New array of planet finds.

A year ago, planetary scientists didn't know of a single sunlike star that possessed a planet. By last month, they had found evidence of five unseen planets, each orbiting a different nearby star similar in mass to the sun (SN: 6/15/96, p. 373). Now, within the past 3 weeks, researchers have reported signs of three additional planets.

With announcements coming this fast and furious, it's probably no surprise that scientists revealed the first of the new finds with little fanfare, on the World Wide Web. Planet hunters reported in mid-June that the back-and-forth motion of the star tau Bootes indicates that a body about 3.8 times as massive as Jupiter orbits it.

With a sensitive spectrograph at Lick Observatory atop Mount Hamilton in California, Geoffrey W. Marcy of San Francisco State University and R. Paul Butler of San Francisco State and the University of California, Berkeley measured the star's wobble along the line of sight to Earth. The planet whips around the star every 3.3 days, they report on their Web site (http://cannon.sfsu.edu/~williams/planetsearch /planetsearch.html/).

A team led by Swiss researchers Michel Mayor and Didier Queloz of the Geneva Observatory, codiscoverers of the first planet detected around a sunlike star (SN: 10/21/95, p. 260), has also measured tau Bootes' periodic wobble and corroborates the find. Turnabout is fair play-last year, Marcy and Butler confirmed the Swiss team's headline-making find, the planet circling the star 51 Pegasi.

Intriguingly, the newly found planet circles tau Bootes at a distance of 6.8 million kilometers, well within the furnace of the star's outer atmosphere. That's one-tenth the distance at which Mercury, our solar system's innermost planet, orbits the sun and closer than any known planet and its parent. Tau Bootes has a higher than average abundance of elements heavier than helium. So does 51 Pegasi, whose planet also orbits tightly.

All of this excites theorist Doug N.C. Lin of the University of California, Santa Cruz. The tight orbits of these planets "make me very happy," Lin says. He believes that no planet could form that close to a star, so the planets must have originated relatively far out in the disk of gas and dust that surrounds young stars and subsequently spiraled inward. Some planets fall into the star, where they may enhance the abundance of heavier elements at the star's surface. At the same time, their death enables other planets traveling behind them to migrate to a niche just short of immolation (SN: 12/16/95, p. 412).

In addition, Lin says, his work predicts that stars with closely orbiting planets also harbor planets much farther out.

Indeed, 2 weeks ago, during a meeting on binary stars and planet formation at the State University of New York in Stony Brook, Marcy reported the tentative discovery of a distant planet orbiting the star 55 rho1 Cancri. Researchers had previously inferred the presence of a planet orbiting the star at a smaller distance, equal to one-third Mercury's distance from the sun (SN: 4/27/96, p. 267). Marcy and Butler calculate that the more distant planet, about five times as massive as Jupiter, lies 35 times farther away, about the same distance as that between Jupiter and our sun. This finding remains uncertain because it would take the planet 20 years to orbit 55 rho1 Cancri and the researchers have measured the star's wobble for only about half that time.

Finally, on June 23, at the annual meeting of the Astronomical Society of the Pacific in Santa Clara, Calif., Marcy announced that the motion of another nearby, sunlike star, upsilon Andromedae, also betrays the presence of a planet. This object would have a mass about 60 percent of Jupiter's and an orbit about 15 percent of Mercury's orbit about the sun.