One of the bodies identified by Marcy and Butler is a very massive object--at least 6.5 times the mass of Jupiter. It orbits around the star 70 Virginis in a highly eccentric, or oval, path quite unlike those of the familiar planets. Although some news reporters optimistically dubbed the planet "Goldilocks," claiming it has just the right temperature for liquid water, this heavyweight is most likely a gaseous world lacking a solid surface on which water could collect.

The final planet discovered by Marcy and Butler (so far) has less extreme attributes. Its three-year orbit takes it on a circular course about 300 million kilometers from its star (corresponding to an orbit between Mars and Jupiter) and its mass is at minimum 2.3 times that of Jupiter. It would not seem terribly out of place in our own solar system.

It comes as no surprise that astronomers are mostly finding giant, short-period planets, for a simple reason: these are the easiest to pick out using the Doppler technique. Detecting a solar system like our own (in which the most massive planet, Jupiter, takes a full 12 years to complete one orbit) would require at least another decade or two of high-precision Doppler observations.

In contrast, the astrometric approach that Gatewood uses is most sensitive to planets in large, leisurely orbits. After scrutinizing 50 years of observations of the star Lalande 21185, he has tentatively deduced the presence of a Jupiter-mass planet in a 5.8-year orbit; the planet would circle at more than twice the earth's distance from the sun. Gatewood also sees evidence of a second planet in a 30-year orbit. (He has released an early abstract describing these results.)

David C. Black of the Lunar and Planetary Institute in Houston, Texas considers Gatewood's sighting to be the only one that would satisfy his definition of what a planet is. "It is not clear that any of the others have anything to do with planets," he says, arguing that they probably formed in a fundamentally different way than the planets that orbit the sun.

Where Do Planets Come From?

Indeed, the question of how these planets (or non-planets) formed is a vexing one that has already generated considerable discussion. Current theory holds that giant planets can form only at comparatively great distances from a star, where cold temperatures allow ice and frozen gases to gather together. What then are Jupiter-mass bodies doing so close to the stellar hearth? Boss suggests that these planets actually formed at much greater distances from their stars but then migrated inward.

One variant of this theory is described by Douglas N.C. Lin of Lick Observatory and his colleagues in a recent issue of Nature. In this view, newborn planets can interact with the disk of material from which they form, causing them to spiral toward the central star. Inner planets (which could have turned out to be more earth-like) might have been destroyed during this early epoch. Whatever the explanation, the surprising attributes of the new planets clearly demonstrate that there are many ways that planetary systems form

And what of planets like the earth--are they out there too? They lie beyond the grasp of most current search techniques. But NASA administrator Daniel S. Goldin has set the detection and study of earth-like bodies around other stars as one of NASA's top priorities. The most likely way to achieve that goal is by building a sophisticated space-based interferometer (as recently described by J. Roger P. Angel and Neville J. Woolf in this magazine). Ground-based tests of optical interferometers are already underway.

The current list of extrasolar planets represents only the tip of the iceberg. Continued observations, careful data analysis and innovative technologies will soon yield many more discoveries, giving us a better sense of the true variety of worlds out there.

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