Here's how to find a planet The Greeks found planets by noticing that certain "stars" seemed to wander around the heavens, out of synch with the rest of the (highly predictable) stars. A couple of millennia later, at the dawn of the Renaissance, Nicholaus Copernicus realized that planets -- including the Earth -- are in orbit around the sun. Over the years, culminating in 1930 with the discovery of frigid Pluto, astronomers found the nine planets in our solar system. But until last year, there were no credible reports of planets orbiting other stars. Was that because there aren't any? Or was it because planets are hard to see, drowned as they are in the glare of their stars, which tend to be 1 billion times brighter?
Detection by indirection Astronomers have concocted two "indirect" methods, and one possible direct method, for finding planets: Deviations in the star's movement (astrometry). This technique, which Gatewood used to detect the companion to Lalande 21185, requires fiendishly accurate measurements, and it's subject to the smallest errors. But since it can be applied to old photographs, it allows astronomers to examine the long data records that will reveal multi-year orbits of planets located in long, slow orbits. We're showing this drawing of a one-planet solar system again because it explains how stars and planets both orbit the center of mass. (right) Left to their own devices, stars would move smoothly across the sky. But the gravitational attraction of orbital companions -- planets or otherwise -- causes this slight, periodic deviation. (below)

Doppler shift. Remember trains? When they pass, the pitch of their whistles changes -- a demonstration of the Doppler effect. The same thing happens with light: when an object approaches, the frequency of its light increases, shifting it toward blue. When an object recedes, the reverse happens, and the light shifts toward red. Faster movement produces more shift (in either direction.) Astronomers use this alteration of light waves to detect the relative movement of celestial objects. They measure the light with a spectroscope (defined). If a planet is tugging a star closer and further from us, the star's light should periodically shift toward blue and toward red. These periodic shifts signal the presence of orbital companions, but they are absent if we are perpendicular to the plane of the orbit. (That's because Doppler shifts measure the movement toward or away from the observer, not absolute movement.) Shading of starlight. If a giant planet orbits between Earth and the star, it will create a regular dimming (a partial eclipse) signalling its presence. But that would require even more luck than the average astronomer can rely on, since it only works if we are in the orbital plane (defined) of the planet. As you can see, each of these techniques favors big planets, which alter a star's movement -- or shade the star --more than a shrimpy planet. Then what? Let's say we figure out how much a star is deviating from its predicted path, and how frequently the deviation occurs. How can we learn about the orbital object? (Warning: Here, we'll assume that one object is orbiting one star; things get considerably more complex when several objects are in one orbital system.) The calculation starts with an estimate of the star's mass (trust me, this can be done). From that mass estimate, and a measurement of the deviation, astronomers can guess at the mass of the companion object. (They could do this more accurately if they could estimate the angle of the orbital plane, but that's not yet possible.) This calculation uses simple orbital mechanics, as worked out by Johannes Kepler about 400 years ago. We found a cool page on Kepler's laws complete with a movie. Bruce Dern plays the swashbuckling astronomer in his battle against the forces of geocentrism (just kidding). By observing the period of the deviation, astronomers can figure the orbital period (the planet's year), again thanks to Kepler. And from that information, they can tell the radius of the planet's orbit -- how far from the star it resides. And that , combined with information on the star's brightness, leads to a calculation of how warm the planet may be. How's that for going a long way on a few scraps of information? But the best may still be in store for the planet-finders, says Dan Werthimer, an astronomer who participates in the search for life in space at the University of California, Berkeley. "If the astrometry people and the doppler shift people ever both detect wiggles in the same star, their combined results can pin down the inclination of the orbits, and thus the mass. This hasn't happened yet -- but it will probably in a year or so. It will give us very high confidence to have two different techniques seeing planets." You can read more about a nifty planet-finder that could be cranked up in a couple of decades or so. They want to put it in orbit near Jupiter, by Jove. (See Searching for Life... in the bibliography.) Warning: Not everybody thinks all these planet announcements are on the money.

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