Largest extrasolar system discovered

A fifth planet has been discovered around a nearby star, making it the largest planetary system known outside our own. The planet appears to be a gas giant like Saturn, but scientists say any large moons it may have could potentially host life, since the planet lies in the "habitable" zone around its star, where liquid water can exist. The planet was discovered around a star called 55 Cancri that is about 41 light years away from Earth and is slightly cooler and dimmer than our own Sun. The 55 Cancri system was already known to include four other planets, including three giant planets that orbit the star closer than Mercury orbits the Sun. The fourth is four times as massive as Jupiter and orbits at about Jupiter's distance from the Sun. All of those planets were discovered by the way their gravity tugs on the parent star, a technique called the radial-velocity method. Now, astronomers have used the same method to discover a fifth planet that lies between the hot, close-in planets and the frigid distant one. The discovery was made by researchers led by Debra Fischer of San Francisco State University in California and Geoff Marcy of the University of California in Berkeley, both in the US. The new planet, called 55 Cancri f, orbits the star at a distance of 117 million kilometres, about 8% farther than Venus is from our Sun, putting it in the right zone for liquid water to exist. Watch an animation of an imaginary journey from our solar system to 55 Cancri, with a tour of the five-planet system that ends at the newly discovered planet in the star's habitable zone.

Beefy Neptune? But the planet itself probably does not boast the right conditions for life. That is because its mass is between that of Neptune and Saturn (or 45 times that of Earth), suggesting it has a gas-rich composition that is unfavourable for life as we know it. "We can only imagine that it might look something like a beefy Neptune-like planet or perhaps a Saturn-like planet with rings and moons around it," Fischer says.