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Third Planet Detected in Alpha Centauri System, Earth’s Closest Stellar Neighbor

By: William Brown, Biophysicist at the Resonance Science Foundation

The discovery shows that our closest stellar neighbor seems to be packed with interesting new worlds, within reach of further study and future exploration,” João Faria, lead researcher and astronomer at the Instituto de Astrofísica e Ciências do Espaço, Portugal.

A third planet has been detected orbiting our closest stellar neighbor Proxima Centauri in the Alpha Centauri solar system [1]. The Alpha Centauri system, at only 4.3 light years from Earth, is turning out to be a rich and diverse solar system: with an Earth-like planet named Proxima b that orbits in the habitable zone around the low-mass M dwarf star Proxima B— which we discussed in the RSF article ‘Potential Habitability of Exoplanets’ [2]— candidate planet Proxima c which has a much larger orbit outside of the habitable zone, and now Proxima d, with a mass about a quarter of that of Earth (twice that of Mars).

Proxima d is the lightest exoplanet ever detected using a radial velocity technique, whereby tiny perturbations, or wobbles, in the motion of a host star is measured—revealing the tiny gravitational pull of a planetary companion. The mass and orbit of the planetary companion can be deduced from the magnitude and frequency of perturbation of the parent star, and Proxima d has such a minute effect that it only causes Proxima Centauri to oscillate at around 40 centimeters per second—which is quite the feat of detection by the team of astronomers at the European Southern Observatory’s Very Large Telescope Interferometer in Chile. The team used  one of the most eagerly anticipated instruments in the astronomical world: ESPRESSO, or the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations.

This artist’s impression shows Proxima d, a planet candidate recently found orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The planet is believed to be rocky and to have a mass about a quarter that of Earth. Two other planets known to orbit Proxima Centauri are visible in the image too: Proxima b, a planet with about the same mass as Earth that orbits the star every 11 days and is within the habitable zone, and candidate Proxima c, which is on a longer five-year orbit around the star. Image Credit: ESO/L. Calçada

The measurements show that Proxima d is very close to the host star Proxima Centauri, and is most likely very inhospitable to life, comparable to Mercury in our own solar system. Yet still, the fact that right next door (so to speak) there is a multi-planetary stellar system with an Earth-like planet in the habitable zone and planetary neighbors not unlike Earth’s own is intriguing and indicates there may be a rich abundance and diversity of planetary systems everywhere in the galaxy and beyond—the exoplanet catalog list 4,914 confirmed exoplanets and 3,686 planetary systems at the date of writing this report [3]. 

The researchers for the study report that "further observations of Proxima will help to complete our understanding of this multi-planetary system and may even unveil the presence of additional planets” P. Faria et al [1].  


[1] J. P. Faria et al., “A candidate short-period sub-Earth orbiting Proxima Centauri,” A&A, vol. 658, p.A115, Feb. 2022, doi: 10.1051/0004-6361/202142337.

[2] William Brown, “Potential Habitability of Exoplanets”, Resonance Science Foundation, 2019;

[3] NASA Exoplanet Exploration, Planets Beyond our Solar System. Accessed February 2022.




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