Alpha Centauri: Nearest Star System to the Sun
A photo of Alpha Centauri.
Credit: MSX/IPAC/NASA

The sun's closest stellar neighbors are three stars in the Alpha Centauri system. The two main stars are Alpha Centauri A and Alpha Centauri B, which form a binary pair. They are an average of 4.3 light-years from Earth. The third star is Proxima Centauri. It is about 4.22 light-years from Earth and is the closest star other than the sun.

Alpha Centauri A and B orbit a common center of gravity every 80 years. The average distance between them is about 11 astronomical units (AU) — about the same distance as the sun is to Uranus. Proxima Centauri 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 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 of about 500,000 years.

Astronomers announced in August 2016 that they had detected an Earth-size planet orbiting Proxima Centauri. The newfound world, known as Proxima b, is about 1.3 times more massive than Earth, which suggests that the exoplanet is a rocky world, researchers said.

The planet is also in the star's habitable zone, that just-right range of distances where liquid water can exist. Proxima b lies just 4.7 million miles (7.5 million kilometers) from its host star and completes one orbit every 11.2 Earth-days. As a result, it's likely that the exoplanet 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.

However, it's unclear just how habitable Proxima b is from today's telescopes. This means that astronomers need to run models and do comparative studies to better understand how habitable the planet might be. As a start, the planet needs a closer look for investigators to look for signs of an atmosphere. From there, the investigators can extrapolate if that atmosphere (if present) allows liquid water to flow on the surface. Even the surface temperature of the planet depends on the atmosphere, which would also play into habitability characteristics. 

Because Proxima b is so close to a red dwarf, habitability problems are already coming to scientists' minds. For one thing, the planet is so close that it likely is tidally locked to the star, meaning that one side of the planet is always facing the star. This means one side of the planet would be very warm, while the opposite side would be very cold – unless winds could distribute the heat around the planet. That makes it hard for life to exist.

But the planet's close distance to the red dwarf presents other problems too. Red dwarfs are unstable stars, particularly when they are young – they have a lot of stellar activity and produce charged particles, which can produce 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.

Studies of red dwarf stars are ongoing to better understand the habitability of Proxima b. In November 2017, another planet in the habitable zone of a red dwarf was discovered that's almost as close to Earth as Proxima b is. Called Ross 128b, this planet orbits a red dwarf star that appears much quieter than that of Proxima b. The research team said that finding out more about its atmosphere will require a next-generation telescope such as the European Extremely Large Telescope, the Giant Magellan Telescope and the Thirty Meter Telescope that all are expected to bbe active in the 2020s. (The James Webb Space Telescope, set to launch in 2019, can't do the search itself since the planet does not transit across the face of its star.)

To the naked eye, the two main stars shine as one, making them the third brightest "star" in our night sky. The two separate stars can be seen through a small telescope; 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. [Infographic: The Nearest Stars to Earth]

By itself, Alpha Centauri A, also known as Rigel Kentaurus, is the third brightest star in the night sky; just a bit dimmer, by 0.02 of a magnitude, than Arcturus. It is a yellow star of the same type (G2) as the sun, and it is about 25 percent larger. Alpha Centauri B is an orange K2-type star, slightly smaller than the sun. Proxima Centauri is a red dwarf about seven times smaller than the sun, or one-and-a-half times bigger than Jupiter. All three stars are a bit older — 4.85 billion years old — than the sun, which is about 4.6 billion years old. [Infographic: Alpha Centauri Stars & Planet Explained: Our Nearest Neighbors]

The system is in the Southern sky and is not visible to observers above the latitude of 29 degrees north — a line that passes near Houston, Texas, and Orlando, Fla. In the Southern Hemisphere, it's 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.

Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.
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Star Quiz: Test Your Stellar Smarts
Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.
Open Star Cluster Messier 50
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Additional reporting by Elizabeth Howell, Space.com contributor