Birth of stars.
Alien worlds are a staple of science fiction but until just a few months ago, they had no place in science fact. Despite decades of earnest searches, astronomers had no direct evidence that planets circle other stars. Then last October, Michel Mayor and Didier Queloz of Geneva Observatory detected a planet circling the star 51 Pegasi--and the floodgates opened. In rapid succession, observers have reported finding at least four more planets circling nearby stars; two of these sightings are so new that they have not yet been formally announced in the literature. "This is a unique era," exclaims Alan P. Boss of the Carnegie Institution of Washington, an expert in modeling planetary formation. "It is the most exciting thing I've seen in my scientific career."

The Hunt is On

For now, the planet-hunting trophy goes to the team led by Geoffrey W. Marcy and R. Paul Butler of San Francisco State University and the University of California at Berkeley. They helped confirm the results announced by Mayor and Queloz, and then swiftly turned up three planets of their own. More discoveries are surely on the way: Marcy reports that "we see hints of planets in a lot of our data." His group has set up a dedicated planet-search Web site to keep up with the rapid progress of the work. A fifth planetary detection will be reported by George Gatewood of the University of Pittsburgh at the upcoming meeting of the American Astronomical Society.

Nobody has actually seen the new planets; they were all identified indirectly, by measuring the way they influenced the motion of their parent stars. As an object orbits a star, its gravitational pull causes the star to wobble back and forth. That motion creates a periodic displacement, or Doppler shift,in the spectrum of the star as seen from the earth. Marcy and Butler, like Mayor and Queloz and several other teams, examine spectra for the tiny displacements that could denote the presence of planets. The Doppler technique can reveal the orbit and the minimum mass of an orbiting body, but no details of its nature.

Gatewood takes a somewhat different approach, one that relies on direct observation of stellar motion. Stars are not fixed in place; they appear to drift across the sky, though very slowly. The same back-and-forth movement that produces Doppler shifts also causes a star's path to appear as a zig-zag rather than a straight line. Gatewood and his colleagues are looking--very carefully--for those telltale wiggles. That precision measurement of stellar positions is known as astrometry.

The success of these techniques has sent ripples of excitement through the astronomical community and captured the public imagination. Despite some overzealous early statements, the newfound bodies are very unlikely to harbor life (much less intelligent life). But they do suggest that planets are common throughout the cosmos, raising the hope that living things flourish somewhere among the multitude of worlds. The planets are also quite unlike anything in our solar system, forcing theorists to reconsider their notions of how stars and planets form

Strange New Worlds

The planet around 51 Pegasi is perhaps the oddest of the bunch. Its mass is at least half that of Jupiter, and yet it orbits just seven million kilometers from its star--less than one eighth Mercury's distance from the sun. At such proximity, the planet's surface should be baked to a theoretical temperature of 1,300 degree Celsius. It whizzes around its star so quickly that its "year" is just four days long.

Marcy and Butler recently posted a notice on their Web site of the discovery of a somewhat similar planet circling the star HR3522, also known as 55 Rho Cancri. (Marcy describes the announcement as an experiment in "publishing by Internet": a way to air a new finding before passing it through peer review.) This object has at least four fifths the mass of Jupiter and orbits at a distance of about 25 million kilometers.

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.