Proxima Centauri

In 1915 the astronomer Robert Innes, working at the Union Observatory in Johannesburg, found a faint red star moving across the sky in step with Alpha Centauri. It turned out to be the nearest star to the Sun, 4.2465 light-years away. It was named Proxima Centauri, from the Latin for nearest.

Too faint to see

Proxima is a red dwarf of spectral type M5.5, with about an eighth of the Sun’s mass and about 15% of its diameter. Its surface is around 3 000 K (2 730 °C), roughly half as hot as the Sun’s, and it gives out only about 0.16% as much energy. From Earth it shines at magnitude 11, about a hundred times too faint for the naked eye. Finding it takes a telescope of at least 8 centimetres, and it can only be seen from south of about 27° north.

Even the Hubble Space Telescope sees Proxima only as a point of light. Its size was measured in 2002 with ESO’s Very Large Telescope Interferometer, which combines the light of separate telescopes to resolve a disc about a thousandth of an arcsecond across.

A star that flares

Proxima is a flare star, first identified as one by Harlow Shapley in 1951. Its magnetic field stores energy and releases it in sudden bursts. In March 2016 the Evryscope telescope array caught a superflare that made the star about 68 times brighter in visible light, briefly bright enough to see with the naked eye. In May 2019 a flare lasting only seconds became more than 14 000 times brighter in far-ultraviolet light, seen by Hubble, and more than 1 000 times brighter at millimetre wavelengths, seen by the ALMA telescope array.

Flares like these matter for anything orbiting Proxima. The team that recorded the 2016 superflare estimated that repeated flares of that size could strip 90% of the ozone from an Earth-like atmosphere on Proxima b within five years.

Two planets, perhaps three

Proxima b, announced in 2016, circles the star every 11.2 days at 0.049 AU. That is far closer than Mercury is to the Sun, but because Proxima is so dim, it lies in the zone where liquid water could exist on a rocky surface. It has at least 1.06 times Earth’s mass. Proxima d, confirmed in 2025 with the NIRPS instrument, has at least a quarter of Earth’s mass and a 5.1-day orbit.

A third candidate, Proxima c, was proposed in 2020 on an orbit of about five years. It is disputed: a 2022 study traced the signal to the instruments, and the 2025 NIRPS analysis called the evidence inconclusive.

The third star of Alpha Centauri

Proxima is bound by gravity to Alpha Centauri A and B, the two Sun-like stars that look like a single bright star in the southern sky. It is now about 13 000 AU from the pair, near the far end of an elongated orbit, and takes roughly 550 000 years to go round once. Seen from a planet around Alpha Centauri A, Proxima would be only a faint fifth-magnitude star.