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By Root 1431 0

Younger Sun-like stars are much more likely to have disks than older ones. In some cases, there’s a hole in the center of the disk as in a phonograph record. The hole extends out from the star to perhaps 30 or 40 AU. This is true, for example, for the disks surrounding the stars Vega and Epsilon Eridani. The hole in the disk surrounding Beta Pictoris extends to only 15 AU from the star. There is a real possibility that these inner, dust-free zones have been cleaned up by planets that recently formed there. Indeed, this sweeping-out process is predicted for the early history of our planetary system. As observations improve, perhaps we will see telltale details in the configuration of dust and dust-free zones that will indicate the presence of planets too small and dark to be seen directly. Spectroscopic data suggest that these disks are churning and that matter is falling in on the central stars—perhaps from comets formed in the disk, deflected by the unseen planets, and evaporating as they approach too close to the local sun.

Because planets are small and shine by reflected light, they tend to be washed out in the glare of the local sun. Nevertheless, many efforts are now under way to find fully formed planets around nearby stars—by detecting a faint brief dimming of starlight as a dark planet interposes itself between the star and the observer on Earth; or by sensing a faint wobble in the motion of the star as it’s tugged first one way and then another by an otherwise invisible orbiting companion. Spaceborne techniques will be much more sensitive. A Jovian planet going around a nearby star is about a billion times fainter than its sun; nevertheless, a new generation of ground-based telescopes that can compensate for the twinkling in the Earth’s atmosphere may soon be able to detect such planets in only a few hours’ observing time. A terrestrial planet of a neighboring star is a hundred times fainter still; but it now seems that comparatively inexpensive spacecraft, above the Earth’s atmosphere, might be able to detect other Earths. None of these searches has succeeded yet, but we are clearly on the verge of being able to detect at least Jupiter-sized planets around the nearest stars—if there are any to be found.

A most important and serendipitous recent discovery is of a bona fide planetary system around an unlikely star, some 1,300 light-years away, found by a most unexpected technique: The pulsar designated B1257+12 is a rapidly rotating neutron star, an unbelievably dense sun, the remnant of a massive star that suffered a supernova explosion. It spins, at a rate measured to impressive precision, once every 0.0062185319388187 seconds, This pulsar is pushing 10,000 rpm.

Charged particles trapped in its intense magnetic field generate radio waves that are cast across the Earth, about 160 flickers a second. Small but discernible changes in the flash rate were tentatively interpreted by Alexander Wolszczan, now at Pennsylvania State University, in 1991—as a tiny reflex motion of the pulsar in response to the presence of planets. In 1994 the predicted mutual gravitational interactions of these planets were confirmed by Wolszczan from a study of timing residuals at the microsecond level over the intervening years. The evidence that these are truly new planets and not starquakes on the neutron star surface (or something) is now overwhelming—or, as Wolszczan put it, “irrefutable”; a new solar system is “unambiguously identified.” Unlike all the other techniques, the pulsar timing method makes close-in terrestrial planets comparatively easy and more distant Jovian planets comparatively difficult to detect.

Planet C, some 2.8 times more massive than the Earth, orbits the pulsar every 98 days at a distance of 0.47 astronomical units* (AU); Planet B, with about 3.4 Earth masses, has a 67-Earth-day year at 0.36 AU. A smaller world, Planet A, still closer to the star, with about 0.015 Earth masses, is at 0.19 AU. Crudely speaking, Planet B is roughly at the distance of Mercury from our Sun; Planet C is midway between the distances