June 24, 2002 Astronomers find planet system that reminds them of home By Robert Sanders, UC Berkeley 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 home solar system. This artist's concept depicts a Jupiter-mass planet orbiting the star, 55 Cancri, about 41 light years from Earth. A possible moon around the planet is shown because such moons are thought to be common around this type of planet, but no moon has been detected. Image: NASA/Lynette Cook. Led by Geoffrey Marcy at UC Berkeley and Paul Butler at the Carnegie Institution of Washington, the team includes two UCSC astronomers, Steven Vogt and Gregory Laughlin. At a press conference June 13 at NASA headquarters, the researchers announced the discovery of a Jupiter-like planet orbiting a sunlike star at nearly the same distance as Jupiter orbits our sun. "This is the first near analog to our Jupiter," said Marcy, a UCSC alumnus. "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 planet-hunting team is funded by grants from the National Science Foundation and NASA. The star, 55 Cancri in the constellation Cancer, was already known to have one planet, a gas giant slightly smaller than the mass of Jupiter whipping around the star in 14.6 days at a distance only one tenth that from the Earth to the sun, announced by Butler and Marcy in 1996. Using the 93 million mile Earth-sun distance as a yardstick, called an astronomical unit or AU, the newly found planet orbits at 5.5 AU, comparable to Jupiter's distance from the sun of 5.2 AU. 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, 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," 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 team of astronomers passed their data along to Laughlin, who 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, an assistant professor of astronomy and astrophysics. "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. Marcy, Butler, and their team announced a total of 13 new planets at the press conference, including the smallest ever detected: a planet circling the star HD49674 in the constellation Auriga at a distance of 0.05 AU and a mass about 15 percent that of Jupiter--nearly half that of Saturn, or 40 times the mass of the Earth. This brings the total number of known planets outside the solar system to more than 90. Discovery of a second planet orbiting 55 Cnc 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. Collaborators on the project include Debra Fischer at UC Berkeley, Greg Henry at Tennessee State University, and Dimitri Pourbaix of the Institut d'Astronomie et d'Astrophysique, Universite Libre de Bruxelles, in addition to Marcy, Butler, Vogt, and Laughlin. Marcy and Butler used a technique that measures 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. By observing over a period of years, they can infer a planet's approximate mass and orbital size and period. The star 55 Cnc is 41 light years from Earth and is about 5 billion years old. Laughlin speculated that the large inner planet probably formed farther from the parent star, where ice could form and rocks accrete to form a solid core, and only migrated inward after it had scooped up a shroud of gas. This inward migration is a characteristic of giant planets in a disk of gas and dust 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 it 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. There also may be another planet around 55 Cnc, because the two known planets do not yet explain all the observed Doppler wobbling. The scientists need more data before they can come to a final conclusion, however. Return to Front Page