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Alpha Centauri

Alpha Centauri is famous as the nearest star to the Sun. More precisely, it is a group of three stars, two of which form a close pair in orbit around each other, and the third, sometimes called "Proxima Centauri", is a wide companion to the other two. Proxima is slightly closer to the Sun than the other two stars, hence its name.

Although Alpha Centauri is close, and very widely studied, no one has seen any planets in orbit around it. It's possible that, as a binary star, there aren't as many possible places a planet could orbit stably. The average separation of the stars in the Alpha Centauri binary system is roughly comparable to that between the Sun and Uranus.

Both members of the Alpha Centauri binary pair are similar to the Sun. The larger and hotter star in the pair, Alpha Centauri A, is about 50% more luminous than the Sun; Alpha Centauri B is about 1/3 the Sun's luminosity. By contrast, Proxima is a red dwarf, like Gliese 876, but even cooler and less luminous.

In principle, planets like the Earth could orbit either Alpha Centauri A or Alpha Centauri B, with relatively little short-term influence from the other star. An Earthlike planet would have to be a bit farther from Alpha Centauri A, and a bit closer to Alpha Centauri B, than the Earth is from the Sun, but that's it. Unfortunately, it's not entirely clear how stable such orbits would be over billions of years in a binary star system like this one.

Renditions of the Alpha Centauri system

  1. Region around the Sun as seen from Alpha Centauri. The Sun is the bright star above center -- just to the right is the "W" shape of Cassiopeia. In fact, essentially all the constellations familiar from Earth are largely unchanged from Alpha Centauri -- experienced sky watchers can find, for example, Perseus stretched out between the Sun and the Pleiades, and part of Auriga in the upper left.

    Proxima Centauri is in this view, but -- despite its close distance to Alpha Centauri A and B, it's inconspicuous. In fact, the Sun (even though it's at a distance of 4.3 light years, vs. 0.2 for Proxima) is much brighter.

  2. The same view with text labels. Now you can find Proxima. :-)

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