Astronomers using NASA’s Transiting Exoplanet Survey Satellite and the SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) telescopes at ESO’s Paranal Observatory have discovered and validated two super-Earth exoplanets transiting LP 890-9, a relatively low-activity M6-type star.
LP 890-9 is located 32 parsecs (104 light-years) away from Earth in the constellation of Eridanus.
Otherwise known as TOI-4306 and SPECULOOS-2, the star is 7.2 billion years old.
It has a mass of only 0.16 solar masses, a radius of 0.12 solar radii, and a temperature of 2,850 K (2,577 degrees Celsius, or 4,671 degrees Fahrenheit).
LP 890-9, which is orbited by at least two exoplanets, is the second-coolest star found to host planets after TRAPPIST-1.
The inner planet is about 30% larger than Earth and completes an orbit around the star in just 2.7 days.
Called LP 890-9b (or SPECULOOS-2b), it was initially identified as a possible planet candidate by NASA’s Transiting Exoplanet Survey Satellite (TESS).
Follow-up monitoring of the system with the SPECULOOS Southern Observatory then led to the validation of this planet and the discovery of the second transiting planet, LP 890-9c, previously undetected by TESS.
“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,” said Dr. Laetitia Delrez, a postdoctoral researcher at the University of Liège.
“However, a follow-up with ground-based telescopes is often necessary to confirm the planetary nature of the detected candidates and to refine the measurements of their sizes and orbital properties.”
Also identified as SPECULOOS-2c, the outer planet is similar in size to the first (about 37% larger than Earth) but has a longer orbital period of about 8.5 days.
Later confirmed with the MuSCAT3 instrument at Haleakala Observatory, its orbital period places LP 890-9c within the conservative habitable zone, very close to its inner limit (runaway greenhouse).
“This second planet receives about the same amount of stellar radiation as our Earth receives from the Sun and could therefore potentially have liquid water on its surface,” said Dr. Robert Wells, an astronomer at the University of Bern and the NCCR PlanetS.
“But we should not get ahead of ourselves. Being in the right spot does not guarantee a palm beach. Our neighbor planet Venus, which is, so to speak, a carbon dioxide-rich near 500-degree-Celsius pressure cooker, is also near this so-called habitable zone around the Sun.”
“The habitable zone is a concept under which a planet with similar geological and atmospheric conditions as Earth, would have a surface temperature allowing water to remain liquid for billions of years,” said Professor Amaury Triaud, an astronomer at the University Birmingham.
“This gives us a license to observe more and find out whether the planet has an atmosphere, and if so, to study its content and assess its habitability.”
“After the TRAPPIST-1 planets, LP 890-9c is the second-most favorable habitable-zone terrestrial planet known so far (assuming for this comparison a similar atmosphere for all planets),” the astronomers concluded.
Their paper will be published in the journal Astronomy & Astrophysics.
L. Delrez et al. 2022. Two temperate super-Earths transiting a nearby late-type M dwarf. A&A, in press; doi: 10.1051/0004-6361/202244041
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