Located
just next door, Alpha Centauri is the closest star system to the Sun. A view
from our interstellar neighbor a mere 4.3 light-years away is shown in this
illustration. The Sun is at the upper right, a bright star against the
background of the Milky Way. The crescent in the foreground is an artist's
rendering of a planet now reported orbiting Alpha Centauri B, making it the
closest known exoplanet. Discovered by astronomer Xavier Dumusque et al. using
the planet hunting HARPS instrument to measure minute shifts in the star's
spectrum for more than four years, the planet has approximately the same mass
as Earth. But it orbits once every 3.2 days, about 0.04 times the Earth-Sun
distance from its parent star. That puts it well outside the habitable zone,
much too close to Alpha Cen B, a star only a little cooler than the Sun. Still,
estimates indicate that planetary orbits would be stable within the habitable
zone of Alpha Cen B, at about half the Earth-Sun distance
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