How often do other stars pass close to the sun? What happens when they do?
Joel Williamson, Newark, California
David Latham, an astronomer at the Harvard-Smithsonian Center for Astrophysics, responds:
We know of only a few stars that have passed within two light-years of the sun in the past million years (the current closest star, Proxima Centauri, is 4.2 light-years away). From what we can tell, the effects are usually small. The typical separation between stars like the sun is 20 million times the star’s diameter. The odds that a star is moving in exactly the right direction to hit the sun are incredibly slim. On the other hand, a passing star does not have to get terribly close to stir things up. If it approaches to less than one light-year away, a passing star could jostle the Oort cloud of comets that hang out at the outer edge of our solar system. If this happens, the star could deflect some of the comets sunward, so that they might cross Earth’s orbit or even hit the planet itself. The impact of a comet could have devastating effects on life here, perhaps causing major extinctions. My colleagues and I have been searching for stars that might pass close enough to have such an effect. The most menacing example we have found is the faint orange dwarf Gliese 710. It is currently 63 light-years away and is moving toward us at some nine miles per second. The star will pass within 1.5 light-years of the sun, but not until about 1.4 million years from now. For the time being, you can breathe easy.