The newfound planet joins four others circling the nearby star 55 Cancri in the constellation Cancer. Although it resides in the star's so-called habitable zone, a place where liquid water and mild temperatures should exist, it is more like Saturn than Earth and therefore not likely to support life.
Still, scientists have not ruled out the possibility of finding an Earth-like planet within the system as technology improves.
"It's a system that appears to be packed with planets," said co-discoverer Debra Fischer, an astronomer at San Francisco State University.
Ranked fourth from 55 Cancri, the latest planet is about 45 times the mass of Earth and has an orbit of 260 days. It was detected after nearly two decades of observations by ground-based telescopes using the Doppler technique that measures a planet's stellar wobble.
The other planets in the 55 Cancri system were discovered between 1996 and 2004. The innermost planet is believed to resemble Neptune, while the most distant is thought to be Jupiter-like.
Scientists have detected about 250 exoplanets, or planets orbiting a star other than the sun. The 55 Cancri star holds the record for number of confirmed planets. Only one other star is known to have four planets, while several others have three or less.
"We can now say there are stars like the sun that have many worlds around them," said planetary scientist Jonathan Lunine of the University of Arizona, Tucson, who had no role in the discovery.
The research will appear in a future issue of the Astrophysical Journal. It was funded by NASA, the National Science Foundation and the University of California.
The latest discovery shows that our solar system is not unique, scientists said.
"When you look up into the night sky and see the twinkling lights of stars, you can imagine with certainty that they have their own complement of planets," said astronomer Geoff Marcy of the University of California, Berkeley, who was part of the research.