Mass, radius, and temperature
The apparent inclination of Proxima Centauri b's orbit has not yet been measured. The minimum mass of Proxima b is 1.27 M⊕, which would be the actual mass if its orbit were seen edge on from the Earth. Once its orbital inclination is known, the mass will be calculable. More tilted orientations imply a higher mass, with 90% of possible orientations implying a mass below 3 M⊕. The planet's exact radius is unknown. If it has a rocky composition and a density equal to that of the Earth, then its radius is at least 1.1 R⊕. It could be larger if it has a lower density than the Earth, or a mass higher than the minimum mass. Like many super-Earth sized planets, Proxima Centauri b could have an icy composition like Neptune, with a thick layer of hydrogen on its surface; the likelihood that this is the case has been calculated to be greater than 10%. The planet has an equilibrium temperature of 234 K (−39 °C; −38 °F).
The planet orbits an M-type red dwarf named Proxima Centauri. The star has a mass of 0.12 M☉ and a radius of 0.14 R☉. It has a surface temperature of 3042 K  and is 4.85 billion years old. In comparison, the Sun is 4.6 billion years old  and has a surface temperature of 5778 K. Proxima Centauri rotates once roughly every 83 days, and has a luminosity about 0.0015 L☉. Like the two larger stars in the triple star system, Proxima Centauri is rich in metals compared with the Sun, something not normally found in low-mass stars like Proxima. Its metallicity ([Fe/H]) is 0.21, or 1.62 times the amount found in the Sun's atmosphere.[note 1]
Even though Proxima Centauri is the closest star to the Sun, it is not visible to the unaided eye from Earth because of its low luminosity (average apparent magnitude of 11.13).
Proxima Centauri is a flare star. This means that it undergoes occasional dramatic increases in brightness and high-energy emissions because of magnetic activity that would create large solar storms. On 18 March 2016, a superflare was observed with an energy of 10^33.5 erg. The March 2016 flare reached about 68 times usual level, thus a little brighter than the Sun. The surface irradiation was estimated to be 100 times what is required to kill even UV-hardy microorganisms. Based on the rate of observed flares, total ozone depletion of an Earth-like atmosphere would occur within several hundred thousand years.
Proxima Centauri b orbits its host star every 11.186 days at a semi-major axis distance of approximately 0.05 astronomical units (7,000,000 km; 5,000,000 mi), which means the distance from the exoplanet to its host star is one-twentieth of the distance from the Earth to the Sun. Comparatively, Mercury, the closest planet to the Sun, has a semi-major axis distance of 0.39 AU. Proxima Centauri b receives about 65% of the amount of radiative flux from its host star that the Earth receives from the Sun – for comparison, Mars receives about 43%. Most of the radiative flux from Proxima Centauri is in the infrared spectrum. In the visible spectrum the exoplanet receives only ~3% of the PAR (400–700 nm) of Earth irradiance – for comparison, Jupiter receives 3.7% and Saturn 1.1%. – so it would usually not get much brighter than twilight anywhere on Proxima Centauri b's surface. The maximum illumination of horizontal ground by twilight at sunrise is about 400 lux, while the illumination of Proxima b is about 2700 lux with quiet Proxima. Proxima also has flares. The brightest flare observed until 2016 had increased the visual brightness of Proxima about 8 times, which would be a large change from the previous level but, at about 17% the illumination of Earth, not very strong sunlight.[note 2] However, because of its tight orbit, Proxima Centauri b receives about 400 times more X-ray radiation than the Earth does.