Two new temperate rocky exoplanets discovered

Astronomers at the University of Birmingham have recently announced the discovery of two “super-Earth” exoplanets orbiting the small, cool star LP 890-9, which is about 100 light-years from our planet.

After the well-known TRAPPIST-1, the star is also known as TOI-4306 or SPECULOOS-2, and is the second-coolest star discovered to host planets. An article about this uncommon discovery will soon appear in the journal Astronomy & Astrophysics.

The inner planet of the system, known as LP 890-9b, is only 2.7 days away from finishing its orbit around the star and is roughly 30% larger than Earth. The NASA Transiting Exoplanet Survey Satellite (TESS), a space mission looking for exoplanets orbiting nearby stars, first recognized this first planet as a potential planet candidate. The SPECULOOS telescopes (Search for habitable Planets EClipsing ULtra-cOOl Stars), one of which is run by the University of Birmingham, confirmed and characterized this candidate. Then, using their telescopes, SPECULOOS researchers looked for additional transiting planets in the system that TESS would have missed.

According to Laetitia Delrez, a postdoctoral researcher at the University of Liège and the lead author of the article, “TESS searches for exoplanets using the transit method by monitoring the brightness of thousands of stars simultaneously, looking for slight dimmings that might be caused by planets passing in front of their stars.”

A follow-up with ground-based telescopes is frequently necessary to confirm that the detected candidates are planets and to improve the measurements of their sizes and orbital characteristics.

This follow-up is very important for very cold stars like LP 890-9, which mostly give off near-infrared light and for which TESS has a low sensitivity.

Thanks to cameras that are extremely sensitive in the near-infrared, the SPECULOOS project’s telescopes, which are located at the ESO’s Paranal Observatory in Chile and on the island of Tenerife, are optimized to observe this type of star with high precision.

According to Michal Gillon, the project’s principal investigator from the University of Liège, “The goal of SPECULOOS is to search for potentially habitable terrestrial planets transiting some of the smallest and coolest stars in the solar neighborhood, such as the TRAPPIST-1 planetary system, which we discovered in 2016.” This strategy is based on the fact that these kinds of exoplanets are perfect for studying their atmospheres in detail and looking for possible chemical signs of life with powerful telescopes like the James Webb Space Telescope (JWST).

The observations of LP 890-9 made by SPECULOOS were successful because they not only helped confirm the existence of the first planet but also helped identify a second, previously undiscovered planet. Although it is slightly larger than the first planet (about 40% larger than Earth), the second planet, LP 890-9c (renamed SPECULOOS-2c by the SPECULOOS researchers), has a longer orbital period of about 8.5 days. This orbital period places the planet in what is known as the “habitable zone” around its star, which was later confirmed by the MuSCAT3 instrument in Hawaii.

Amaury Triaud, a professor of exoplanets ology at the University of Birmingham and the head of the SPECULOOS working group that planned the observations that led to the discovery of the second planet, explains that the habitable zone is a concept under which a planet with similar geological and atmospheric conditions as Earth would have a surface temperature that would allow water to remain liquid for billions of years. This gives us permission to keep looking at the planet and see if it has an atmosphere. If it does, we can look at its makeup and decide if it’s a good place to live.

The next step will be to study this exoplanets atmosphere, possibly with the help of the JWST. LP 890-9c seems to be the second-most promising target among the known potentially habitable terrestrial planets, behind only the TRAPPIST-1 planets, which Professor Triaud also helped find.

Professor Triaud continued, “It is crucial to find as many temperate terrestrial worlds as possible to study the variety of exoplanets climates and, eventually, to be able to measure how frequently biology has emerged in the cosmos.”