Proxima Centauri, the Sun's Closest Neighbor, Is Small, Faint and Fiery

Nearest, but not easy to spot

Proxima Centauri sits about 4.2 light-years from the Sun, making it the closest star to the solar system.^[2][5] It is a red dwarf, a small, dim, low-mass star with about one-eighth of the Sun's mass.^[1][2] Its brightness is so low that it cannot be seen with the unaided eye. EarthSky lists its magnitude as about +11 and places it in the constellation Centaurus, best observed from the Southern Hemisphere.^[2]

Despite its quiet looks, Proxima is a flare star. In 2021, astronomers saw a burst of radiation from it that was 100 times more powerful than any flare seen from the Sun.^[2] ESA/Hubble adds that it should remain on the main sequence for another four trillion years, some 300 times the current age of the universe.^[1]

It is also part of the Alpha Centauri system, which ESA/Hubble describes as a triple star system, with Alpha Centauri A and B as its other members.^[1] EarthSky points out that some scientists question whether Proxima is actually bound by gravity to the other two.^[2]

A planet next door

In August 2016, a team announced Proxima b in the journal Nature. It was found by the radial velocity method, which detects a star's tiny wobble; in this case the star's motion changed by only about 5 kilometers per hour.^[3] The planet has a minimum mass of 1.3 Earths and circles its star every 11.2 days.^[3][4] Eos adds that its orbit is only about 5 percent of the Earth-Sun distance.^[4] EarthSky lists three confirmed planets: Proxima b (2016), Proxima c (2020, with an orbit of 1,907 days) and Proxima d (2022, orbiting in five days).^[2] Britannica notes that they are the nearest known planets beyond the solar system.^[5]

Is it habitable?

Proxima b lies in the habitable zone, where liquid water could in theory exist on a surface. Eos relays a researcher's view that "temperate zone" is the better term, since temperature alone allows water but not necessarily life.^[4] The French research agency CNRS says the planet received about 250 times more X-rays and 15 times more extreme ultraviolet light than Earth over its history, while Eos gives 100 to 400 times the X-ray dose.^[3][4] Both sources say it could be tidally locked, with one side permanently dark.^[3][4] The announcement team said the world cannot be considered an Earth twin, since its formation and history were different.^[3] Whether it kept any water or atmosphere depends on its formation and flare history, which remain uncertain.^[3][4]

Researcher Ansgar Reiners, quoted by Eos, described it as "the closest planet outside our solar system that will ever be found."^[4]

The discovery paper, led by Guillem Anglada-Escudé, reports a planet with a minimum mass of about 1.3 Earth masses and an orbital period of roughly 11.2 days at about 0.05 astronomical units from the star, which lies 1.295 parsecs away.^[6] ESO's announcement of August 24, 2016, links the result to the Pale Red Dot campaign and gives the orbital distance as about 7 million kilometers. It describes the star's regular wobble of around 5 kilometers per hour, comparable to walking pace, repeating every 11.2 days, and says the paper appeared in Nature on August 25, 2016.^[7]