Proxima Centauri

Proxima Centauri holds the title of humanity's nearest stellar neighbour — 4.2 light-years away, or roughly 265,000 AU from the Sun. It is almost certainly a gravitationally bound member of the Alpha Centauri AB binary system, orbiting that pair on an immense 500,000-year path.

Despite its cosmic proximity, Proxima is utterly invisible to the unaided eye, shining at apparent magnitude 11.0. It is an M5.5 red dwarf with only 12% of the Sun's mass, 15% of its diameter, and barely 0.16% of its luminosity — most of that energy pours out as infrared rather than visible light. Like many M dwarfs, it is a violent flare star, unleashing X-ray and UV bursts capable of stripping an unshielded atmosphere in a geological eyeblink.

In 2016 astronomers confirmed Proxima b, a planet of at least 1.06 Earth masses sitting in the habitable zone with an 11.2-day orbit. A sub-Earth candidate, Proxima d (≥0.26 Earth masses, 5.1-day orbit), was announced in 2022. Whether any of these worlds can harbour life remains an open question given the star's flare activity.

Proxima has captivated the public imagination ever since it was identified as the nearest star in 1915 by Robert Innes at the Union Observatory in Johannesburg. It is the primary target of the Breakthrough Starshot laser-sail concept, which aims to cross the 4.2-light-year gulf in roughly 20 years.