SAN DIEGO -- The world's most prolific planet-hunting team has done it again, this time announcing the discovery of what may be the weirdest solar systems observed so far -- one harboring two humongous planets and another with planets in synchronized orbits.

Geoff Marcy of the University of California in Berkeley, with Paul Butler, Debra Fischer and other colleagues, announced on Tuesday the two unique planetary systems as part of their ongoing survey of nearby stars -- systems that appear to challenge current theories of how planetary systems form.

"We thought we understood the lay of the land in terms of planetary systems," said Marcy, whose team has found more than half of the 50-plus extrasolar planets discovered so far. "We were becoming a bit cocky -- we thought we knew the full diversity of planets."

But the two new systems -- known as HD168443 and Gliese 876, "depart quite dramatically from our expectations."

The team presented its findings Tuesday at the winter meeting of the American Astronomical Society in San Diego.

"Frankly, that one is so massive, we don't even know if the term 'planet' is appropriate for it," Marcy said in an interview. But he added that it would be presumptuous to call the object a brown dwarf because it most likely formed in a manner similar to that of the giant planets in our own solar system.

The "whopper" and its companion likely formed as an accretion disk swirled around the host star, Marcy said, and in the process "somehow, gobbling up so much mass that they became humongous planets."

HD168443 is located about 123 light-years from Earth in the direction of the southern constellation Serpens.

The star's 17-Jupiter-mass companion "is about twice as massive as anything we've previously seen," said Paul Butler of the Carnegie Institution in Washington, D.C. But he agrees with Marcy that it makes more sense to call the object a planet rather than a star.

"To say it's a brown dwarf is just sweeping all of the mysteries under the rug," he said.

The two large planet-like objects "presumably both formed in a disk around the star, as conventional planets do. But the fact that they're so massive suggests perhaps some new mechanism."

Computer simulations suggest that multiple-planet systems may be common -- however, most models suggest that having two ultra-giant planets, especially in such tight orbits about their host star, would be rare. The two systems "remind us of the extraordinary diversity of planetary systems," Butler said.

He also said the observations are a positive sign that Earth-sized planets may be out there, awaiting detection.

"I would say these discoveries represent a thumb's up for terrestrial planets," Marcy said. "We're finding more and more planetary systems, a great diversity of planets. The bottom line is, stars are typically attended by a complement of planets, some of which are probably Earth-sized."

Some observers were more critical of that idea, however, saying that the gravitational tug from numerous giant planets could lead to solar systems which are unlikely to harbor Earth-like siblings.

Systems like those announced by Marcy's team on Tuesday "probably won't be forming many terrestrial planets," said Hal Levison of the Southwest Research Institute in Boulder, Colorado.

"And if they are, they're probably closer to their sun than the habitable zone [the distance range from the star in which liquid water could be sustained on a planet's surface]."

Marcy's team also includes Steve Vogt of Santa Cruz and Jack Lissauer of the NASA Ames Research Center in California. The findings have been submitted to the Astrophysical Journal for publication.

The research is part of an ongoing to project to search for planets around 1,100 stars within 300 light-years of Earth. The work is supported by NASA, the National Science Foundation and Sun Microsystems.