Hunting planets with a gravitational lens.

Does the Milky Way harbor planetary systems other than our own? A definitive answer has eluded astroners for decades. But now, using a novel search technique, two research groups are beginning separate scans of the heavens that in a few years might reveal the presence of Jupiter-size planets orbiting distant stars within our galaxy.

The new studies rely on a fundamental principle of Einstein's theory of gravitation: Mass bends light. In fact, a massive object acts as a lens, dramatically distorting the image of a more distant star. If an observer, the massive object and the distant star are perfectly aligned, the lens will alter the star's image into a ring, an effect predicted by Albert Einstein and first observed in 1979. If alignment is slightly out of killer, the lens can split the distant star into pairs of images that may appear as a single, brighter image, depending on the resolution of the telescope used to view the object.

Researchers have used entire galaxies as gravitational lenses -- both to examine the properties of quasars billins of light-years beyond our galaxy and to study the lensing galaxies. Closer to home, individual stars that lie halfway to the center of the Milky Way should act as tiny gravitational lenses, distorting the light from stars near our galaxy's center, says Bohdan Paczynski of Princeton University. Moreover, if the foreground star happens to have a planet circling it, the character of the lensing may betray the planet's presence.

Viewed from Earth, a solar-mass star midway to the heart of our galaxy spends about two months moving in and out of alignment with a resident of the galactic core. During that time, the distant star appears progressively brighter and then dimmer.

But if the foreground star possesses a Jupiter-size planet, observers should see a second, measurable lensing effect. The more distant star undergoes additional brightening for a day or two as the planet further distorts the stellar image, Paczynski and his Princeton colleague Shude Mao reported in the June 20, 1991 ASTROPHYSICAL JOURNAL LETTERS. Thus, the ephemeral brightening of a star near the heart of our galaxy may spotlight a planetary system lurking among the swarms of stars that reside between the distant object and Earth.

These notions might have remained idle speculation, since lensing by a planetary system would occur rarely, says Paczynski. But large-scale electronic light detectors now enable astronomers to monitor the brightness of 10 million stars near the Milky Way's center each night, boosting the odds of discovering a lens at work. And Andrew Gould and Abraham Loeb of the Institute for Advanced Study in Princeton calculate that a Jupiter-like planet orbiting a solar-mass star that lies midway to the galactic core would further distort light from a core star in some 20 percent of lensing events. They will report their work this fall in the ASTROPHYSICAL JOURNAL.

Taking advances in both theory and experiment to heart, researchers are now looking for lenses in our galaxy. Last month, Paczynski and his colleagues began their search with a 14-inch telescope at Las Campanas Observatory in La Serena, Chile. First on the agenda, he says, is to identify those stars at our galactic center that intrinsically vary in brightness, so his team won't mistake such changes for lensing.

And next month at the Mount Stromlo Observatory near Canberra, Australia, a team led by Charles Alcock of the Lawrence Livermore (Calif.) National Laboratory will set up shop with the largest electronic light detector ever built for astronomy. This study aism to uncover small black holes, brown dwarfs and other unseen objects in our galaxy's halo. But the researchers will also turn their attention to the heart of the Milky Way and expect to detect planetary systems if they lurk there, Alcock says.