Discovery of a third planet around Proxima Centauri
Just over four light years away is Alpha Centauri, the closest star system to the Sun. And in August 2016, researchers announced that they had found a long-sought planet around Proxima Centauri, the smallest and closest component of this three-star system. Then, in January 2020, astronomers spotted a second world around Proxima Centauri. And now the star’s family tree seems to be growing again: a third terrestrial planet has been discovered orbiting the closest star to Earth.
A new addition
In a study published on February 10 in Astronomy & Astrophysics, astronomers announced the discovery of Proxima d. This small planet, weighing barely a quarter of the mass of Earth, orbits Proxima Centauri every five days at a distance of about 2.5 million miles (4 million kilometers), or less than a tenth of the distance from Mercury to our own Sun. But because Proxima Centauri is a red dwarf with only about 12% of the Sun’s mass and 14% of its diameter, that puts Proxima d in the star’s habitable zone, where conditions are ideal for water liquid potentially exists on its surface.
Proxima d was discovered using the radial velocity method, during which astronomers carefully observe a star to look for subtle changes in position, which occur when an orbiting planet’s gravity pulls on its star. This is the same method used to detect the other two planets of Proxima Centauri, Proxima b and c. But these planets are more massive – in fact, Proxima d is the lightest exoplanet ever discovered using this method.
So how does this new addition to the family compare to its siblings?
Proxima b, the first planet discovered around this star, has about the same mass as Earth and orbits every 11.2 days at a distance of 4.3 million miles (7 million km), or about 5 % of the distance around which the Earth revolves around the Sun. But again, because Proxima Centauri is much smaller than our star, Proxima b also orbits in its habitable zone.
Proxima is different: it’s a super-Earth weighing about six times the mass of our planet, with an orbital period of 5.2 years. This places its orbit, about 130 million miles (224 million km), beyond the habitable zone.
The researchers discovered Proxima d using a new instrument called the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, or ESPRESSO, on the European Southern Observatory’s Very Large Telescope in Chile. Now that the instrument has proven itself, astronomers hope to use it to discover many other terrestrial worlds, perhaps much like our own, both near and far from home.