Dr Brad Carter
13 June, 2002
New-found planetary system starts to look like home
An international team including two Australian astronomers has finally found a planetary system that reminds them of our own solar system.
At a NASA news conference in Washington D.C. today [1 P.M. Thursday, Eastern US time] the team said they had found a Jupiter-like planet orbiting a Sun-like star at nearly the same distance as Jupiter orbits our Sun.
The announcement was made by Geoffrey W. Marcy, Professor of Astronomy at the University of California, Berkeley, and director of UC Berkeley's Center for Integrative Planetary Science, and his colleague, Paul Butler of the Carnegie Institution of Washington.
"This is the first near analog to our Jupiter," Marcy said. "All other extrasolar planets discovered up to now orbit closer to the parent star, and most of them have had elongated, eccentric orbits. This new planet orbits as far from its star as our own Jupiter orbits the sun."
The star, 55 Cancri in the constellation Cancer, was already known to have one planet, announced by Butler and Marcy in 1996. That planet is a gas giant slightly smaller than the mass of Jupiter. It whips around the star in 14.6 days at a distance only one-tenth that from Earth to the Sun.
Using as a yardstick the 93-million mile Earth-Sun distance, called an astronomical unit or AU, the newfound planet orbits at 5.5 AU, comparable to Jupiter's distance from our Sun of 5.2 AU. It is 3.5 to 5 times the mass of Jupiter. Its slightly elongated orbit takes it around the star in about 13 years, comparable to Jupiter's orbital period of 11.86 years.
"We haven't yet found an exact solar system analog, which would have a circular orbit and a mass closer to that of Jupiter. But this shows we are getting close, we are at the point of finding planets at distances greater than 4 AU from the host star," said Butler. "I think we will be finding more of them among the 1 200 stars we are now monitoring."
"There's no place like home, but we're definitely coming closer," said Dr Chris Tinney of the Anglo-Australian Observatory.
The team announced the discovery of 13 new planets today. The new planet around 55 Cancri was found from data taken with the 3-m telescope of the Lick Observatory in California. Another six planets were found solely by one of the 10-m Keck telescopes in Hawaii, and three were found through joint Keck and Lick observations. Two of the 13 were found by the 3.9-m Anglo-Australian Telescope in New South Wales, which also recently bagged a further two.
This haul brings the total number of known planets outside our solar system to more than 90.
One of the planets, found with the Keck telescope, is the smallest ever detected: a planet circling the star HD49674 in the constellation Auriga at a distance of .05 AU, one-twentieth the distance from Earth to the Sun. Its mass is about 15 percent that of Jupiter and 40 times that of Earth.
Finding the second planet around 55 Cancri took 15 years of patient observations.
The team of astronomers passed their data on 55 Cancri along to theoretical astronomer Greg Laughlin, Assistant Professor of Astronomy and Astrophysics at UC Santa Cruz. He conducted dynamical calculations that show an Earth-sized planet could survive in a stable orbit between the two gas giants.
"We tried a hypothetical configuration of a terrestrial planet in the habitable zone around one AU from the central star and found it very stable," said Laughlin. "Just as the other planets in our solar system tug on the Earth and produce a chaotic but bounded orbit, so the planets around 55 Cancri would push and pull an Earthlike planet in a manner that would not cause any collisions or wild orbital variations."
For the foreseeable future, any such planet in the habitable zone around 55 Cancri will remain speculative.
"Nevertheless, this planetary system will be the best candidate for direct pictures when the Terrestrial Planet Finder is launched later this decade," said UC Berkeley astronomer Debra A. Fischer, referring to NASA's planned space-borne imaging telescope designed to take pictures of Earth-sized planets.
There also may be another planet around 55 Cnc, because the two known planets do not yet explain all the observed Doppler wobbling. The team needs more data before they can come to a final conclusion, but one possibility is a Saturn-mass planet orbiting about 0.24 AU from the star, or one-quarter the Earth-sun distance.
Marcy and Butler developed a sensitive technique for measuring the slight Doppler shift in starlight caused by a wobble in the position of a star, a periodic shift due to a planet yanking on the star as it orbits. From measurements over a period of years, they are able to infer the period, its approximate mass and the size of its orbit. Uncertainties arise because there is no way to determine the orientation of the orbit - whether we are seeing it edge on, or tilted to face toward us.
The astronomers are monitoring 300 stars with the Lick telescope, 250 with the Anglo-Australian Telescope and 650 with the Keck Telescope. They hope that within a few years they can use the 6.5-meter Magellan telescopes at Las Campanas Observatory in Chile to ramp up to 2 000 stars, all within 50 parsecs (150 light-years) of Earth.
In addition to Marcy, Butler, Fischer and Laughlin, collaborators on the project include Steve Vogt, professor of astronomy and astrophysics at UC Santa Cruz; Greg Henry of the Center of Excellence in Information Systems at Tennessee State University, Nashville; Dimitri Pourbaix of the Institut d'Astronomie et d'Astrophysique, Universite' Libre de Bruxelles; Hugh Jones of the Astrophysics Research Institute at Liverpool John Moores University in the United Kingdom; Chris Tinney of the Anglo-Australian Observatory in Australia; Chris McCarthy of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington; Brad Carter of the University of Southern Queensland, Australia; and Alan Penny of the Rutherford Appleton Laboratory in the United Kingdom.
The planet-hunting research is supported by NASA, the US National Science Foundation, the Australian government, and the UK government (through PPARC).
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