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This breakthrough was made possible by SPIRou, a very high-precision infrared spectropolarimeter developed by an international consortium led by IRAP in Toulouse, and a new innovative method that corrects for disturbances related to Earth's atmosphere.
M-type stars, or red dwarfs, are the most numerous in the Galaxy. Small, cold, and dim, they are ideal targets for exoplanet detection because their low mass makes the motions induced by orbiting planets more perceptible. It is around one of them, Gliese 725 B, located 11.4 light-years from Earth, that a system of two exoplanets was discovered thanks to SPIRou, a very high-precision infrared spectropolarimeter installed on the Canada-France-Hawaii Telescope and developed by an international consortium led by IRAP in Toulouse.
The discovery relies on the radial velocity method, which measures the oscillations of a star due to orbiting planets. This technique is particularly effective in the infrared for red dwarfs, which emit most of their light in this range. But observing in the infrared from the ground is complicated: water vapor and other components of Earth's atmosphere leave traces in the data, which can mask planetary signals.
To overcome this obstacle, researchers developed an innovative method called Wapiti, capable of correcting these disturbances. Thanks to this approach, a first planetary signal, relatively weak, was detected with an orbital period of 4.8 days.
However, this signal is not statistically significant enough to confirm the existence of a planet. The associated object, Gl 725 Bb, is therefore for now considered only as an exoplanet candidate. The Wapiti method also revealed a more massive and better-characterized planet, Gl 725 Bc, with a mass at least 3.4 times that of Earth and an orbital period of 37.9 days.
This planet lies in the habitable zone of its star, meaning that if this planet is rocky and has water in its composition, then that water should be in liquid form on its surface, one of the essential prerequisites for the possible emergence of life. Gl 725 Bc receives an amount of energy comparable to that received by Mars, a planet on which water was present before the loss of most of its atmosphere, and it now constitutes the second closest planet in the habitable zone to Earth.
Although it does not transit in front of its star, which limits the direct study of its atmosphere, its proximity and characteristics make it a prime target for next-generation instruments. This planet indeed has characteristics that make it the second potentially rocky planet in the habitable zone that is least complex to study, after Proxima Centauri b, which is our closest exoplanet in the habitable zone.
As an example, an instrument like LIFE could enable such observations. LIFE is a space telescope project designed to directly study the atmospheres of nearby exoplanets by analyzing their infrared radiation, with the goal of searching for signatures of molecules like water or other potential indicators of conditions favorable to life.
The study of this planet will allow, in the near future, a better understanding of the diversity of exoplanets likely to harbor liquid water on their surface, an essential prerequisite for the possible emergence of life elsewhere than on Earth. This discovery highlights the potential of high-precision infrared measurements in the search for habitable worlds around the closest stars.