The newest planet found orbiting 55 Cancri is a gas giant roughly half the size of Saturn (shown here in an image taken by the Cassini spacecraft). Image courtesy of NASA/JPL.
According to lead author Debra Fischer, assistant professor of astronomy at San Francisco State University, the fifth planet is within the star's habitable zone in which water could exist as a liquid. Though the planet is a giant ball of gas, liquid water could exist on the surface of a moon or on other, rocky planets that may yet be found within the zone. "Right now, we are looking at a gap between the 260-day orbit of the new planet and the 14-year orbit of another gas giant, and if you had to bet, you'd bet that there is more orbiting stuff there."
"We haven't found a twin of our solar system, because the four planets close to the star are all the size of Neptune or bigger," Marcy said, but he added that he's optimistic that continued observations will reveal a rocky planet within five years.
It's only been about 15 years since the first extra-solar planet was discovered. Since then over 200 have been found orbiting other stars. 55 Cancri is the first star (other than our own) known to have more than 4 planets.