Among all the objects in the solar system, tiny Vesta and Ceres don't exactly turn heads. They orbit the sun in the asteroid belt, amid the solar system's refuse. They lack rings, red spots, or other adornments. They're just there.

But what Vesta and Ceres lack in majesty they make up for in clues to how planets form, astronomers say. Moreover, Ceres may host an underground ocean suitable for simple forms of life.

On Thursday, the National Aeronautics and Space Administration is set to launch Dawn, a spacecraft designed to orbit Vesta, then Ceres, beginning in August 2011. Armed with a camera and two other instruments, the craft will gather a wealth of chemical, mineral, and other data on these two starkly contrasting cases of planetary arrested development.

"The differences between Ceres and Vesta are a real puzzle," says Thomas McCord, a planetary scientist at the Bear Fight Center in Winthrop, Wash., and a member of the Dawn science team.

Although they swing around the sun inside the asteroid belt, which falls between Mars and Jupiter, Vesta and Ceres are not asteroids, astronomers note. Rather, they are fully formed objects that grew as asteroids – chips knocked off of other old blocks – clumped together into planet wannabes.

Instead of a planet, an asteroid belt

European astronomers discovered the duo in the early 1800s as they searched for a planet predicted to orbit between Mars and Jupiter. Today, astronomers calculate that the asteroid belt holds enough raw material to form a full-scale planet. But within the solar system's first 10 million years, Jupiter stretched its gravitational reach far enough to ensure that chaos would reign among the asteroids. Ceres and Vesta managed to form, but Jupiter stunted their growth. Ceres, which orbits about 38 million miles farther from the sun than does Vesta, is a little less than 600 miles across. Vesta averages about 330 miles wide.

Despite their small sizes, these "protoplanets have experienced planetary processes," says Dr. McCord. By that he means that as these objects collected more material, they trapped heat generated by the decay of radioactive elements the material contained. Material melted, then separated – and the heavier material collected toward the objects' centers and the lighter material moved toward the surface.

Page: 1 | 2