Proxima Centauri is a small red star, and as of 2026 the closest known star to the Sun, about 4.2 light-years away. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Proxima Centauri is about 210,000 kilometers (130,000 miles) across, about 15% of the Sun's width. That figure is the width of the star's visible surface, and estimates of it differ by several percent.

Size

Proxima Centauri is a red dwarf: a star much smaller, cooler and dimmer than the Sun. Its surface is only about half as hot as the Sun's, and it has about one-eighth of the Sun's mass. Although it is so close, it is too faint to see with the unaided eye.

Its width has been measured directly with interferometry, which combines the light of several telescopes to see finer detail than any one of them could. The most precise measurement, published in 2019, used the Very Large Telescope Interferometer in Chile. It found the star to be about 214,000 kilometers across.

Earlier measurements with the same observatory, published in 2003 and 2009, gave slightly smaller widths of about 196,000–202,000 kilometers, with larger uncertainties. A 2017 study worked out the width a different way, from the star's brightness, and also found about 215,000 kilometers.

Size comparisons

• The Sun is about 6.5 times as wide as Proxima Centauri and has about eight times its mass.
• Proxima Centauri is about one and a half times as wide as Jupiter, but about 130 times as massive.
• Wolf 359, another nearby red dwarf and the next smaller object in the explorer, is about the same width. Studies that measured both stars the same way found them within a few percent of each other. TrES-4b, the next larger, is a planet about 1.2 times as wide as Proxima Centauri.
• The star's width is a little more than half the average distance from Earth to the Moon.
• If the Sun were shrunk to the size of a basketball, Proxima Centauri would be a ball about 3.7 centimeters (1.5 inches) across, about 6,900 kilometers (4,300 miles) away.

A star in a triple system

Proxima Centauri is the smallest of three stars that travel through space together. The other two, Alpha Centauri A and B, orbit each other about 4.4 light-years from Earth. Alpha Centauri A is much like the Sun, and B is slightly cooler.

Proxima Centauri lies about 13,000 astronomical units from the pair, where one astronomical unit is the average distance from Earth to the Sun. A 2017 study showed that it is held by the pair's gravity, and that it circles them about once every 550,000 years.

Planets

In 2016 astronomers announced a planet orbiting Proxima Centauri, found with the European Southern Observatory's 3.6-meter telescope at La Silla, Chile. Called Proxima b, it has about the mass of Earth and circles the star every 11.2 days, about 7 million kilometers out. That is only 5% of Earth's distance from the Sun. Because the star is so faint, the planet still lies in its habitable zone, where liquid water could exist on the surface.

In 2022 astronomers reported a third, lighter candidate planet, Proxima d. It has about a quarter of Earth's mass and orbits every five days. An earlier candidate, Proxima c, may circle much farther out, taking about five years. The planets are also exposed to strong flares of ultraviolet light and X-rays from the star, far more intense than those that reach Earth from the Sun.

Sources

• Proxima's orbit around Alpha Centauri. Kervella et al., Astronomy & Astrophysics, 2017.
• A discontinuity in the Teff-radius relation of M-dwarfs. Rabus et al., Monthly Notices of the Royal Astronomical Society, 2019.
• Mass-radius relation of low and very low-mass stars revisited with the VLTI. Demory et al., Astronomy & Astrophysics, 2009.
• First radius measurements of very low mass stars with the VLTI. Ségransan et al., Astronomy & Astrophysics, 2003.
• Planet Found in Habitable Zone Around Nearest Star. European Southern Observatory, 2016.
• New planet detected around star closest to the Sun. European Southern Observatory, 2022.
• A candidate short-period sub-Earth orbiting Proxima Centauri. Faria et al., Astronomy & Astrophysics, 2022.
• The M-dwarf Ultraviolet Spectroscopic Sample I. Determining Stellar Parameters for Field Stars. Pineda et al., The Astrophysical Journal, 2021.
• The GAPS Programme with HARPS-N@TNG VI: The Curious Case of TrES-4b. Sozzetti et al., Astronomy & Astrophysics, 2015.
• Measuring the Universe. International Astronomical Union.
• Sun Fact Sheet. NASA, 2024.
• Jupiter Fact Sheet. NASA, 2024.
• Moon Fact Sheet. NASA, 2024.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.