890-9c provides a unique opportunity to better understand and limit habitability conditions around the smallest and coldest stars in our solar region.”

Astronomy and Astrophysics.

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