Discovery of a third planet orbiting the star closest to our solar system

Nicknamed Proxima Centauri d, the new planetary candidate falls into the category of sub-Earths, much lighter than Earth and Venus.

Astronomers say they have discovered a third planet orbiting Proxima Centauri, circling the star outside its habitable zone in space, essentially closing the gates to find liquid water and possible living conditions. Located in the constellation Centaurus, the star is barely more than four light-years from Earth, but due to its low luminosity, it is not visible to the naked eye. Occasionally it has been documented to give off bright bursts of starlight.

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Interestingly, Proxima Centauri is part of a three-star system that has two other stellar companions named Alpha Centauri A and B. Scientists have previously known of two planets surrounding the star, calling them Proxima Centauri b and Proxima Centauri c. But in 2019, a team of international astronomers with experts from the University of Geneva discovered signs of a third planet with a predicted mass of around 0.29 Earth masses.

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A study published in Astronomy & Astrophysics now presents evidence of a third planet, called Proxima Centauri d, which falls into the subterrestrial category of planetary bodies due to their mass. Observed using the Very Large Telescope at the European Southern Observatory (ESO’s VLT) in Chile, the third planet discovered circling Proxima Centauri has a mass of about a quarter (0.26 ± 0.05) of that of Earth and would be one of the lightest exoplanets ever found. It orbits Proxima Centauri at a distance of about 4.3 million kilometers, less than a tenth of Mercury’s distance from the Sun, measured at 57,900,000 kilometers according to NASA data.


Great for exploration, but probably no life

What’s really fascinating is that Proxima Centauri d is that it takes about five days to complete a circle, but lives outside the habitable zone of its host star. For those unaware, the habitable zone is basically the range of distances from a star in which liquid water can exist on the surface of an orbiting planet. For a body like Proxima Centauri, the habitable zone is much closer to the star. In the case of the Sun, the only planet falling into its habitable zone is Earth. Mercury and Venus are too close to host liquid water, while Mars is too far. However, recent research suggests that the Red Planet once had running water and even recorded massive flash floods.


The researchers placed Proxima Centauri D’s equilibrium temperature somewhere around 85 degrees Celsius, much higher than cold landscapes in Earth’s solar system and much lower than hellish exoplanets spotted in other star systems. But there is a catch here. The team behind the discovery still treats Proxima Centauri d as just a “planetary candidate” body. This means that they need independent verification of the data to confirm if it belongs to the category of planets. Nonetheless, it’s a special case, primarily because Proxima Centauri’s proximity to Earth makes it one of the best candidates for exploring planets outside of Earth’s own solar system.


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Source: Astronomy and Astrophysics, ESO, NASA

Arline J. Mercier