Exoplanètes : après Proxima b et c, voici Proxima d
free members
Proxima d is 4 million kilometers from its star, less than one tenth the distance of Mercury from the Sun. It is located at the limit of the habitable zone.
Astronomers have detected a new planet orbiting the star Proxima Centauri, the closest to our solar system, according to a study published Feb. 10, 2022, in Astronomy & Astrophysics. They named it Proxima d.
A planet difficult to detect
With only a quarter of the mass of the Earth, it is one of the lightest of the catalog of exoplanets, rich of nearly 5,000 planets orbiting stars other than the Sun. Proxima Centauri is located only four light years from our Sun. Much smaller and twice as cold as our Sun, it belongs to the category of "red dwarfs".
But even though the star is close and not too big - and therefore not very bright - it is difficult to detect planets in its lap. Because when we observe the "Proxima system", the Earth, the exoplanet and its star are not on the same plane: we cannot therefore apply the "easy" transit method, which consists of capturing the variations in luminosity caused by the passage of a planet in front of its host star, like a micro-eclipse. It was therefore necessary to use the radial velocity method, which consists in measuring the speed of the star, by observing if it approaches or moves away from us. If planets orbit around it, this speed will vary slightly.
A revolution of five days around its star
This is how Proxima b, with a mass similar to the Earth and located in the "habitable" zone (neither too close nor too far from the star), and Proxima c, smaller, were found a few years ago. "When they were discovered, we suspected a signal" from a third planet, tells AFP Baptiste Lavie, from the astronomy department of the University of Geneva, member of the team using the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. But this signal was so weak that it was necessary to continue the observations with the ESPRESSO spectrograph installed on the VLT. Which finally confirmed a tiny variation in the speed of the star (1.4 km per hour) caused by a nearby planet.
Proxima d is 4 million kilometers from its star, less than one tenth the distance of Mercury from the Sun. Located at the edge of the habitable zone, it takes only five days to make its revolution. It is "certainly a rocky planet, its mass being too low to retain gas", as do the gas giants of the solar system that are Jupiter, Saturn or Neptune, according to the astronomer Baptiste Lavie.
"This extremely important discovery shows the potential of the method of radial velocities to reveal populations of light planets, like ours, which would be the most abundant in our galaxy and likely to harbor life," said Pedro Figuiera, head of the ESPRESSO instrument in Chile, quoted in a statement from ESO.