Jupiter-Size Planet Found Orbiting Star in Big Dipper
The planet is the second found to orbit the star 47 Ursae Majoris (47 UMa) in the Big Dipper, also known as Ursa Major or the Big Bear. The new planet is at least three-fourths the mass of Jupiter and orbits the star at a distance that, in our solar system, would place it 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 team member Debra Fischer of the University of California at Berkeley. "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 period, planetary systems that look more like our solar system."
The planet-search team, which is supported by NSF and NASA, has been instrumental in finding a majority of the extrasolar planets. Besides Fischer, the team includes Geoffrey Marcy, also of Berkeley; Paul Butler of the Carnegie Institution of Washington; Steve Vogt of the University of California at Santa Cruz; and Gregory Laughlin of NASA's Ames Research Center. Their report on the new planet has been submitted to Astrophysical Journal.
A few years ago, Marcy and Butler discovered a planet more than twice the mass of Jupiter in a circular orbit around the same star. The star is one of 100 that the scientists have targeted since 1987 in their search for evidence of extrasolar planets. They use the 3-meter and 0.6-meter telescopes at the University of California's Lick Observatory to measure Doppler-shifted light reaching the Earth from stars. Regular changes in the Doppler shift, they believe, signal the presence of a planet periodically pulling the star toward or away from Earth.
Fischer was able to see the periodic wobble from the second planet, smaller and farther from the star than the first, because of improved instrumentation that can measure motions as small as three meters per second.
The star is a yellow star similar to the Sun, probably about seven billion years old and located about 51 light years from Earth.
"47 UMa itself appears quite faint to the naked eye. It was chosen because it is a star which is both relatively nearby, and is also very similar to our own Sun in all of its characteristics," said Laughlin.
Although 47 UMa has a planet similar to Jupiter in mass and orbital distance, there is no evidence that this star system also has an Earth-like planet. In fact, the existence of two gas giant planets in orbit around 47 UMa makes it more difficult for such terrestrial planets to form within the star's habitable zone.
"The overall behavior of the 47 UMa system seems to be very similar to that of the Jupiter-Saturn pair," said Laughlin. "The gravitational effects of Jupiter and Saturn make it difficult to put together a large planet in the asteroid belt. By the same token, it is probably difficult to assemble a large planet in the habitable zone of 47 UMa."
To find an Earth-like planet suitable for the development of advanced life, scientists have to be able to assess the planet's proximity to the parent star, the planet's mass, whether the orbit is circular (thus avoiding seasonal extremes), and whether there is a massive outer planet to shield against incoming asteroid or comet strikes.
"The Doppler shift technique is really the best available method for finding Jupiter-sized planets in Jupiter-like orbits," said Laughlin. "Unfortunately, the Doppler technique will have great difficulty in finding planets that are less than about 10 Earth masses (Jupiter, by contrast is about 300 Earth masses). To find terrestrial planets will require a space mission such as the "Kepler Mission" being proposed by Ames."