Proxima Centauri is the closest known star to the Sun, located approximately 4.25 light-years from Earth. Although this small red dwarf is too faint to see with the naked eye, it produces powerful stellar flares and hosts two confirmed planets, including the approximately Earth-mass Proxima b in its habitable zone.
The Closest Star Beyond the Sun
Proxima Centauri lies approximately 4.25 light-years away, making it slightly closer than the bright Alpha Centauri A and B pair at 4.37 light-years. Its name comes from the Latin word proxima, meaning “nearest.” The star was discovered in 1915 by astronomer Robert Innes, who noticed its large movement across photographic plates and its apparent association with Alpha Centauri. Later distance measurements confirmed that Proxima is the nearest known star to our solar system.
Too Faint to See With the Naked Eye
Despite being our nearest stellar neighbor, Proxima Centauri has an apparent magnitude of about 11.1—roughly 100 times fainter than the dimmest stars normally visible without optical aid. This is because red dwarfs release very little visible light. Proxima lies in the southern constellation Centaurus, a little over two degrees from Alpha Centauri A and B, but requires a telescope and detailed star chart to identify. It never rises from locations north of approximately 27° N, including the UK and most of the United States.
Only About One-Seventh the Sun’s Width
Proxima Centauri is a small M-type red dwarf with approximately 12% of the Sun’s mass and 15% of its diameter. If the Sun were reduced to the size of a basketball, Proxima would be roughly the size of a ping-pong ball. Its surface temperature is approximately 3,000 kelvins, compared with about 5,800 kelvins for the Sun, and it produces only around 0.17% as much total energy. Its low temperature and luminosity explain why the nearest star is so difficult to see. They also allow Proxima to consume its nuclear fuel exceptionally slowly, meaning it should continue shining for trillions of years—far longer than the Sun.
Proxima Produces Powerful Stellar Flares
Proxima Centauri may be small, but it is far from quiet. Its magnetic field frequently releases sudden bursts of energy that can make the star brighten dramatically for several minutes while producing intense ultraviolet and X-ray radiation. Some eruptions are powerful enough to be classed as superflares. Any planet orbiting close to Proxima would therefore experience repeated radiation storms capable of altering or gradually stripping away its atmosphere—one of the greatest obstacles to habitability in the system.
The Distant Third Member of Alpha Centauri
Proxima Centauri belongs to the same triple-star system as Alpha Centauri A and B, but it lies far beyond the bright central pair. Approximately 13,000 astronomical units—or about 0.2 light-years—separate Proxima from them, compared with only 11 to 36 AU between A and B during their orbit. Proxima probably takes roughly 550,000 years to complete a single journey around the pair. Its enormous separation explains why the three stars do not resemble a close group in the sky.
Two Confirmed Planets Orbit Proxima Centauri
Astronomers have confirmed two planets around Proxima Centauri by measuring the tiny gravitational wobble they produce in the star. Proxima d is the innermost, completing an orbit every 5.1 days and having a minimum mass of roughly one-quarter to one-third that of Earth. It orbits too close to Proxima to lie within the star’s habitable zone. Farther out, Proxima b completes an orbit every 11.2 days and has a minimum mass similar to Earth’s. A much more distant world called Proxima c was previously proposed, but newer observations do not support its existence.
Proxima b Lies in the Habitable Zone
Proxima b orbits only about 7.3 million kilometers (4.5 million miles) from its star. Because Proxima is so faint, however, the planet receives roughly two-thirds as much stellar energy as Earth and lies within the region where temperatures could permit liquid water. This does not mean the planet is known to be habitable. It may be tidally locked, its radius and composition remain uncertain, and no atmosphere has been detected. Any atmosphere would also need to survive the star’s repeated flares and intense high-energy radiation. Proxima b is therefore a compelling nearby target, but calling it a second Earth would be premature.
Even Our Nearest Star Remains Far Beyond Reach
Light from Proxima Centauri takes approximately 4.25 years to reach Earth, but a spacecraft traveling at Voyager 1’s speed would require more than 70,000 years to cover the same distance. Proposed projects such as Breakthrough Starshot have explored using powerful lasers to accelerate tiny probes to a significant fraction of the speed of light, potentially reducing the journey to a few decades. No spacecraft capable of such a flight currently exists, but Proxima’s proximity and potentially rocky planets make it a natural first destination if humanity eventually begins exploring other star systems.