Discovery of two new temperate rocky worlds

An international research team including astronomers from the University of Birmingham has just announced the discovery of two “super-Earth” planets orbiting LP 890-9, a small, cool star located about 100 light-years from Earth. Earth.

The star, also called TOI-4306 or SPECULOOS-2, is the second coolest star found to host planets, after the famous TRAPPIST-1. This rare discovery is the subject of an upcoming publication in the journal Astronomy & Astrophysics.

The system’s inner planet, called LP 890-9b, is about 30 percent larger than Earth and orbits the star in just 2.7 days. This first planet was initially identified as a possible candidate planet by NASA’s Transiting Exoplanet Survey Satellite (TESS), a space mission to search for exoplanets orbiting nearby stars. This candidate has been confirmed and characterized by the SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) telescopes, one of which is operated by the University of Birmingham. The SPECULOOS researchers then used their telescopes to search for other transiting planets in the system that would have been missed by TESS.

“TESS searches for exoplanets using the transit method, simultaneously monitoring the brightness of thousands of stars, looking for slight obscurations that could be caused by planets passing in front of their stars,” explains Laetitia Delrez, postdoctoral researcher at the University. of Liege. , and the main author of the article.

“However, follow-up with ground-based telescopes is often needed to confirm the planetary nature of detected candidates and refine measurements of their orbital sizes and properties. »

This tracking 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.

The telescopes of the SPECULOOS project, installed at ESO’s Paranal Observatory in Chile and on the island of Tenerife, are optimized to observe this type of star with great precision, thanks to very sensitive cameras in the near infrared. .

“The objective of SPECULOOS is to search for potentially habitable terrestrial planets passing through some of the smallest and coldest stars in the solar neighborhood, such as the planetary system TRAPPIST-1, which we discovered in 2016”, recalls Michaël Gillon, from the University of Liège, and principal researcher 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 to the search for possible chemical traces of life with large observatories, such as the James Webb Space Telescope (JWST). »

Observations of LP 890-9 collected by SPECULOOS proved successful as they not only confirmed the first planet, but were critical for the detection of a previously unknown second planet. This second planet, LP 890-9c (renamed SPECULOOS-2c by SPECULOOS 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.

“The habitable zone is a concept according to which a planet with geological and atmospheric conditions similar to Earth would have a surface temperature allowing water to remain liquid for billions of years” explains Amaury Triaud, professor of exoplanetology at the University of Birmingham and leader of the SPECULOOS working group which programmed the observations leading to the discovery of the second planet. “It gives us a license to observe further and find out if the planet has an atmosphere, and if so, to study its contents and assess its habitability. »

The next step will be to study the atmosphere of this planet, for example with the JWST, for which LP 890-9c appears to be the second most favorable target among the potentially habitable terrestrial planets known to date, surpassed only by the TRAPPIST – 1 planets (of which Professor Triaud was also co-discoverer).

“It is important to detect as many temperate terrestrial worlds as possible in order to study the diversity of climates of exoplanets, and ultimately to be able to measure the frequency of emergence of biology in the Cosmos”, adds Professor Triaud.

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