A planet this size and distance from its star produces slight long-period wobbles in the motion of the star that until now have been impossible to detect.

The UC Berkeley astronomers found the planet, which is at least three-quarters the size of Jupiter, orbiting the star 47 Ursae Majoris (47 UMa) in the Big Dipper - Ursa Major or the Big Bear. The star is known already to have one orbiting planet 2.5 times bigger than Jupiter. Both planets are in nearly circular orbits, and in our solar system would be located beyond Mars but within the orbit of Jupiter.

"For the first time we have detected two planets in nearly circular orbits around the same star," said Debra Fischer, an assistant research astronomer working with UC Berkeley astronomy professor Geoffrey Marcy and researcher Paul Butler of the Carnegie Institution of Washington. "Most of the 70 planets people have found to date are in bizarre solar systems, with short periods and eccentric orbits close to the star.

"As our sensitivity improves we are finally seeing planets with longer orbital periods, planetary systems that look more like our solar system."

The star 47 UMa is one of the 100 stars that Marcy and Butler first targeted in 1987 when, in search of evidence for planets, they began collecting data on stellar wobbles. The 13 years' worth of data were obtained at Lick Observatory with the 3-meter Shane and 0.6-meter Coude Auxiliary telescopes hooked to the high-resolution Hamilton spectrograph - all instruments shared with other astronomers.

Fischer said that the UC Berkeley team's success in finding extrasolar planets highlights the need for a telescope dedicated to planet searches and available to do the nightly monitoring necessary to plot the motion of bright, nearby stars and detect distant companions.

"We hope to raise $5 million to purchase a dedicated two-meter telescope that will give us the same or better efficiency, when coupled to the Hamilton spectrograph, as the three-meter telescope we use today," Fischer said. The telescope would be constructed on Mt. Hamilton as part of Lick Observatory.

"Our technique has the precision to detect tiny wobbles in stars and this discovery demonstrates our ability to keep a steady hold on that exquisite precision over many years," she said. "With a dedicated telescope we could begin to detect much lower mass planets - perhaps as low as 20 Earth masses - and Jupiter-sized planets in Jupiter-sized orbits."

Fischer, Marcy, Butler and their colleagues described the new planet in a paper recently submitted for publication in Astrophysical Journal. Their team included Gregory Laughlin, a theoretician based at NASA Ames Research Center, and Steve Vogt, professor of astronomy and astrophysics at UC Santa Cruz.

"When we discovered the first planet around 47 Ursae Majoris five years ago, I never dreamed that we would find yet another planet orbiting the same star," said Marcy. "Every new planetary system reveals some new quirk that we didn't expect. We've found planets in small orbits and wacky eccentric orbits. With 47 Ursae Majoris, it's heartwarming to find a planetary system that finally reminds us of our solar system."

"The discovery of planets in circular orbits is exciting because they are so rare," Butler added. "Of the published planets with orbital periods longer than a month, only HD 27442 and this system are in circular orbits. From this we can make a preliminary guess that about five percent of planetary systems are in circular orbits."

The star 47 UMa is a yellow G0V star very similar to the Sun, probably about 7 billion years old and located about 51 light years from Earth. Fischer noted that the so-called habitable zone around the star -a region approximately equivalent to that between the orbits of Venus and Mars in our solar system, an area that includes Earth - is devoid of large gaseous planets. This means it potentially could harbor an Earth-sized rock we can't yet see, and won't be able to see until NASA launches the next generation of planet hunting missions from space.

Based on dynamical computer simulations by Laughlin, Fischer cautioned that "it could be difficult for an Earth-mass terrestrial planet to form in a stable orbit within that habitable zone" because of the proximity of the two outer gas giants.

The inner planet circling 47 UMa was first discovered in 1996 by Marcy and Butler using their new and precise technique for measuring Doppler-shifted light from stars. Regular changes in the Doppler shift, they thought, signaled the presence of a planet periodically pulling the star toward or away from Earth.

Fischer was able to see the periodic wobble from a second planet, smaller and farther from the star, because of improved precision - down to 3 meters per second - in measuring motion along the line of sight to the star.

"Among the published planets, only the outer planet in this system and HD 16141 have Doppler amplitudes smaller than Jupiter on the Sun," which induces a wobble of 11 meters per second, said Butler. "Our long range goal remains the detection of true Jupiter analogs - bona fide solar system analogs - to allow us to compare our solar system to other planetary systems."

In addition, 13 years of data from the star allowed Fischer to tease out a wobble with a 7.1-year period, the time it takes the newly discovered outer planet to orbit the star. The inner, larger planet orbits in 2.99 years.

The two-planet system bears another intriguing resemblance to our own. The new measurements by Fischer and her colleagues peg the mass of the inner planet at 2.5 times that of Jupiter, at least, while the newly discovered outer planet has a mass at least 3/4 that of Jupiter, yielding a mass ratio of 3.3. The mass ratio of Jupiter to Saturn is also 3.3.

The average distance from the star to the inner planet is 2.09 times the average distance of the Earth from the Sun, a unit of measure called an astronomical unit or AU. The outer planet is 3.73 AU from the central star. For comparison, Jupiter and Saturn are at distances of 5.203 and 9.555 AU, respectively.