The closest star to Earth is a triple-star system called Alpha Centauri.
The two main stars are Alpha Centauri A and Alpha Centauri B, which form a binary pair. They are about 4.35 light-years from Earth, according to NASA (opens in new tab). The third star is called Proxima Centauri or Alpha Centauri C, and it is about 4.25 light-years from Earth, making it the closest star other than the sun.
According to NASA, Alpha Centauri A and B are on average about 23 astronomical units (AU) from each other — a little more than the distance between the sun and Uranus. (An astronomical unit is the average distance between Earth and the sun, which equals 92,955,807 miles or 149,597,870 kilometers.) The closest the two stars ever come to each other is 11 astronomical units, according to NASA (opens in new tab), and the two stars orbit a common center of gravity every 80 years.
Proxima Centauri, meanwhile, is about one-fifth of a light-year or 13,000 AUs from the two other stars, a distance that makes some astronomers question whether it should even be considered part of the same system.
Proxima Centauri may be passing through the system and will leave the vicinity in several million years, or it may be gravitationally bound to the binary pair. If it's bound, it has an orbital period around the other two stars of about 500,000 years.
Meet the stars of Alpha Centauri
To the naked eye, the Alpha Centauri A and B shine as one, making them the third brightest "star" in our night sky. The two separate stars can be seen through a small telescope, making the system one of the finest binary stars that can be observed. Proxima Centauri is too faint to see unaided, and through a telescope it appears about four diameters of the full moon away from the other two.
Alpha Centauri A, also known as Rigel Kentaurus, is a yellow star of the same type (G2) as the sun, although slightly larger, according to NASA (opens in new tab). It is three times closer to Earth than the next nearest star like our sun, according to NASA (opens in new tab).
Alpha Centauri B is an orange K1-type star slightly smaller than the sun. Proxima Centauri is a red dwarf about one-eighth the size of the sun, according to NASA (opens in new tab).
The system is in the Southern sky and is not visible to observers above the latitude of 29 degrees north, according to EarthSky.org (opens in new tab) — a line that passes near Houston and Orlando, Fla. In the Southern Hemisphere, the Alpha Centauri system is easy to find because the cross-piece of the Southern Cross (from Delta to Beta Crucis) points the way. Its right ascension is 14h 39m 41s and its declination is minus 60 degrees 50 minutes 7 seconds.
Are there planets in the Alpha Centauri system?
Astronomers announced in August 2016 that they had detected an Earth-size planet orbiting Proxima Centauri. The planet, known as Proxima b, is about 1.3 times more massive than Earth, which suggests that the exoplanet is a rocky world, researchers said. And Proxima b's relatively small distance from Earth makes it a particularly appealing target for scientists.
The planet is also in the star's habitable zone, that just-right range of distances from a star where liquid water can exist on the surface of a body. Proxima b lies just 4.7 million miles (7.5 million km) from its host star and completes one orbit every 11.2 Earth-days.
However, despite Proxima b's size and location, it's unclear from today's telescopes just how habitable the planet might be. Astronomers need to run more models and do more comparative studies to better understand how habitable the planet might be.
As a start, scientists need to be able to look for signs of an atmosphere. From there, the investigators can extrapolate whether that atmosphere (if present) allows liquid water to flow on the surface. Even the surface temperature of the planet, which would also affect habitability characteristics, depends on the atmosphere.
The planet may also be so close to its star that it is tidally locked, meaning it always shows the same face to its host star, just as the moon shows only one face (the near side) to Earth. This arrangement would make one side of the planet very warm and the other very cold unless winds could distribute the heat around the planet. If that stark temperature difference does exist, it would be a severe challenge to any life.
And Proxima Centauri's status as a red dwarf also likely reduces habitability. Red dwarfs are unstable stars, particularly when they are young — such stars produce a lot of stellar activity and emit charged particles, which can cause intense radiation on nearby planets. Some of this radiation can strip molecules off the top of a planet's atmosphere and thin it over time, according to 2017 studies led by the NASA Goddard Space Flight Center in Greenbelt, Maryland.
Scientists are continuing to study red dwarf stars to better understand the habitability of worlds like Proxima b. (NASA's Transiting Exoplanet Survey Satellite, or TESS, is an exoplanet-hunter particularly adept at spotting planets around this category of star.)
In November 2017, scientists discovered Ross 128b, another planet in the habitable zone of a red dwarf that is nearly as close to Earth as Proxima Centauri is, but that appears to be a much quieter star.
However, finding out more about its atmosphere will require a next-generation ground-based telescope. (The James Webb Space Telescope, set to launch in late 2021, can't gather the necessary observations because the planet does not transit across the face of its star.)
In 2019, scientists spotted what might be a second planet orbiting Proxima Centauri, although the discovery has not yet been confirmed.
This story was updated Nov. 5, 2021, and March 20, 2019.