Proxima Centauri is the closest star to the Sun, about 4.2 light-years away in the Alpha Centauri system. As a red dwarf star—the most common type of star—Proxima Centauri is about seven times smaller than the Sun and a little more than half as hot, at 3,100 kelvins. In fact, the tiny star is only 50 percent larger than the planet Jupiter. Red dwarf stars burn their hydrogen fuel very efficiently, ensuring lengthy life spans. Astronomers predict that Proxima Centauri will remain in its current phase for well over four trillion years (by comparison, the universe is roughly 13.8 billion years old). The Alpha Centauri system can be seen in the constellation Centaurus, but Proxima Centauri itself is normally invisible to the naked eye. The star was discovered in 1915 by the Scottish astronomer Robert Innes.
Proxima Centauri orbits the other two stars in the system, Alpha Centauri A and B (also known as Rigil Kentaurus and Toliman, respectively), which in turn circle each other for 80-year periods. It takes 550,000 years for Proxima Centauri to complete an orbit of A and B. With an apparent visual magnitude of 11, Proxima Centauri is the dimmest of the three stars, Alpha Centauri A and B having magnitudes of 0.0 and 1.4, respectively. However, Proxima Centauri’s brightness can fluctuate by more than one magnitude in just a few minutes; thus, it is classified as a flare star, prone to enormous solar flares. These flares, which occur multiple times daily, can be strong enough to make Proxima Centauri visible to the human eye. In 2019 Proxima Centauri unleashed the largest solar flare ever recorded in the Milky Way Galaxy, shining 14,000 times brighter than average in ultraviolet wavelengths.
Three planets are thought to orbit Proxima Centauri: Proxima Centauri b, c, and d. These are the closest planets outside the solar system. Since red dwarf stars are relatively small and cool, planets can orbit very close to them and still be considered within the habitable zone—the distance range from a star within which a planet’s water, if present, could be in liquid form, theoretically enabling life to exist there.
The European Southern Observatory (ESO) discovered Proxima Centauri b in 2016 and estimated the planet to contain about 17 percent more mass than Earth and thus likely to be a rocky planet as well. With an orbital period of 11.2 days, the planet lies within Proxima Centauri’s habitable zone. However, scientists theorize that the planet could be tidally locked and is unlikely to support an atmosphere because of its proximity to the star’s radiation—two conditions that may spell doom for the possibility of life.
The candidate planet Proxima Centauri c is posited to complete an orbit of its star in 5.2 years, far outside the habitable zone. First thought to have been discovered in 2019, the super-Earth planet, if it exists, is at least 5.8 times the mass of Earth, although it appears to shine much brighter than expected for that size—indicating that it may be shrouded in dust clouds or circled by a ring system. Astronomers estimated the proposed planet to be extremely cold, possibly near −233 °C (−388 °F). However, a 2022 study argued that Proxima Centauri c’s existence was a false reading because of failings in the discovery method and the low signal-to-noise ratio in the possible images of the planet.
In 2022, astronomers at the ESO announced the possibility of a third planet: Proxima Centauri d. If confirmed, it would be one of the lightest exoplanets discovered, with a mass a little more than a quarter of Earth’s. This size points to a rocky composition. If it exists, it completes its orbit in 5.1 days, making it too close to the star, and therefore too hot, to be in the habitable zone.