Matter and the universe

Two planets around a red dwarf

In 2020, the “SAINT-EX” Observatory, led by scientists from the National Centre of Competence in Research NCCR PlanetS of the University of Bern and the University of Geneva, detected two exoplanets orbiting the star TOI-1266. The Mexico-based telescope thus demonstrates its high precision and enables an important step in the quest to find potentially habitable worlds.

Red dwarfs are the coolest kind of star in the universe. As such, they potentially allow liquid water to exist on planets that are quite close to them – unlike the sun. In the search for habitable worlds beyond the borders of our solar system, this is a big advantage: The distance between an exoplanet and its star is a crucial factor for its detection. The closer the two are, the higher the chance that astronomers can detect the planet from Earth.

“The red dwarves are rather small and emit little light compared to most other stars, such as our Sun,” explains Brice-Olivier Demory, lead author of the study that was published in October 2020 and Professor of Astrophysics at the University of Bern. These factors make them challenging to observe in detail. Without the proper instruments, any planets that might orbit them could easily be overlooked – especially terrestrial planets, like Earth, that are comparatively small.

A dedicated telescope

One instrument with which it is possible to study red dwarves and their planets closely is the Mexico-based SAINT-EX telescope, co-operated by the NCCR PlanetS (see info box). SAINT-EX is an acronym that stands for Search And characterIsatioN of Transiting EXoplanets. The project has been named in honor of Antoine de Saint-Exupéry (Saint-Ex), the famous writer, poet and aviator.

The SAINT-EX Observatory is a fully robotic facility hosting a 1-metre telescope. It is equipped with sensors enabling high-precision detection of small planets orbiting cool stars. This specialization has started to pay off: At the beginning of 2020, the telescope was able to detect two exoplanets orbiting the star TOI-1266, located around 120 light years from Earth.

A peculiar pair

The study provides a first impression of the characteristics of the two exoplanets. Compared to the planets in our solar system, TOI-1266 b and c are much closer to their star – it takes them only 11 and 19 days respectively to orbit it. However, as their host star is much cooler than the Sun, their temperatures are not very extreme: The outer planet has approximately the temperature of Venus (although it is seven times closer to its star than Venus is to the Sun). The two planets are of similar density, possibly corresponding to a composition of about one half rocky and metallic material and the other half water. This makes them about half as rocky as Earth or Venus but also far rockier than Uranus or Neptune.

In size, the planets clearly differ from each other. The inner planet, TOI-1266 b, measures a little under two-and-a-half times the Earth’s diameter. This makes it a so-called “sub-Neptune”. The outer planet, TOI-1266 c, is just over one-and-a-half times the size of our planet. Thus, it belongs to the category of “super-Earths”. 

“Being able to study two different types of planets in the same system is a great opportunity to better understand how these different sized planets come to be,” says Brice-Olivier Demory.