It was not all that long ago that binary star systems were thought to preclude planets, and I can remember reading as a boy that stars like Tau Ceti and Epsilon Eridani were far more likely to have planetary systems than the close binaries Centauri A and B. Now, of course, all that has changed, and we know from theoretical work that stable planetary orbits are possible around both Centauri A and B, though naturally constrained to orbits in the inner systems of both (which includes, satisfyingly enough, their habitable zones). But what can we make out not just from theory but observation? A new study of the 131 planetary systems detected by radial-velocity measurements (as of July 1, 2005) has come up with interesting results. 23 percent of these exoplanetary systems have stellar companions. Many of these had been recognized before as binary systems, but the international team behind this work, led by Deepak Raghavan and Todd Henry (Georgia State) also found six stars in five systems...

Imagine the supernova leading to the formation of a pulsar. Also called a 'neutron star,' a pulsar is the remnant that survives the catastrophe. The explosion of a star at least 1.4 times the mass of the Sun leaves behind an object with a diameter of 10 miles, so dense that a teaspoon of its matter would weigh about two billion tons. It's hard to see such a pulsar as the forming ground for a new planetary system. But we know that such things happen, as at least one pulsar planetary system has already demonstrated. The pulsar PSR B1257+12 was found to have three planets orbiting it back in the early 1990s, two of them the size of the Earth. These were the first exoplanets ever discovered, and it seemed even then that they must have been created out of some kind of debris disk. After all, any planets originally orbiting a star that goes supernova will doubtless be incinerated, so pulsar planets are not survivors but entirely new worlds. Image: This artist's concept depicts the pulsar...