A publication in Astronomy & Astrophysics

ULiège astronomers contribute to the discovery of a new exotic world

Credit : ESA/Hubble, M. Kornmesser

An international research team led by the University of Bern and the University of Liège, discovers a new exotic world in the vicinity of the solar system : TOI-2257b. This fascinating exoplanet, about twice the size of the Earth, is a temperate “mini-Neptune” orbiting a cool star on a highly-elliptical 35 days orbit. This research is published in the journal Astronomy & Astrophysics.


nitially identified by NASA's Transiting Exoplanet Survey Satellite (TESS), a space mission searching for planets around nearby and bright stars, the existence of this planet had to be confirmed by ground-based telescopes, including those managed by researchers from the University of Liege. "TESS is performing a planet-hunting using the transit method, that is, monitoring the stellar brightness of thousands of nearby stars awaiting for slight dimmings, which might be caused by the passing of a planet between the star and the observer. However, despite its power to detect new worlds, the TESS mission needs ground-based support to confirm the planetary nature of the detected signals. In this context, our research group at the University of Liège is playing a major role, being one of the most active teams in this domain", explains Michaël Gillon, FNRS Senior Researcher at the Astrobiology Research Unit of the University of Liege (ULiege) and co-author of the scientific article recently published in Astronomy & Astrophysics.

The planet was confirmed simultaneously using the TRAPPIST-North and SAINT-EX telescopes on April 20th, 2021. SAINT-EX is a 1 meter robotic telescope operated by a consortium led by the University of Bern and including ULiege. It is located at the San Pedro Martir Observatory (Mexico). On the other hand, TRAPPIST-North is a 0.6-m robotic telescope operated by ULiege and located at the Oukaimeden Observatory in the Altas mountains of Morocco, comments Emmanuel Jehin FNRS Senior Researcher from ULiege and co-author of this study. "The TRAPPIST telescopes are used about every night to observe exoplanet candidates. They are also important for the students of the Master in Space Sciences of our University to learn how to perform and analyze astronomical observations in real conditions to become future astronomers."  

"The particular case of TOI-2257b was very challenging", adds Francisco J. Pozuelos, postdoctoral researcher within the ULiege Research Units Astrobiology and STAR and second author of the study. "Indeed, TESS was not able to constrain the orbital period of the planet, and an accurate coordination between the TRAPPIST-North and the SAINT-EX telescopes, located at different time zones, was critical to fully confirm the planet", Francisco Pozuelos continues.  

The research team concluded that TOI-2257b is a temperate planet 2.2 times larger than the Earth, with a highly-elliptical orbit. "In fact, it is the mini-Neptune with the most elliptical orbit around a cool star ever discovered", explains Nicole Schanche of the Center for Space and Habitability CSH of the University of Bern and lead author of the study. "A possible explanation for this surprising orbit is that further out in the system, a giant planet is lurking and disturbing the orbit of TOI-2257b. Further observations measuring the star's radial velocity will help confirm the ellipticity and search for possible additional planets that could not be detected in transit."

While its 35-day orbital period places TOI-2257b in the habitable zone of its host star (where liquid water is possible to exist and hence favorable conditions for the emergence of life), its size suggests that the planet is rather gaseous, with high atmospheric pressure not conducive to life as we know it. However, TOI-2257b is one of the most attractive mini-Neptunes where to search for signs of water vapor in the atmosphere using the successfully recently launched James Webb Space Telescope (JWST), which will revolutionize research into exoplanet atmospheres, concludes the study.

Scientific reference

Schanche N. & al., TOI-2257 b: A highly eccentric long-period sub-Neptune transiting a nearby M dwarf, Astronomy & Astrophysics, 7 January 2022.


Fransisco J. Pozuelos

Michaël Gillon

Emmanuel Jehin

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