As astronomers detect more and more planets outside our solar system, they are finding that many of them have more than one sun, unlike Earth and its companions.

Imagine a world where it is always daytime. The notion of sunrise and sunset would not exist. According to a study led by Georgia State University graduate student Deepak Raghavan, this could be true of many planets discovered in the past decade in other solar systems because they have multiple suns.

"Over one fifth of the extrasolar planetary systems are in multiple star environments," Raghavan said. "So if you lived on any of these planets, you would have at least two suns in your sky."

Raghavan told the American Astronomical Society meeting in Washington that he and colleagues painstakingly sifted through data already collected on 131 foreign planetary systems and backed up some of it with observations using a telescope in Chile. He found that 26 planetary groups orbit twin stars and three of them circle triplet stars.

In situations where it was unclear whether a planet had multiple suns, his team looked for evidence that stars close to each other traveled together.

"If we find two stars in the same piece of the sky moving in the same direction, we think that there is a good chance that they are related to each other and actually orbiting each other," he added. "If we find that the distances to the two stars are also of the same order, then we are ready to conclude that they are actually gravitationally bound to each other."

Astronomers were once unsure whether planets could form in so-called binary star systems. They thought the strong gravitational forces from one star might interfere with planet formation around the other.

But, as Raghavan's survey emphasizes, recent planet discoveries have shown otherwise, and work at the Carnegie Institution in Washington provides a theoretical explanation. It shows that gas giant planets like Jupiter can form from the disk of gas and dust surrounding two stars as they do around single stars if the gravitational interaction between them does not heat the matter up too much. The planet-forming disk would remain cool enough for ice grains to remain solid and permit the growth of solid cores that form the center of big gaseous planets.

Deepak Raghavan says his findings that planets can grow in multiple-sun systems reaffirm his belief that life is common in the universe. "For us folks that like extraterrestrial life and planets, these results show that planets are quite hardy," he said. "They do form and survive in a variety of environments."

Raghavan's study shows that some of the planets in the double and triple star systems are at a safe enough distance from the primary star to allow for liquid water. He says a planet in constant sunlight conjures up images of life very different from our own.