Washington, D.C. - After 15 years of observation and lots of patience, the world's premier planet-hunting team has finally found a planetary system that reminds them of our own solar system.
At a press conference today (Thursday, June 13) at National Aeronautics and Space Administration (NASA) headquarters, University of California, Berkeley, astronomer Geoffrey W. Marcy and Carnegie Institution of Washington astronomer Paul Butler announced the discovery of a Jupiter-like planet orbiting a sun-like star at nearly the same distance as Jupiter orbits our sun.
"This is the first near analog to our Jupiter," said Marcy, professor of astronomy and director of UC Berkeley's Center for Integrative Planetary Science. "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, which was announced by Butler and Marcy in 1996. That planet is a gas giant slightly smaller than the mass of Jupiter, and whips around the star in 14.6 days at a distance only one tenth that from the Earth to the sun.
Using the 93-million-mile Earth-sun distance as a yardstick, called an astronomical unit or AU, the newly found planet orbits at 5.9 AU, comparable to Jupiter's 5.2 AU distance from the sun. With a mass between 3.5 and 5 times that of Jupiter, the planet has a slightly elongated orbit that carries it around the star in about 13 years (5,360 days), comparable to Jupiter's orbital period of 11.86 years. The team also reported today a third planet around 55 Cancri, a gas giant with an orbital radius of 0.24 AU and a mass of about 0.21 Jupiter masses. That planet orbits in 44.28 days.
"We haven't yet found an exact solar system analog, with a planet in a circular orbit and a mass closer to that of Jupiter," Butler said. "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. And we found this planet among the 107 stars we first targeted when we started looking for planets at Lick Observatory in 1987, so I think we will be finding more of them among the 1,200 stars we are now monitoring."
The planet-hunting team, funded by grants from the National Science Foundation and NASA, announced a total of 15 new planets today, including the smallest ever detected: a planet circling the star HD49674 in the constellation Auriga at a distance of 0.05 AU - one-twentieth the distance from the Earth to the sun. Its mass is about 15 percent that of Jupiter, or nearly half that of Saturn, and 40 times the mass of the Earth. This brings the total number of known planets outside the solar system to more than 90.
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, who conducted dynamical calculations that show an Earth-sized planet could survive in a stable orbit between the two inner gas giants and the outer planet.
"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, who also is associated with Lick Observatory. "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.
The star 55 Cnc is 12.5 parsecs (41 light years) distant and a middle-aged, 4-7 billion-year-old G8 star rich in heavy elements like carbon, iron, silicon and sulfur, Fischer said. The sun is about 5 billion years old, with half that amount of heavy metals.
Laughlin speculated that the large, inner planets probably formed farther from the parent star, where ice could form and rocks accrete to form a solid core, and only migrated inward after they had scooped up a shroud of gas. This inward migration is a characteristic of giant planets in a disk of gas and dust that is typical of forming planetary systems, he said. They create a spiral wake that actually tugs on the planet, slowing it down and sending it spiraling inward toward the star.
"To me, the question is why they stopped before crashing into the star," Laughlin said. Numerous giant extrasolar planets have been found in very short-period orbits - 3 to 3.5 days - when, by all rights, they should have spiraled to a flaming death.
Marcy and Butler originated 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.
Discovery of a planet orbiting 55 Cnc at the distance of Jupiter is the culmination of 15 years of observations using the 3-meter telescope at Lick Observatory, which is owned and operated by the University of California. Four of the 15 newly found planets were discovered at the 3.9-meter Anglo-Australian Telescope in New South Wales, Australia.
In addition to the 300 stars the team monitors with the Lick telescope, the astronomers are following another 650 with the 10-meter Keck Telescope in Hawaii and another 250 southern hemisphere stars with the 3.9-meter AAT. Within a couple of years, they hope to 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.
"This will cover all the good candidates out to 50 parsecs, so we will know where to look when we have the Terrestrial Planet Finder and the Space Interferometry Mission, which will do the first reconnaissance to identify Earth-like planets," Butler said.
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 Telescope; 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.