A dozen new planets ... and still counting.

Just last month, the number of known planets orbiting stars similar to the sun stood at 10--one more than within the solar system (SN: 8/8/98, p. 88). The official count has now risen to an even dozen, with the likelihood that several other candidate objects will soon boost the population to 15 or 20.

"We may be announcing a new planet every 2 months," says R. Paul Butler of the Anglo-Australian Observatory in Epping, Australia. Too dim to be seen, planets betray their presence by their tug on the stars they orbit.

In a separate study, new images of dust around one star indicate that it hosts a full planetary system rather than a single planet.

The two latest planets, which Butler announced at the Carnegie Institution of Washington (D.C.) on Sept. 9, each have a special niche. A planet orbiting the star HD210277, discovered by the Keck 1 Telescope atop Hawaii's Mauna Kea, is the first whose average distance from its parent star is nearly the same as Earth's distance from the sun.

Often used as a yardstick, the Earth-sun separation is defined as 1 astronomical unit (AU). The new planet lies at an average distance of 1.15 AU from its host star. The planet is far heavier than Earth, at least 1.36 times as massive as Jupiter, and has a much more elongated orbit. The planet ventures closer to its host star than Venus' average distance from the sun and farther away than Mars' average distance. The parent star is 68 light-years from Earth.

The other new planet, orbiting the star HD187123, is closer to its host than any other planet found so far. The star is 156 light-years from Earth. Butler and his colleagues, including Geoffrey W. Marcy of San Francisco State University and the University of California, Berkeley, will report details in PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC. The orbiting body, which has at least half the mass of Jupiter, whips around the star once every 3.095 days and lies 0.042 AU from its parent, one-ninth the average distance of Mercury from the sun.

That makes the object a real hotty--along with three other recently found planets that orbit within the blistering outer atmosphere of their host stars. Like all planets, these objects arose from disks of gas, dust, and ice that surrounded the parent stars in their youth. According to a popular theory, massive planets arise at Jupiterlike distances from their star and can migrate inward by flinging material toward the outer part of the disk. This suggests that stars with migrating planets have unusually dusty disks that should be easy to detect.

That's just what David E. Trilling and Robert H. Brown of the University of Arizona in Tucson found when they examined the region surrounding the star 55 [Rho.sup.1] Cancri, already known to harbor a closely orbiting planet.

Using NASA's Infrared Telescope Facility atop Mauna Kea, the researchers blocked out the star's bright light and spied a dusty disk extending at least 40 AU from the star. That's roughly the same distance at which the Kuiper belt, the solar system's reservoir of dusty comets, lies from the sun. The disk around 55 [Rho.sup.1] Cancri, however, appears to contain about 10 times as much material. The excess, says Trilling, "is a nice confirmation of the theory ... that the planet migrated in" and pushed dust out.

"Although I haven't seen the data ... the explanation seems very plausible to me," says Michael Jura of the University of California, Los Angeles.

"We're trying to be very cautious," Trilling adds, "but a Kuiper belt is almost certainly what we're looking at." The disk, which may represent debris from the formation of several planets, is the first seen around a middle-aged, ordinary star, he says. "There's real evidence that this is a fully mature planetary system."

The team plans to unveil an image of the disk next month at a meeting of the American Astronomical Society in Madison, Wis. A description of the team's work appears on the Web site "The Extrasolar Planets Encyclopaedia" (http://wwwusr. obspm.fr/departement/darc/planets/ encycl.html).