Astronomers discover Jupiter-sized planet in vicinity of Big Dipper

By Robert Sanders, Public Affairs

This artist's concept of the 47 Ursae Majoris system shows how the star would appear from a moon of the outer Jupiter-sized planet orbiting the star. The artist has imagined a ring around the planet, similar to the rings around Saturn and Uranus. The inner gas giant is visible nearer the star, while a hypothetical Earth-like planet is just visible in the star’s glare. Lynette Cook artwork, copyright 2001

22 August 2001 | With the help of improved measurement techniques, Berkeley planet hunters have been able to detect a Jupiter-sized planet orbiting a nearby star at a distance comparable to that of Jupiter in our solar system.

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

“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.”

“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,” noted 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 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 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.