An international team of researchers has just announced the discovery of two ‘supertierras’ They orbit around LP 890-9, a small, cool star about 100 light-years from Earth. After the famous TRAPPIST-1, the star is the second coldest found so far with planets. The finding is published today in the journal ‘Astronomy & Astrophysics’.
The innermost planet in the system, called LP 890-9b, is about 30% larger than Earth and completes one orbit around the star in just 2.7 days. This first world had already been classified as a ‘planet candidate’ by NASA’s TESS (Transiting Exoplanet Survey Satellite) space mission, which seeks exoplanets around nearby stars.
Now, the candidate has been confirmed and characterized by the SPECULOOS (Search for Habitable Planets Transiting Ultra Cool Stars) telescopes. The researchers used the occasion to also search for other possible planets around the same star that TESS might have missed.
“TESS – explains Laetitia Delrez, from the University of Liège and lead author of the article – searches for exoplanets using the transit method, by monitoring the brightness of thousands of stars simultaneously, looking for slight dimming that could be caused by planets passing in front of them. However, follow-up with ground-based telescopes is often necessary to confirm the planetary nature of detected candidates and refine measurements of their sizes and orbital properties.”
Such tracking is especially important for very cool stars, such as LP 890-9, which emit most of their light in the near infrared and are beyond the capabilities of TESS instruments. However, 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 high precision, thanks to very sensitive cameras that act precisely in the near infrared.
“The goal – says Michaël Gillon, Principal Investigator of the SPECULOOS project – is to look for potentially habitable terrestrial planets transiting some of the smaller, cooler stars in the solar neighborhood, such as the TRAPPIST-1 planetary system, which we discovered in 2016. This strategy is motivated by the fact that these planets are particularly suitable for detailed studies of their atmospheres and for the search for possible chemical traces of life with large observatories, such as the James Webb Space Telescope.”
Zona habitable
The new observations of LP 890-9 have proven to be a success, as they not only confirmed the first planet, but managed to detect a previously unknown second planet as well. Called LP 890-9c (and renamed SPECULOOS-2c by SPECULOOS astronomers), it is similar in size to the first (about 40% larger than Earth) but has a longer orbital period of about 8.5 days, which places it in the so-called ‘habitable zone’ of its star.
As explained by Amaury Triaud, Professor of Exoplanetology at the University of Birmingham and leader of the SPECULOOS Working Group that programmed the observations, “the habitable zone is a concept according to which a planet with geological and atmospheric conditions similar to Earth would have a surface temperature which allows the agua stay in liquid state for billions of years. Which gives us license to look further and find out if the planet has an atmosphere and, if so, study its content and assess its habitability.”
The atmosphere, next step
The next step, in fact, will be to study the planet’s atmosphere with other instruments, such as the James Webb Space Telescope. In fact, LP 890-9c is the second most favorable target among potentially habitable terrestrial planets known so far, second only to the TRAPPIST-1 worlds (of which Professor Triaud was also a co-discoverer).
“It is important -adds the scientist- to detect as many temperate terrestrial worlds as possible to study the diversity of exoplanet climates and finally be in a position to measure how often biology could have arisen in the Cosmos”.