The closest known exoplanet to Earth is just four light-years away
In 2016, astronomers discovered Proxima Centauri b, a roughly Earth-sized world orbiting Proxima Centauri, the Sun's nearest stellar neighbor at 4.24 light-years away. It orbits within the star's habitable zone, where temperatures could allow liquid surface water. However, living there would be punishing: Proxima Centauri is an active red dwarf whose violent flares subject the planet to hundreds of times more ultraviolet and X-ray radiation than Earth receives.
A Neighbor in the Cosmic Dark
Proxima Centauri sits roughly 4.2 light-years from our solar system in the southern constellation of Centaurus. Despite being the closest known star to the Sun, it is completely invisible to the unaided eye. The star is a red dwarf—a low-mass, cool M-dwarf star with a surface temperature of roughly 3,000 Kelvin and an energy output that is only a tiny fraction of the Sun's luminosity. For decades, astronomers scrutinized this faint neighbor to determine whether it harbored planets, but the star's intrinsic dimness and stellar variability made definitive detections extraordinarily difficult.
That search culminated in August 2016 with the formal announcement of Proxima Centauri b. Uncovered through an intensive observing campaign known as Pale Red Dot, the discovery confirmed that our nearest stellar neighbor hosts a world of roughly terrestrial mass. The effort brought together an international team of astronomers using instruments across the globe, led by data from the European Southern Observatory's facilities in Chile. The confirmation transformed Proxima Centauri from an obscure stellar neighbor into the most heavily studied target in modern exoplanet science.
Detecting the Invisible Wobble
Detecting Proxima Centauri b required remarkable precision because the planet cannot be observed directly with existing imaging technology. Instead, researchers relied on the Doppler method, also known as the radial velocity technique. As an exoplanet orbits a star, its gravitational pull causes the host star to wobble back and forth along the line of sight to Earth. This subtle movement shifts the star's spectral features slightly toward the blue end of the spectrum as it approaches and toward the red end as it recedes.
The Pale Red Dot campaign utilized the High Accuracy Radial Velocity Planet Searcher (HARPS), a specialized spectrograph mounted on the ESO 3.6-metre telescope at the La Silla Observatory in Chile. The data revealed that Proxima Centauri moves toward and away from Earth at a speed of about 5 kilometers per hour—roughly the pace of a walking human—in a repeating cycle of 11.2 days. This periodic wobble established the presence of an orbiting world with a minimum mass of approximately 1.3 Earth masses, circling its star at a distance of about 7 million kilometers, or just 5 percent of the distance between Earth and the Sun.
The Geometry of the Habitable Zone
Despite orbiting exceptionally close to its parent star, Proxima Centauri b lies squarely within the circumstellar habitable zone. In planetary science, the habitable zone is defined as the orbital envelope where an Earth-like planet with appropriate atmospheric conditions can theoretically support liquid water on its surface. Because Proxima Centauri produces far less visible light and radiant heat than the Sun, its habitable zone is pulled inward by a factor of dozens compared to our own solar system.
At a distance of roughly 7 million kilometers, Proxima Centauri b receives approximately 65 percent of the total radiant flux that Earth receives from the Sun. If the planet had no atmosphere at all, its calculated equilibrium temperature would be around minus 39 degrees Celsius—roughly equivalent to Earth's theoretical temperature without its natural greenhouse effect. If a substantial atmosphere exists to trap heat and circulate warmth, surface temperatures could permit regions where water remains liquid, making the planet a compelling target for astrobiological study.
The Fury of a Red Dwarf
The label "habitable zone" accounts solely for orbital distance and equilibrium temperature; it does not guarantee a benign planetary environment. Red dwarf stars are notoriously turbulent during their long lifespans, and Proxima Centauri is an active flare star. Driven by strong internal magnetic fields, the star periodically erupts with violent flares that discharge bursts of high-energy radiation and intense streams of charged particles into surrounding space.
Because Proxima Centauri b is parked so close to its star, it experiences high-energy ultraviolet and X-ray radiation hundreds of times more intense than what Earth receives from the Sun. This constant barrage poses a critical threat to planetary habitability. High-energy radiation can heat and ionize upper atmospheric gases, driving them into space through hydrodynamic escape. Unless Proxima Centauri b formed with an unusually deep volatile reservoir or maintains an extraordinarily powerful magnetic field to shield itself, its atmosphere and surface water could have been stripped away over billions of years.
Day and Night Frozen in Place
The planet's tight 11.2-day orbit also exerts immense gravitational tidal forces on its interior. Over millions of years, these gravitational interactions drain rotational energy, slowing the planet's spin until its rotation becomes synchronized with its orbit. As a result, Proxima Centauri b is widely suspected to be tidally locked, presenting the same face to its star indefinitely, just as the Moon does to Earth. Alternatively, it might be caught in a 3:2 spin-orbit resonance, completing three rotations on its axis for every two orbits around the star, similar to Mercury.
A tidally locked configuration creates extreme environmental asymmetry. The star-facing hemisphere suffers permanent daylight and relentless thermal exposure, while the opposite side is cast into eternal darkness and sub-zero temperatures. If the planet has little or no atmosphere, volatile compounds could migrate to the dark side and freeze out permanently into massive ice sheets. However, if the planet maintains an atmosphere of sufficient density, atmospheric winds could distribute heat across the hemispheres, creating a temperate, habitable band along the border between permanent day and night.
Unresolved Questions and Future Exploration
Despite its status as the closest known exoplanet, several fundamental properties of Proxima Centauri b remain unknown. The radial velocity method provides only a minimum mass, because the measurement depends on the angle at which the planetary orbit is tilted relative to our view from Earth. Unless the planet's orbital inclination is precisely established, its true mass could be higher. Furthermore, because Proxima Centauri b does not transit the disc of its star as seen from Earth, astronomers cannot easily measure its radius or determine its exact bulk density.
Answering whether Proxima Centauri b possesses an atmosphere, surface water, or biosignatures will require advanced observational facilities. Upcoming instruments, including the Extremely Large Telescope (ELT) currently under construction in Chile, aim to resolve the planet directly from the glare of its parent star. By analyzing reflected starlight and thermal emission, future astronomers hope to finally determine whether this neighboring world is a barren, radiation-scoured rock or an enduring, water-bearing outpost on the doorstep of the solar system.
Key takeaways
•Proxima Centauri b orbits our closest stellar neighbor, a red dwarf located 4.24 light-years from Earth, with an orbital period of just 11.2 days.
•The planet was discovered in 2016 using the radial velocity method, detecting a stellar wobble of just 5 kilometers per hour caused by a world with a minimum mass of 1.3 Earth masses.
•Although Proxima Centauri b sits in the habitable zone and receives about 65 percent of Earth's solar energy flux, it is bombarded by hundreds of times more high-energy X-ray and ultraviolet radiation than Earth.
•Due to tidal gravitational forces from its close orbit, the planet is likely tidally locked, keeping one hemisphere in perpetual daylight and the other in permanent darkness.