A third planet has been detected orbiting the star Proxima Centauri, the closest to the Solar System, according to a study published Thursday in the journal Astronomy & Astrophysics.
At just a quarter of the mass of Earth, Proxima d is one of the lightest in the set of exoplanets discovered so far, which has around 5,000 bodies orbiting other stars.
The star Proxima Centauri is located four light-years away (equivalent to more than 125 million times the distance of the Moon). It is much smaller and twice as cold as the Sun, and belongs to the category of “red dwarfs”.
Detecting exoplanets is very difficult as they are very distant and reflect little light.
In the case of the Proxima Centauri system, when observed from Earth, the bodies orbiting the star are in another plane, which prevents the application of the so-called “transit” method – the easiest – that detects planets by measuring variations in the star’s luminosity caused by by the passage of a body, causing a micro eclipse.
It was therefore necessary to resort to the method of radial velocities, which is based on the measurement of the star’s oscillations due to the micro effect of the gravity of an orbiting body.
This is how Proxima b, similar in mass to Earth, and located in the “habitable” zone (not too close to the star, not too far from the star), and smaller Proxima c, were found a few years ago.
Suspicion of a third planet
“When we discovered them, we suspected an influence” from a third planet, Baptiste Lavie, from the department of astronomy at the University of Geneva, a member of the team using the Very Large Telescope (VLT, literally “very large telescope”), told AFP. from the European Southern Observatory (ESO) in Chile, in the Atacama Desert.
But this signal was so weak that it was necessary to continue the observations with the help of the ESPRESSO spectrograph installed on the VLT. These observations confirmed a tiny variation in the star’s speed (1.4 km per hour) caused by a nearby planet.
Proxima d is 4 million kilometers from its star, less than a tenth of the distance from Mercury to the Sun.
Located on the edge of the habitable zone, Proxima d takes just five days to complete one revolution around its star.
“Without a doubt it is a rocky planet, because its mass is too weak to hold the gas”, as with the gas giants of the Solar System, Jupiter and Saturn, according to astronomer Baptiste Lavie.
“This major discovery demonstrates the potential of the radial velocity method to reveal the existence of light planets such as this one. […]capable of harboring life”, says Pedro Figuera, head of the ESPRESSO spectrograph in Chile, quoted in the ESO statement.
