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Our Nearest Celestial Neighbor? An Exotic 3-Star System

Animation shows Alpha Centauri A and B orbiting each other and, farther out, Proxima Centauri, orbited by a planet, Proxima b.
Richard Barkus, NASA-JPL/Caltech

A journey of 4.25 light-years will bring you to our nearest neighboring star: Proxima Centauri, or Proxima Cen for short. At least one planet, and possibly two more, orbit the star, making the planet – or planets – the closest outside our solar system.

Yet Proxima is itself part of another star system. It is locked in a distant orbital relationship with two other stars, Alpha Centauri A and B (“Rigil Kentaurus” and “Toliman,” as designated by the International Astronomical Union). These two stars, admittedly, orbit each other at a significant separation: It takes them 80 years to complete one orbit. But to their distant sibling, this would look like a close embrace. Proxima Cen takes more than half a million years to orbit the other two.

Together, the three stars are known as the Alpha Centauri system, or just Alpha Centauri.

Whether the planet first discovered around this nearest star, nicknamed Proxima b, might be potentially habitable is so far unknown. It is a “super-Earth,” or a planet likely to be rocky like ours but notably heftier – in this case, 1.27 times the mass of Earth. Proxima b orbits its star closely, with a “year” lasting only 11 days. But because Proxima Cen is smaller and cooler than our Sun, the planet still dwells in the habitable zone, the distance from a star that could allow liquid water on a planet’s surface.

That is, all things being equal. In this case, however, things might be far from equal. Liquid water requires a suitable atmosphere, and so far none of our telescopes, in space or on the ground, has the capability to check whether Proxima b has any sign of an atmosphere at all. Another major issue: the periodic flaring of the star itself. Proxima Cen, a red-dwarf star, frequently erupts in bursts of intense ultraviolet radiation; this might be enough to strip away any atmosphere the planet held onto after it formed.

Our nearest neighboring star system, spanning far more cosmic real estate than our solar system, turns out to be something of a laboratory for stellar – and planetary – orbital relations.

‘Other Stars, Other Worlds’ — other stories

A retro looking travel poster for the exoplanet Kepler-16b shows a human standing on a rocky world with two suns large in the foreground. There is a larger white-yellowish sun, with a smaller orang sun. The person, seen from behind, has two criss-crossing shadows between an outcropping of rocks, reminiscent of the American Southwest. Like Luke Skywalker's planet "Tatooine" in Star Wars, Kepler-16b orbits a pair of stars. Kepler-16b is a gas giant, like Saturn, so it would have no solid surface to stand on. The view here is of and from an imagined nearby moon. Prospects for life on this unusual world aren't good, as it has a temperature similar to that of dry ice.

Learn about Kepler-16 b and the twin suns in its sky.

Not one, not two, but three stars in this system.

A system of seven Earth-sized exoplanets has the potential for liquid water.

The TRAPPIST-1 star and its clutch of rocky planets — are any capable of life?

Kepler-90

A Sun-like star surrounded by eight planets — just like us! But look again...

infographic showing the configuration of a Sextuple System

Six stars, paired up, twirling around and between each other.